Abstract
Little is known about how the miscarriage rate has changed over the past few decades in the United States. Data from Cycles IV to VI of the National Survey of Family Growth (NSFG) were used to examine trends from 1970 to 2000. After accounting for abortion availability and the characteristics of pregnant women, the rate of reported miscarriages increased by about 1.0% per year. This upward trend is strongest in the first seven weeks and absent after 12 weeks of pregnancy. African American and Hispanic women report lower rates of early miscarriage than do whites. The probability of reporting a miscarriage rises by about 5% per year of completed schooling. The upward trend, especially in early miscarriages, suggests awareness of pregnancy rather than prenatal care to be a key factor in explaining the evolution of self-reported miscarriages. Any beneficial effects of prenatal care on early miscarriage are obscured by this factor. Differences in adoption of early-awareness technology, such as home pregnancy tests, should be taken into account when analyzing results from self-reports or clinical trials relying on awareness of pregnancy in its early weeks.
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Notes
For example, the National Library of Medicine of the National Institutes of Health advises, “Miscarriages are less likely if you receive early, comprehensive prenatal care and avoid environmental hazards. . . .” (MedlinePlus n.d.).
Although we do not address fetal death, defined as fetal loss after 20 weeks of pregnancy, the rate of fetal death among pregnant women receiving no prenatal care is more than five times that among those receiving at least some such care (Hoyert 1996).
The sample size was 8,450 in 1988. It increased to 10,847 in 1995 and decreased in 2002 to 7,643. We use sampling weights but rescale the weights so that the weighted number of observations for each survey equals the actual number.
Although incomplete pregnancies are recorded separately, pregnancies ending in miscarriage are significantly shorter than those ending in abortion or birth. Given that the last year is not fully included (surveys took place in the first part of the year), including the last two years of each survey would cause miscarriages to be overrepresented and would bias our results upward. This is particularly a concern regarding pregnancies occurring in 2001 and 2002 because these years are recorded in a single survey. As expected, including the last two years of each survey increases the occurrence of miscarriages and produces a significantly steeper estimate of the trend for middle miscarriages.
“Still pregnant” is not applicable in our case because we restrict the sample to pregnancies at least two calendar years prior to the survey.
A small number of births are also reported at implausibly early dates. We have not removed them from the data.
We restrict our analysis to time-invariant variables because the NSFG does not collect information on characteristics that can change during pregnancy.
A simple example may help. Suppose that of every four pregnancies, in the absence of induced abortion, one would end in an early miscarriage, one in a late miscarriage, and two in a live birth. The true miscarriage rate is therefore 50%. Now suppose that women would choose to terminate half of pregnancies and that all such terminations occur in the middle of pregnancy (after early miscarriages would occur but before late miscarriages would occur) and that the probability of an induced abortion is unrelated to miscarriage risk. Of every four pregnancies, on average, one will end in early miscarriage, one in birth, one and a half in induced abortion, and one-half in late miscarriage. The presence of induced abortion reduces the miscarriage rate to three-eighths. Conditional on no abortion, the miscarriage rate is three-fifths. Neither of these captures medical risk.
The AIC is defined as 2 k – 2 L, where k is the number of parameters in the model and L is the log-likelihood of the statistical model. Lower values of the AIC indicate better fit.
This confidence interval and its counterparts for different durations of miscarriage do not correct for clustering.
The sample period is 1976–2004. Standard errors are each less than 0.1.
The coefficients shown are relative to “other,” which is primarily, but not exclusively, Asians.
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Acknowledgments
This paper was written in part while Lang was a visiting fellow at the Collegio Carlo Alberto and the University of New South Wales. He gratefully acknowledges their support and hospitality. The research was funded in part by Grant Number R03 HD05605601 from the National Institute of Child Health and Human Development, NIH. We have benefited from helpful discussions with and comments from Michael Greene, Karen Norberg, Mark Pasternack, Fred Wang, and the referees and editors. The responsibility for any errors of fact or interpretation in this article is ours alone.
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Lang, K., Nuevo-Chiquero, A. Trends in Self-reported Spontaneous Abortions: 1970–2000. Demography 49, 989–1009 (2012). https://doi.org/10.1007/s13524-012-0113-0
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DOI: https://doi.org/10.1007/s13524-012-0113-0