Hormonal Contraceptive Failure and Its Risk Factors

Subject: Family Planning
Pages: 18
Words: 5321
Reading time:
18 min
Study level: College


Hormonal contraceptives are some of the best methods of controlling unintended pregnancies among the sexually active individuals. Hormonal contraceptives are very effective but there are instances when they fail to produce the desired results. This is because of the risk factors, which make them perform unintended purpose. This would consequently result to unintended pregnancies that would have negative impacts both socially and economically to the concerned individuals. One of the risk factors leading to the failure of hormonal contraceptive is lack of knowledge on how to use the contraceptives. This would result to poor usage. This may entail wrong timing when using the contraceptive, wrong administration of the contraceptive or general abuse of the contraceptive by the concerned individual. Another risk factor that would result in failure of the contraceptive is the irregularity of the menses. This is because it makes it difficult for an individual to predict the exact days to use the contraceptive. Obesity and general overweight is another risk factor that would result to unintended pregnancies, despite the use of hormonal contraceptive. This study is a systematic literature review of the risk factors of hormonal contraceptive failure resulting to unintended pregnancy. It is established in the study that many women carry unintended pregnancies owing to the fact that they fail to follow procedures related to the use of contraceptives. Some would even use the contraceptives unsuitably. All these factors contribute to ineffective performance of contraceptives


There is a rising concern over the rate of failure of hormonal contraceptives resulting in unintended pregnancies. According to Cochrane Gebbie and Loudon (2012, p. 4), the need to prevent unintended pregnancies is always great among the women aged 16- 24. They have the greatest need for contraceptives because at this age, they have just been introduced to sex and are probably not in any marriage. They would therefore want to avoid pregnancy at all cost because of the possible social and economic consequences. This is specifically so in Sub Sahara Africa which has the leading cases of unintended pregnancies.

The same is the case in central and southeast Asia, and parts of South America. Those in marriage aged about 25 to 44 years would also want to put to check, the number of children they bear and the interval with which this comes. Of the unintended pregnancies in Sub Sahara Africa involving women aged below 24 years, sixty percent would result in abortion, 85 percent of which are unsafe abortions (Toh, S Mitchell, A Anderka, M De Jong-Van Den Berg L & Hernandez-Diaz 2011, 422). One out of every twenty individuals who carry out unsafe abortion perishes in the process according to the report by this scholar. His study further shows that most of the incidents would occur in the developing countries.

According to Cochrane, Gebbie and Loudon (2012, p. 69) one in a sample of five married couples in the developed would have unintended pregnancy each year. This ratio would be raised to one out of a sample of three married couples in the developing countries. Such pregnancies not only have financial constrain on the concerned individual but also social problems. The economic constrain is not only on the concerned individual and her family, but also on the government which would be forced to provide healthcare for the unsuccessful abortions or such other related cases. There are also psychological tortures that a woman would be subjected to when a woman gets unintended pregnancy. As such, there is every need to ensure that such pregnancies are eliminated if not then minimized as much as possible.

As Mousavifar, Hafizi and Ashraf (2011, p. 5) say, contraceptives therefore come as the best solution to these women because the abstinence method has failed among many individuals, and may not be applicable to the married couple. Hormonal contraceptives are the most popular form of contraceptives used by women of this age group, besides condom. However, there is the concern of failure of the hormonal contraceptives in which case, there would result an unintended pregnancy.

Previous scholars have singled out as risk factors that would lead to failure of the hormonal contraceptives some factors. These scholars have noted that because of this, many individuals get unintended pregnancies leading to the above stated consequences. There is an increasing need to analyze critically these risk factors keenly to determine how to develop mitigation measures.

The hormonal contraceptives can be categorized into four sub-groups, namely: injectables (Depoprovera), progestin only pills also referred to as the mini-pill, Implantable also referred to as the Norplant, and combination oral contraceptives or the COCs. They are used under varied circumstances.

These hormones work based on three methods of actions as stated below:

  1. They primarily inhibit the ovulation process through suppression of pituitary or the ovarian axis by suppressing gonadotrophin, which releases hormone from hypothalamus.
  2. They can make cervical mucus very thick thereby inhibiting movement of sperm cells through cervix.
  3. They can also alter the uterine lining to deter any process of implantation, in case the process of fertilization occurred.

These hormonal contraceptives are not always perfect ways of preventing pregnancies. In some cases, these contraceptives would fail to prevent pregnancies as was expected.

The importance of this Review

After a carefully review of the existing literatures, it came to the realization of the researchers that there were some gap in the literatures of risk factors of hormonal contraceptives that would lead to unintended pregnancies. Most of the existing literatures were discussing other aspects of contraceptive use and talked just briefly about failure of hormonal contraceptives. The researchers were therefore realized that there was a need to conduct a review of these literatures and come up with a research paper that would specifically address the issue of failure of hormonal contraceptives leading to unintended pregnancies.

This study seeks to analyze systematically literature of risk factors of hormonal contraceptive failure, which would result in unintended pregnancy.


Given the above background, the following are some of the objectives that the researchers had when conducting this study

  1. The primary objective of this research is to critically analyze risk factors of hormonal contraceptive failure resulting in unintended pregnancy
  2. The secondary objective of this research is to analyze the effect of unintended pregnancies the affected woman and people around her in economic and social fronts.


Hormonal contraceptives are very important to women who are sexually active, as Trussell (2009, p. 205) states. There is need to ensure that all unintended pregnancies are avoided as much as possible. The aim of this study is to ensure that there is a comprehensive measure to avoid the possibility of failure of the contraceptives. It seeks to do this through a comprehensive and systematic analysis of literatures in order to bring to fore some of the past incidents that involved unintended pregnancies. The paper would also aim at showing how past cases were dealt with in order to yield better results. Through this, the researchers hope to compile a report that would help couples and other professionals realize that the contraceptives are effective but not to the totality and therefore, it is important that an individual takes the responsibility in case such an unexpected and unintended pregnancy is to be avoided.

The research also seeks to achieve the following:

  1. To demonstrate that although the hormonal contraceptives are effective, there are loopholes that would leave a possibility of an individual being pregnant
  2. To come up with mitigation measures that would ensure that the loopholes are sealed as much as possible, which would help eliminate all the possibilities of having the unintended or unplanned pregnancies.

Research Question

As stated above, hormonal contraceptives are not perfect ways of preventing pregnancies though their service in avoiding the same is invaluable. To help focus this research, the researchers developed the research question below:

  1. To come up with mitigation measures that would ensure that the loopholes are sealed as much as possible, which would help eliminate all the possibilities of having the unintended or unplanned pregnancies.


Type of study

This research utilized quantitative research methods in conducting the study and collecting data. Quantitative research was used all through because it aims at summarizing data mathematically. In this regard, the research took the form of a literature review to gather data. The study utilized deductive scaffold because some concepts were used to give a certain picture of the study. Deductive reasoning starts by analyzing some concepts before moving to the available data, in this case the secondary data was used to confirm the claims. The researchers analyzed some of the concepts related to contraceptives, their usage, and affectivity before moving to collect data. The researchers did not use non-randomized control studies because this research is not an intervention.

Secondary data for analysis in this project came from various publications. These included reports, journal articles, and research publications. Most of it related to the work that other researchers in the field of human sexuality and contraceptive use undertook. The other areas where the literature review covered include theories relating to the use of contraceptive and their affectivity to many people around the world given the differences in socio-economic structure of people around the world. The nature of material used varied. Journals dealing with specific aspects of human sexuality and contraceptive use provided specific information on specific research elements investigated by researchers.

The researchers strictly considered using secondary sources that were published within the last ten years. This was so because hormonal contraceptives are a comparatively new technology. It is true that the genesis of these contraceptives can be traced to years beyond the given timeline. However, it is within the last ten years that medical researchers have advanced in this field and developed a clear understanding of the hormonal contraceptives. In this study, the researchers excluded condom use as one of the most popular contraceptives. This is because the technology used in condoms is very different from that used in the hormonal contraceptives. The researchers appreciate its role in this field but given the specific focus of the study, it could not be included in the study.

The researchers also excluded post coital emergency contraceptives, otherwise known popularly as the morning pills. This is because their efficacy and functionality or even their failure rates are presented from the other hormonal contraceptives, which are under this investigation. The study also excluded the use of intra-uterine device (IUD) because the technology involves in this contraceptive is different from that used in the hormonal contraceptives. However, the study included studies whose relevant measures were discontinuation. This was to help determine method specific risk factors for failure.

This review comprehensively covers studies from different socio-economic around the world. The researchers limited the study to English speaking nations only. Because the researchers did not go to the field to gather data directly from the respondents, primary research was not considered as part of this study.

Type of Participants

This study specifically targeted sexually active women. Although it has been scientifically proven that a woman as young as twelve years is capable of conceiving, this are always abnormal cases and many studies before have proven that such young girls would have difficulties of carrying such pregnancies to term. Their cases would be considered unnatural and it is not likely that they are in a position to access hormonal contraceptives even if they were in the capacity to. A research of this type concerning individuals of this age may also be challenging because it would not be easy to determine a sexually active twelve or thirteen year old girl. As such, they were not included in this study. The researchers included women who are sexually active and between the ages sixteen to forty four years. Inasmuch as it is true that women who are aged between forty five to forty eight have the capacity of getting pregnant because they have not reached the menopause, chances of this happening is always minimal. They do not fall in the category of those who are at risk of getting unintended pregnancies as those captured in this study. The participant must be sexually active and must have used at least one form of the hormonal contraceptives such as the pill.

Type of intervention

In this study, the researchers mainly considered the use of hormonal contraceptives among the sexually active women. The research would consider the use of such hormonal contraceptives as transdermal, injectibles, implantable, transvaginal and oral contraceptives such as the combined oral contraceptive pill. Although there are some intrauterine devices that are hormonal, the study strictly covered the hormonal contraceptive and ignored the device. The device was therefore considered as implantable form of hormonal contraceptives.

Type of outcome Measures

Primary Outcome

The primary outcomes were contraceptive failure, which would lead to unintended pregnancies.

Secondary Outcomes

From the above primary outcomes, the secondary outcome are abortions, adverse clinical events, economic complications, risk of rejection from members of the family and even friends, and reasons for discontinuity.

Search Methods for Identification of Studies

In this study, the electronic search was done MEDLINE, POPLINE, CENTRAL (Cochrane Central Register of Controlled Trials), EMBASE and CINAHL.

EMBASE via Ovid

  1. (contracept$ or sub-dermal implant$ or rhythm method or hormonal contracept$ or birth control$ or family planning or pillS or IUD$ or condom$ or coil$).ti,ab.
  2. (risk factor$ or risk$ or associat$ or correlatS).ti,ab.
  3. (pregnan$ or unintended pregnan$ or conception$ or fail* contracept*or faulty contracept*).ti,ab.

Limits: human, female and year-2002-current.

MEDLINE via Ovid

  1. exp oral contraception/ or exp treatment failure/ or exp intrauterine contraceptive device/ or exp contraception/ or exp contraceptive agent/ or exp oral contraceptive agent/ or exp family planning/
  2. exp unwanted pregnancy/ or exp pregnancy/ or exp pregnancy outcome/ or exp unplanned pregnancy/
  3. (contracept$ or sub-dermal implant$ or rhythm method or hormonal contracept$ or birth control$ or family planning or pillS or IUD$ or condom$ or coil$).ti,ab.
  4. (fail$ or faulty or breakdown$).ti,ab.
  5. (risk factor$ or associat$ or correlatS).ti,ab.
  6. (pregnan$ or unintended pregnan$ or conception$ or breakthrough pregnan$ or gravid$).ti,ab.
  7. 3 and 4 and 5 and 6
  8. 1 and 2 and 7
  9. limit 8 to (female and humans and yr=”2002 -Current”)

Cochrane Central Register of Controlled Studies

(contracept*, Oral contracept*, Hormonal contracept*, IUD, birth control, pill*, family planning, coil*, condom*) and (fail*, breakdown, problem*) and (correlat*, risk factor*, risk*, associat*) and (pregnancy, unintended pregnancy) and (female) from 2002 to 2012


(contracept*/hormonal contracept*/oral contracept*/sub-dermal implant*/rhythm method/birth control*/pregnancy prevent*/family planning/pill*/IUD*/coil*) & (fail*/faulty/breakdown) & (risk factor*/correlat*/associat*) & (pregnan*/unintended pregnan*/breakthrough pregnan*)


  1. “contraceptive failure”/all topical subheadings/all age subheadings
  2. “breakthrough pregnancy”/all topical subheadings/all age subheadings
  3. “unintended pregnancy”/all topical subheadings/all age subheadings
  4. “pregnancy”/all topical subheadings/all age subheadings
  5. explode “Contraception”/ all topical subheadings / all age subheadings
  6. explode “Contraceptive drug”/ all topical subheadings / all age subheadings
  7. explode “Contraceptive-Devices”/ all topical subheadings / all age subheadings
  8. explode “Family-Planning”/ all topical subheadings / all age subheadings
  9. ((family near6 planning) or contracept* or hormonal contracept* or oral contracept* or pill* or IUD* or (pregnan* near6 prevent*) or (birth near6 control*)) in ti,ab
  10. #1 or #2 or #3 or #4
  11. #5 or #6 or #7 or #8 or #9
  12. (risk* or factor* or risk factor* or risk-factor* or correlat* or associat* or relat*) in ti,ab
  13. #10 and #11 and #12

Limits – Human, Sex, Published date from January 2002 to July 2012.

Searching other Resources

The researchers examined the list of references of the relevant literatures for the purposes of additional citation. The researchers made no attempt of searching grey literatures (the unpublished studies) because there were enough literatures that were published in this field.

Data Collection and Analysis

Selection of Studies

The researchers will asses all titles and abstracts that were identified in the literature searches for the purpose of inclusion, limiting them to those in English language. One author will review independently, the search results in order to identify the reports for the purposes on inclusion or exclusion in this study. The second author would ensure that any disagreement are in the identified reports are resolved and appropriately categorized as per the criteria of this protocol. The researchers did not include all study designs. For instance, this research did not include randomized control trials (RCTs). The researchers then considered post hoc analysis of the literatures from the identified studies, which met the set criteria for consideration in this study.

Data extraction and management

One of the researchers would extract relevant data from the identified abstracts and enter the information into RevMan. Another researcher would conduct a follow-up data extraction for the verification of correctness data entry. This second researcher would take note of all discrepancies or conflicts between his entry and that of the previous researcher. The researchers would discuss all the noted discrepancies and in some occasions where the discrepancies were not obvious enough to be resolved by the two researchers, then they would call in a third party who would help them iron out the issue. The data was strictly taken from the studies of the past ten years. Within this year, the researchers were keen to identify data from the literatures that were available. In cases that the missing data was considered vital for this study, the researchers contacted relevant authorities for further clarification. In other cases, the researchers tried to construct the missing data from the data, which was available.

Assessment of Risk of Bias in the Included Studies

Because the researchers did not use randomized control trials as a method of selection of studies, its principles could not be applied in this study. The researchers therefore had to go for other principles, which would be relevant given the methodology used in this study. This included concealment selection method, application o sequentially numbered sealed envelopes and the centralized system. This would help eliminate any form of bias that may result when the researchers are allowed to choose the studies on a free will (Xu, Peipert, Zhao, Madden & Secura 2011, p. 37). The researcher ensured that the selected studies included all individuals targeted in this study in regard to age, body weight, health conditions and socioeconomic status. All the limitations in the design were amicably presented in Risk of Bias in the included studies, and were given consideration in the interpretation of the results.

Assessment of Heterogeneity

In this study, the study population, design and the interventions were heterogeneous, as would be expected. In order to ensure uniformity in this study, the researchers avoided studies that had application of different contraceptives. For instance, the study did not involve the use of condoms as one the contraceptives.

Data Synthesis

In this research, Peto odds ratio (OR) was used for the possible dichotomous outcomes. The confidence interval was 95 percent. Such examples included women who got unintended pregnancies even after using the hormonal contraceptives. Most of the hormonal contraceptives have always registered the efficacy of over 95 percent Trussell (2009, p. 201). It therefore means that chances of an individual getting pregnant while using them is very law. Mousavifar, Hafizi and Ashraf (2011, p. 6) say that Peto OR is very effective when the events are no so common, or the effects are relatively small. The approach was taken because it would perform very well various circumstances unless the arm sizes of the study are unbalanced (Xu, Peipert, Zhao, Madden & Secura 2011, p. 34). This method was considered appropriate because require zero events correction. Given the rarity of this event, it is possible that there can be a zero event. Under normal circumstances, random effects and fixed effects would similar results in case heterogeneity does not exist (Dinger,Minh, Buttmann, and Bardenheuer 2011, p. 67). The researchers therefore did a test for possible statistical heterogeneity of the data. The result would be examined the specific contraceptive method in the study.

Sensitivity Analysis

The researchers considered only those studies that had been approved by GRADE (Grading of Recommendations, Assessment, Development, and Evaluation system. The studies must have also confirmed that there are risk factors of hormonal contraceptives that would lead to unintended pregnancies. The study must have used references that would be considered as appropriate for the study.


Included Studies

The researchers chose eight of the studies that were presented for the study, which passed the predetermined specifications. Their study had at least 3668 women. The median size of the sample size was 3024. The studied hormonal contraceptives varied as demonstrated below.

A study by Trussell 2009 showed that contraceptive failure is a major source of unintended pregnancies. Moreau, Trussell, Rodriguez, Bajos, and Bouyer 2007 looked at the failure of the contraceptive among the active users especially in the United States and France. They looked at various contraceptives and then determined their rates of failures, especially among the initial users. Dinger,Minh, Buttmann, and Bardenheuer 2011 analyzed effectiveness of the oral contraceptive pills, especially in the US cohort closely comparing regimen and progestogen, basing their study on 73,269 women.

Xu, Wade, Peipert, Zhao, Madden, and Secura 2012 analyzed the rate of contraceptive failure of Entonogestrel Subdermal Implants in women who are overweight or obese. Their study included in 8445 participants who used different types of hormonal contraceptives. Richman 2010 looked at the failure of BMI and OCP. The study of this researcher is of interest because it exonerates obesity as a risk factor for hormonal contraceptives. Brunner and Toth 2007 focused their study on contraceptive failure which is associated with progestogen-only pill. Kaneshiro and Edelman 2011 analyzed consideration of contraceptives among the teens.

Excluded Studies

The researchers were able to amass a large volume of literatures in this field. Not all of them could be analyzed in this research. Such literatures were disqualified due to the year they were published or their impact based on the number of respondents that were involved in the research. Pharmacokinetic studies were also excluded in this study.

Risk of Bias in Included Studies

Although the researchers made great efforts to ensure that bias was eliminated as much as possible, it was clear that some studies could not avoid some form of bias. Chandler and Nash 2010 noted some of the participants were blinded by the kind of assignments that they were given. Two of the reports had a clearly incomplete outcome data (Richman 2010; Trussell 2009). In the study of Dinger,Minh, Buttmann, and Bardenheuer 2011, the participants were expelled from the study if they missed out on four consecutive session, reducing the number of participants by 13 percent.



The study by Xu, Wade, Peipert, Zhao, Madden, and Secura 2007 involved 8445 participants. The study showed that women who were obese would not have any higher risk of experiencing contraceptive failure due to their body mass. The participant used various types of implantable. The researchers used chi test the t test and Kaplan-Meier to analyze the data. Their study shows that there is no relationship between failures of the implantable and obesity. This study concludes that implantable are safe for the obese women. This was confirmed by Kaneshiro and Edelman (2011, p. 124) who note that the implantable are completely safe for the obese teenagers.

However, the study by Chandler and Nash (2010, p. 45) shows that there is some relationship between obesity and failure of the progestogen only pills, otherwise known as POP. According to their report, women who had body mass index of 36 had higher chances of failure of the hormonal contraceptives. The study relates the failure of this specific hormonal contraceptive to the body weight.


According to Trussell (2009, p. 67), failure of the hormonal contraceptives that would lead to unintended pregnancies do not have any serious relationship with the implantable as form of hormonal contraceptive. This scholar notes that the relationship that may exist in this regard may be experience when it comes to the application. This study shows that age had no relationship with the failure of the injectibles. Cochrane Gebbie and Loudon (2012, p. 79) note that unintended pregnancies in older women, especially if they are obese have significant effect to their health.


Body Mass Index has also been considered as one of the risk factors that would lead to failure of the hormonal contraceptives. Moreau, Trussell, Rodriguez, Bajos, and Bouyer (2007, p. 56) note that American women were at higher risks of experiencing failure of the injectibles as a form of hormonal contraceptives. This was the case in France. These researchers registered a higher rate of failure of the hormonal contraceptives (injectibles) among the women who are sexually active.

Social Status of the woman

The study by Dinger, Minh, Buttmann and Bardenheuer (2011, p. 58) analyzes the relationship between economic status of women and failure of the hormonal contraceptives. The study shows that economic statuses of women do not have any direct association with failure of the hormonal contraceptives, especially the oral pills. Their study did not register any difference in the efficacy of the oral contraceptives based on the level of wealth of an individual.


Some of the most common methods of preventing pregnancies have been the use of hormonal contraceptives. Hormonal contraceptives are currently considered the best way of avoiding unintended pregnancies in case abstinence cannot be applied. They offer the best solution in case they are used properly. The failure of hormonal contraceptives depends on various factors, as is seen from the above meta-analysis. One of the most common reasons why hormonal contraceptives fail is the wrong timing. Some of the users of these contraceptives fail to follow the correct time-line procedures that should be followed when administering the contraceptives.

Those contraceptives that are administered to the body through injection have a clear guideline of the time specifications when it should be administered. This is because the hormone needs to be active in the body by the time sperm is released into the vagina during the coital process. When it is administered earlier than is expected, the contraceptive may fail because by the time of coital, it may be too weak to perform as was expected. As such, the individual would end up with a pregnancy that was not intended. When the administration is done too soon prior to the coital, there are chances that the contraceptive may not be active in the body as at the time it is expected to. By the time this contraceptive becomes active in the body, it may be too late to perform its intended purpose. The concerned individual would therefore result with unintended pregnancy while well aware that all precautionary measures are taken to prevent the same.

Another factor that clearly comes out from the above analysis as a reason for failure of the hormonal contraceptives is lack of proper knowledge of how to administer the contraceptive. Some of the active users of the contraceptives lack the knowledge of how to use the contraceptives. Dinger, Minh, Buttmann and Bardenheuer (2011, p. 76) notes that trans-vaginal contraceptives and the implantable are some of the hormonal contraceptives that are commonly abused by various users. There is a position where the trans-vaginal hormones should be applied. Before reaching the uterus, the sperm must pass through this contraceptive. The rational in this case is that the sperm should be killed before it reaches the uterus. It is therefore important that the application of the hormone be done on the end part of the virginal opening to ensure that the sperms would pass through the contraceptive. Very few people know how to administer this contraceptive, according to a study by Moreau, Brunner and Toth (2007, p. 1311). This scholar notes that when this fails, the unintended pregnancy would be unavoidable.


Contraceptives remain the best method of containing unwanted pregnancies. According to the systematic literature review given above, it is very clear that the use of contraceptives has been very popular among the sexually active individuals, both in the developed and developing nations around the world. This rejects our first null hypothesis, which proposed otherwise. However, the use of the contraceptives is not a guarantee that the individual user is 100 percent sure that the contraceptive is safe. This is a confirmation of our second null hypothesis that proposed that hormonal contraceptives 100 percent effective. There are some risk factors of hormonal contraceptive, which would result in unintended pregnancy.

Some of the risk factors are related to the use of the contraceptives. This includes wrong timing by the users of the contraceptives. When the contraceptives are not used at the right time, then there is a risk of a pregnancy. Another risk factor is the wrong usage of the contraceptive. All the four groups of contraceptives have a clear direction on how they should be administered. This procedure should be adhered to very strictly if the concerned individual expects good results from the contraceptive. In some occasions, the risk factors are due to the physiological process of a person. Another major risk factor of hormonal contraceptive failure is obesity or overweight in general. This study has demonstrated that obese women have higher chances of failure of the hormonal contraceptives leading to unintended pregnancies. The situation is even worse in the developing counties as opposed to developed countries. This is due to the low levels of literacy in the third world.


The above systematic literature review of risk factors of hormonal contraceptive failure resulting in unintended pregnancy has unearthed some facts about efficacy of the hormonal contraceptives. It has managed to separate some of the commonly held beliefs, which are not factual from facts about hormonal contraceptives. The following are some of the recommendations that the researchers would give in respect to the findings of this study.

This study has compiled literatures that have addressed the issue of hormonal contraceptive failure leading to unintended pregnancies. Because of some disagreement that the researchers noted in some of the existing literatures, it is recommended that further research be done on the risk factors in order to clarify the observations that are identified in this review. This study affirms that they are likely factors that need to be further researched on further for clarification.

List of references

Brunner, R & Toth, J 2007, Obesity and oral contraceptive failure, Findings from the 2002 National Survey of Family Growth American Journal of Epidemiology, Vol. 166 no. 11, pp. 1306-1311.

Chandler, J & Nash, K 2010, Contraceptive failure and the progestogen-only pill: the issue of body weight, Journal of Family Planning and Reproductive Health Care, Vol. 36 no. 3 pp. 167-8.

Cochrane, R Gebbie, A & Loudon, J 2012, Contraception in obese older women, Maturitas, 2012.

Dinger, J Minh, T Buttmann, N & Bardenheuer, K, 2011, Effectiveness of oral contraceptive pills in a large U.S. cohort comparing progestogen and regimen, ZEG, Berlin Center for Epidemiology Studies, Vol. 11 no. 5, pp. 10-115.

Kaneshiro, B & Edelman, A 2012, Contraceptive considerations in overweight teens, Current Opinion In Obstetrics and Gynecology, Vol. 8 no. 15, pp. 101-155.

Moreau, C Trussell, J Rodriguez, G Bajos, N & Bouyer, 2007J, Contraceptive failure rates in France: Results from a population-based survey, Human Reproduction, Vol. 22 no. 9 pp. 2422-2427.

Mousavifar, N Hafizi, L & Ashraf, H 2011, Determining the rate of oral contraceptive pills failure in unintended pregnancies, Iranian Journal of Obstetrics, Gynecology and Infertility, Vol. 14 no. 2 pp. 1-6.

Richman, S 2010, BMI and OCP failure, Contraception, Vol. 81 no. 2 pp. 1-175.

Toh, S Mitchell, A Anderka, M De Jong-Van Den Berg L & Hernandez-Diaz, S 2011, Antibiotics and oral contraceptive failure – A case-crossover study, Contraception, Vol. 83 no. 5 pp 418-425.

Trussell, J 2009, Understanding contraceptive failure: Best Practice and Research, Clinical Obstetrics and Gynaecology, Vol. 8 no. 16 pp. 199-209.

Xu, H Wade, J Peipert, J Zhao, Q Madden, T & Secura, M, 2012 Contraceptive Failure Rates of Etonogestrel Subdermal Implants in Overweight and Obese Women, Obstetrics and Gynecology, Vol. 117 no. 1 pp. 33-40.