Study Design and Key Measures

The methods we used for this analysis are similar to those used in previously published studies.15,17 Among all U.S. females and key population subgroups, we determined the total number of pregnancies that ended in birth, miscarriage (i.e., fetal loss or stillbirth), and induced abortion and calculated the percentages of each of these pregnancy outcomes that were unintended; we then divided the total number of unintended pregnancies by the population of women and girls 15 to 44 years of age to obtain a rate of unintended pregnancy per 1000 in this age group.

Data Sources and Definitions

The numbers of U.S. births, miscarriages, and abortions reported or estimated in 2011 and 2008 were derived from several sources. The numbers of births were obtained from NCHS,18,19 which tabulates data from birth certificates to obtain birth counts at the national level. Because there is no recognized best estimate of the number — or method to obtain the number — of miscarriages in a given year, we followed a procedure that was established by researchers at NCHS20 using that center’s National Survey of Family Growth (NSFG), a nationally representative in-home survey that collects information on pregnancy and childbearing: we calculated the ratio of miscarriages to births that were reported in the NSFG and multiplied that ratio by the actual number of U.S. births to obtain our estimates of the number of miscarriages. The total number of abortions, including both surgical and medication abortions, for each year was obtained from a periodic census of all known abortion providers that was conducted by the Guttmacher Institute.21 This census is considered to be the most comprehensive source of data on the incidence of abortion in the United States.22

Pregnancy intention was defined according to a respondent’s answers to a series of retrospective survey questions about her desire to become pregnant right before each pregnancy occurred. If she reported that she did not want to become pregnant at the time the pregnancy occurred, but wanted to become pregnant in the future, the pregnancy was categorized as mistimed. If a respondent reported that she did not want to become pregnant then or at any time in the future, the pregnancy was categorized as unwanted. We classified a pregnancy as unintended if it was either mistimed or unwanted; an intended pregnancy was one that was desired at the time it occurred or sooner.

Data on pregnancy intentions (often called intendedness) were obtained from two nationally representative sources. The percentages of births and miscarriages that resulted from unintended pregnancies were calculated from the 2011–2013 NSFG. We evaluated 1975 pregnancies that ended between 2009 and 2013 (with 2011 as the central or reference year), as reported by the respondents; a respondent could report more than one pregnancy. The percentages of abortions that followed unintended conceptions were calculated from the 2008 Abortion Patient Survey that was conducted by the Guttmacher Institute.23 This paper-and-pencil survey gathered information from a representative sample of 9493 women who had abortions in the United States and is the most recent data set available of its kind. The questions about pregnancy intention in the Abortion Patient Survey were modeled on those in the NSFG. For both data sets, the pregnancy outcomes were weighted to represent all pregnancies in the United States in 2011.

Statistical Analysis

The percentages of births, miscarriages, and abortions that resulted from unintended pregnancies were applied to the counts of each respective pregnancy outcome and then summed to determine the total number of unintended pregnancies. To calculate rates, we obtained population counts according to age and according to race and ethnic group from the U.S. Census Bureau.24 All other distributions of population subgroups were derived from the Annual Social and Economic Supplements of the U.S. Census Bureau’s Current Population Survey,25 except for religious affiliation, which was derived from the NSFG. Poor females were defined as those with incomes below 100% of the federal poverty level, and low-income females were those whose incomes were between 100% and 199% of the federal poverty level.

When calculating the percentage of unintended pregnancies that ended in abortion, we excluded miscarriages in order to assess only pregnancies in which the outcome was determined by the respondent. The rates of unintended pregnancy according to educational attainment were limited to women 20 years of age or older; this age cutoff excluded most females who had not yet completed schooling, yet still included young women, who have had historically high rates of unintended pregnancy. We also updated the rates of unintended pregnancy for 1981, 1987, 2001, and 2008 — years that the NSFG was fielded — to take into account updated population estimates and recent improvements in our analytic approach. Data on pregnancy intendedness were also collected in the 1995 survey of the NSFG but were excluded owing to concerns about the accuracy of the pregnancy intendedness data from that year.26

We performed analyses at an aggregate level and separately for each population subgroup: we combined data on pregnancy intention, pregnancy outcomes, and populations from several different sources to calculate rates, which made it difficult to assess the reliability of our estimates and of the change over time. Because most of the uncertainty around the rate estimates was attributable to the percentage of pregnancies that were unintended (since the numbers of pregnancies and population denominators are based largely on generally complete census data), we performed a supplementary analysis to calculate 95% confidence intervals for the percentage of pregnancies that were unintended using a merged data set that combined the sample of births and miscarriages from the NSFG with the sample of abortions from the Abortion Patient Survey. We then used this range of percentages to calculate the 95% confidence intervals around the rate estimates. Although these percentages are expected to be less accurate than the ones calculated in the aggregate manner, the 95% confidence intervals around these percentages should represent the variance around the rate estimates.

The above approach uses two different data sources for pregnancy intention. We also used a single data set, the NSFG, to calculate a test statistic for the change between 2008 and 2011 in the percentage of pregnancies that were unintended. Using the NSFG alone for all pregnancy outcomes allows for a simple calculation of the test statistic. Abortions are underreported in the NSFG, and therefore the percentages calculated using this approach were expected to be lower than those in our main analysis. Nonetheless, we considered this analysis of trends to be reasonable, because the underreporting of abortions has not changed substantially over time.27,28