Study Enrollment

The Contraceptive CHOICE Project is a prospective cohort study with the primary goal of promoting the use of long-acting reversible contraceptive methods as a means of reducing unintended pregnancies.13 From August 2007 through September 2011, we enrolled 9256 participants at risk for unintended pregnancy. All participants chose a contraceptive method and received it at no cost. Study participants were a convenience sample of women and adolescents in the St. Louis region, recruited by means of referrals from medical providers, newspaper reports, study flyers, and word of mouth.

The study protocol was approved by the local institutional review board before recruitment was initiated. All participants provided written informed consent.

Eligible participants were 14 to 45 years of age, were not currently using a contraceptive method or were willing to switch to a new reversible contraceptive, had no desire for pregnancy for at least the next 12 months, were sexually active or were planning to become sexually active with a male partner during the next 6 months, resided in the St. Louis region, and spoke English or Spanish. Persons were excluded if they had undergone a hysterectomy or sterilization procedure.

Study Design

Before enrollment in the study, all potential participants were read a standardized script regarding long-acting reversible contraceptive methods that stated that the two IUDs and the subdermal implant were the most effective methods of contraception (see the Supplementary Appendix, available with the full text of this article at NEJM.org). If they were eligible for enrollment, participants received contraceptive counseling regarding all reversible contraceptive methods, including their effectiveness, side effects, risks, and benefits. Each participant was then provided with a reversible contraceptive method of her choice at no cost for 3 years (first 5090 participants) or 2 years (remainder of cohort). Participants were permitted to discontinue or switch methods as many times as desired during the follow-up period. A comprehensive baseline interview was performed, and participants were screened for sexually transmitted infections.

Participants were followed prospectively, with telephone interviews at 3 and 6 months and every 6 months thereafter for the duration of follow-up. Participants received a $10 gift card for every completed follow-up interview. In the baseline and follow-up interviews, we collected comprehensive information on demographic characteristics and reproductive history.

This analysis includes the first 7486 participants who used an IUD, implant, DMPA injection, pills, patch, or ring during the study. Periods of condom use or other contraceptive method use (e.g., diaphragm or natural family planning) were excluded from the analysis of contraceptive failure. At each survey, participants were asked about missed menses and possible pregnancy. Any participant who thought she might be pregnant was asked to come in for urine pregnancy testing. A pregnancy log was used to record all pregnancies. Participants who had a pregnancy were asked if it was intended and what contraceptive method (if any) they were using at the time of conception. The conception date was estimated from the date of the last menstrual period or from the gestational-age assessment on ultrasonography. Contraceptive-method failure was defined as conception that occurred during a period when the contraceptive method was used. If the participant stated that she had stopped using the method, this was listed as “no method” and was not considered a contraceptive-method failure. We excluded conception that occurred after a participant stopped using a method owing to a desire to conceive (intended pregnancy).

Information about periods of contraceptive use, including start and stop dates, was collected from three sources: scheduled telephone interviews; pharmacy data obtained from the partner pharmacy where participants obtained pills, patch, or ring; and the participant contraceptive-method log, which documented in our research records when the participant initiated or discontinued use of a method or switched to another method (i.e., insertion or removal of an IUD or implant; receipt of an initial pill supply, patch, or ring; and DMPA injection). A participant was considered to have used DMPA for the 3-month interval after a record of an injection. In the case of expulsion of an IUD, if the participant knew the device had fallen out and she became pregnant, the unintended pregnancy was attributed to “no method” (unless an alternative method was used). However, if the participant was unaware that the device had fallen out, the pregnancy was attributed to IUD failure.

At each follow-up interview, we asked participants if they were still using the same contraceptive method, if they had stopped using it, and their start and stop dates. For participants who did not answer these questions, we reviewed their contraceptive-method log and pharmacy-refill records to confirm their status. If the data from the log, follow-up interview, and pharmacy records were conflicting, we used the data source with the most detailed information. If multiple contraceptive methods were used simultaneously (e.g., pills and condoms), the most effective method was assigned.9 Data from participants who were lost to follow-up were censored at the time of their last completed interview.

Study Outcomes

The primary outcome of the study was contraceptive failure. We also evaluated pregnancy rates by age group as a secondary outcome. Our hypotheses were, first, that participants using pills, patch, or ring would have higher rates of contraceptive failure than those using long-acting methods and, second, that the rate of failure with the pills, patch, or ring would be higher among younger women and adolescents (<21 years of age) than among older women.

Statistical Analysis

Statistical analyses were performed with the use of Stata software, version 11 (StataCorp). The significance level (alpha) was set at 0.05. To describe the demographic characteristics of the study participants, we used means, standard deviations, frequencies, and percentages. For the comparison among users of different contraceptive methods, Student's t-test was used for continuous variables and a chi-square test was performed for categorical data.

We measured distinct segments of contraceptive-method use that represented the months each participant used the method. We calculated participant-years of use for each method by capturing all segments of use. Thus, participants who switched methods contributed distinct segments to multiple methods. Cox proportional-hazard models were used to estimate the hazard ratios for unintended pregnancy with different methods. We used clustering of variance–covariance estimation methods to account for the effect of correlation among different periods of contraceptive use and multiple pregnancies for the same participant. Effect modification was assessed by including an interaction term between the method and the covariate of interest in the model. The final multivariate model included adjustment for confounders and other variables with the potential to influence the outcome.