American Journal of Epidemiology Vol. 152, No. 3 : 287-289
Copyright © 2000 by The Johns Hopkins University School of Hygiene and Public Health
LETTERS TO THE EDITOR |
RE: "ESTIMATES OF THE ANNUAL NUMBER OF CLINICALLY RECOGNIZED PREGNANCIES IN THE UNITED STATES, 1981–1991"
Division of Research Kaiser Permanente Medical Care Program Oakland, CA 94611-5714
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INTRODUCTION |
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We were pleased to see the recent Journal article by Saraiya et al. (1
), in which the authors attempted to estimate the number of clinically recognized pregnancies that occur annually in the United States. Because pregnancies ending in live birth are determined easily from state vital statistics, this task reduces to estimating the number that end in some form of fetal loss. Because many forms of fetal loss are not routinely reported, the authors had to rely on innovative uses of sample studies to derive their numbers.
To derive the number of spontaneous abortions, they used data from the well-known Wilcox et al. study (2) based on the menstrual cycles of a large number of young women followed up over many years. From this study, Saraiya et al. (1) extracted age-specific rates of spontaneous abortion, applying them to the assumed maternal age distribution in the United States during an 11-year study period, 1981–1991.
The Wilcox et al. study (2), however, occurred 10–50 years before the Saraiya et al. (1) time period of interest. Spontaneous abortion rates may well have changed. More important, the Wilcox et al. study occurred well before the legalization of induced abortion, when the pressure from this competing pregnancy outcome was slight. In addition, the Wilcox et al. study population included only a single social class of White, college-educated women and their offspring. Despite these limitations and others (3), the Wilcox et al. study was the only one available to Saraiya et al. that provided spontaneous abortion rates by maternal age.
To update and/or confirm the Wilcox et al. study's (2) age-specific rates and therefore the Saraiya et al. study's (1) extrapolations of them, we reanalyzed data from a previous investigation (4). Our investigation was conducted within the Saraiya et al. study period; featured a multiracial, working class population; and included induced abortion as a likely pregnancy outcome (table 1). In our investigation, we followed a large cohort of pregnancies from the time of an initial pregnancy test (at gestational week 5 and beyond) until pregnancy outcome. We used fetal life table methodology to calculate the incidence of spontaneous abortion, excluding ectopic pregnancy and treating induced abortion as a censoring event.
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The good news is that we were able to confirm two of the assumptions made by Saraiya et al. (1
). First, we found that the incidence of spontaneous abortion increased with maternal age by roughly the same degree as found in the Wilcox et al. (2) study. Second, we found little discernible difference in spontaneous abortion rates by maternal race (table 2).
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Our overall incidence of spontaneous abortion, however, was lower than that of the Wilcox et al. study (2
), 0.11 versus 0.16 (0.14 in the Saraiya et al. study (1) was due to age adjustment of the Wilcox et al. rates). Our lower incidence could have resulted from lower underlying population risks, different time periods of investigation, or different measuring abilities. (Note that our 0.11 incidence excludes the first week after the missed menses; that is, it begins at 5 completed weeks from the last menstrual period and goes approximately through 20 weeks of fetal development.)
In any case, we suggest that the Saraiya et al. study (1) estimates are high even if our more recent data are not considered. Spontaneous abortion rates from the Wilcox et al. study (2), calculated in the absence of induced abortion, need to be adjusted downward in the Saraiya et al. study in the presence of induced abortion as a substantial, competing risk (5–7) (table 3). Without an adjustment, the estimated number of spontaneous abortions may be inflated by as much as 1 million.
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The magnitude of the risk of spontaneous abortion has been, and remains, elusive. Recent prospective investigations that follow early pregnancy by measuring maternal human chorionic gonadotropin show that incidences of spontaneous abortion in the clinical period (after 4 completed weeks from the last menstrual period), in the absence of induced abortion as a competing risk, range from 10 to 18 percent (8
–12). Between-studies differences are at least partially attributed to difficulties in distinguishing between categories of occult and clinical pregnancies around the time of the missed menses (11). |
REFERENCES |
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TOPINTRODUCTIONREFERENCESINTRODUCTION REFERENCES |
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Saraiya M, Berg CJ, Shulman H, et al. Estimates of the annual number of clinically recognized pregnancies in the United States, 1981–1991. Am J Epidemiol 1999;149:1025–9.
[Abstract/Free Full Text] -
Wilcox AJ, Treloar AE, Sandler DP. Spontaneous abortion over time: comparing occurrence in two cohorts of women a generation apart. Am J Epidemiol 1981;114:548–53.
[Abstract/Free Full Text] - Wilcox AJ, Gladen BC. Spontaneous abortion: the role of heterogeneous risk and selective fertility. Early Hum Dev 1982;7:165–78.[Web of Science][Medline]
- Goldhaber MK, Fireman BH. The fetal life table revisited: spontaneous abortion rates in three Kaiser Permanente cohorts. Epidemiology 1991;2:33–9.[Medline]
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Susser E. Spontaneous abortion and induced abortion: an adjustment for the presence of induced abortion when estimating the rate of spontaneous abortion from cross-sectional studies. Am J Epidemiol 1983;117:305–8.
[Abstract/Free Full Text] -
Figa-Talamanca I, Repetto F. Correcting spontaneous abortion rates for the presence of induced abortion. Am J Public Health 1988;78:40–2.
[Abstract/Free Full Text] - Hammerslough CR. Estimating the probability of spontaneous abortion in the presence of induced abortion and vice versa. Public Health Rep 1992;107:269–77.[Web of Science][Medline]
- Edmonds DK, Lindsay KS, Miller JF, et al. Early embryonic mortality in women. Fertil Steril 1982;38:447–53.[Web of Science][Medline]
- Whittaker PG, Taylor A, Lind T. Unsuspected pregnancy loss in healthy women. Lancet 1983;1:1126–7.[Web of Science][Medline]
- Wilcox AJ, Weinberg CR, O'Connor JF, et al. Incidence of early loss of pregnancy. N Engl J Med 1988;319:189–94.[Abstract]
- Zinamon MJ, Clegg ED, Brown DD, et al. Estimates of human fertility and pregnancy loss. Fertil Steril 1996;65:503–9.[Web of Science][Medline]
- Bonde JP, Hjollund NH, Jensen TK, et al. A follow-up study of environmental and biologic determinants of fertility among 30 Danish first-pregnancy planners: design and methods. Reprod Toxicol 1998;12:19–27.[Web of Science][Medline]
The First Two Authors Reply
Division of Cancer Prevention and Control National Center for Chronic Disease Prevention and Health Promotion Centers for Disease Control and Prevention Atlanta, GA 30341-3724
Pregnancy and Infant Health Branch Division of Reproductive Health Centers for Disease Control and Prevention Atlanta, GA 30341-3724
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INTRODUCTION |
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TOPINTRODUCTIONREFERENCESINTRODUCTION REFERENCES |
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The letter from Goldhaber and Fireman (1
) nicely characterizes many of the complexities of estimating the number of spontaneous abortions, particularly in a situation in which induced abortion presents a significant competing risk. We also are pleased that they were able to confirm many of the assumptions we made in our study (2), especially that the estimated spontaneous abortion rates do not vary for women of different races.
Their letter (1) raises two important issues about spontaneous abortion rates. When comparing data from the Wilcox et al. study (3) (whose data we used to calculate our spontaneous abortion rates) and reanalyzing their Kaiser study data from 1981–1982 (4), Goldhaber and Fireman found differences in overall spontaneous abortion rates–16 percent for the Wilcox et al. study and 11 percent for the Kaiser reanalysis (1). The Kaiser study identified their cohort of women retrospectively through urine pregnancy tests or prenatal care registration at three clinics. While some pregnancies were identified as early as 5 weeks, the majority of women were not entered into the study until later in the pregnancy. Women who had pregnancy tests or spontaneous abortions outside the Kaiser system would not have been captured by their method. On the other hand, the longitudinal study reported by Wilcox et al. followed women prospectively, increasing the likelihood of identifying spontaneous abortions that occurred even outside the use of formal health care. Thus, we are not surprised by the difference in the reported spontaneous abortion rates and feel that the Wilcox et al. data more accurately reflect the actual rate.
Goldhaber and Fireman (1) suggest that we should have adjusted for the competing risk of induced abortions. When reviewing the methodological papers (5–7) for such adjustment, we encountered several approaches that included assumptions about the gestational age distribution of induced abortions compared with that of spontaneous abortions. Since we had limited information on induced abortions and the corresponding gestational distribution, we were not able to apply these methods. However, we discussed this issue in our paper (2) and acknowledged it as a limitation of our analysis. We agree with Goldhaber and Fireman that this is a difficult issue and that no consensus exists on how to correct for the competing risks of these two pregnancy outcomes.
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REFERENCES |
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TOPINTRODUCTIONREFERENCESINTRODUCTION REFERENCES |
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Goldhaber MK, Fireman BH. Re: "Estimates of the annual number of clinically recognized pregnancies in the United States, 1981–1991." Am J Epidemiol 2000;152:287–8.
[Free Full Text] - Saraiya M, Berg CJ, Shulman H, et al. Estimates of the annual number of clinically recognized pregnancies in the United States, 1981–1991. Am J Epidemiol 1999;149:1025–9.
- Wilcox AJ, Treloar AE, Sandler DP. Spontaneous abortion over time: comparing occurrence in two cohorts of women a generation apart. Am J Epidemiol 1981;114:548–53.
- Goldhaber MK, Fireman BH. The fetal life table revisited: spontaneous abortion rates in three Kaiser Permanente cohorts. Epidemiology 1991;2:33–9.
- Susser E. Spontaneous abortion and induced abortion: an adjustment for the presence of induced abortion when estimating the rate of spontaneous abortion from cross-sectional studies. Am J Epidemiol 1983;117:305–8.
- Figa-Talamanca I, Repetto F. Correcting spontaneous abortion rates for the presence of induced abortion. Am J Public Health 1988;78:40–2.
- Hammerslough CR. Estimating the probability of spontaneous abortion in the presence of induced abortion and vice versa. Public Health Rep 1992;107:269–77.
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 M. K. Goldhaber, B. H. Fireman, M. Saraiya, and C. J. Berg RE: "ESTIMATES OF THE ANNUAL NUMBER OF CLINICALLY RECOGNIZED PREGNANCIES IN THE UNITED STATES, 1981-1991" Am. J. Epidemiol., August 1, 2000; 152(3): 287 - 289. [Full Text] [PDF]
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