The fetal life table revisited: spontaneous abortion rates in three Kaiser Permanente cohorts

A cohort of 9,055 singleton pregnancies was identified by conventional urine tests or physician examination at the Kaiser Permanente Medical Care Program in Northern California in 1981-1982. Using life table methods, we estimated the cumulative risk of spontaneous abortion or fetal death through the end of pregnancy as 13.0% of all pregnancies surviving 1 week or more past the time of the first missed menses. This value was lower than the estimates of 15.1% and 16.0%, respectively, from two previous fetal life table studies conducted at Kaiser Permanente in prior decades, but similar to estimates from recent, smaller studies of early pregnancy diagnosed by human chorionic gonadotropin. The major difference in survival between the three Kaiser Permanente cohorts was in the earliest gestational week of observation, week 5 from the last menstrual period, where the older data were sparse and potentially biased. High loss rates during this week accounted for one-fourth to one-third of the cumulative risk observed in the older studies. Although the older cohorts were larger, the 1981-1982 cohort included five times and three times as many pregnancies under observation during week 5 from the last menstrual period, yielding more stable estimates for this period. Because of improved reliability of early pregnancy testing and an emphasis on early prenatal care, the mean gestational age at entry to the 1981-1982 cohort was 10.4 weeks from the last menstrual period compared to 14.3 weeks and 13.7 weeks for the older studies. All three studies showed a peak for risk of spontaneous abortion around weeks 10-12 from the last menstrual period.