First, maternal age is positively correlated with pregnancy loss rates. An illustration of the declining loss rate with increasing week of gestation is shown in Figure 1.Ĭlinical loss rates reflect many factors, but two associations are worth emphasizing. This can be deduced from loss rates being only 1% in women confirmed by ultrasound to have viable pregnancies at 16 weeks. Most pregnancy losses after 8 weeks occur in the following 2 gestational months. That almost all losses are retained in utero for an interval before clinical recognition means most losses are “missed abortions”. 6, 7, 8, 9 Given the accepted clinical loss rate of 10–12%, fetal viability must cease weeks before maternal symptoms appear thus, most fetuses aborting clinically at 9–12 weeks have died weeks previously. 5 Studies involving obstetric registrants were very similar. This conclusion was first reached on the basis of cohort studies showing that only 3% of viable pregnancies are lost after 8 weeks' gestation. After ultrasonography became widely available, it was shown that fetal demise actually occurred weeks before the time overt clinical signs are manifested. Information in older studies was based on clinical pregnancy losses that traditionally were not appreciated until 9–12 weeks' gestation, at which time bleeding and passage of tissue (products of conception) occurred. Most clinically recognized losses occur before 8–9 weeks. There is a tendency for recurrent losses to occur around the same time of gestation (e.g., first trimester or other).
5 Higher clinical loss rates reported in some older studies may have reflected misclassification, unwittingly including surreptitious illicit abortions. 4 The total loss rate (preclinical and clinical) in the NICHD cohort was lower, at 16%.Ĭlinically recognized first-trimester fetal loss rates of 10–12% are well documented in both retrospective and prospective cohort studies. Our cohort was ascertained approximately 10 days later than the date of ascertainment in the sample of Wilcox and colleagues. These rates are consistent with data gathered by us and colleagues 4 in a National Institute of Child Health and Human Development (NICHD) collaborative study using serum β-hCG assays performed 28–35 days after the previous menses. The clinically recognized loss rate in this cohort was 12% (19/155). Of pregnancies detected in this fashion, 31% (61/198) were lost the preclinical loss rate was 22% (43/198). To determine the frequency of losses before clinical recognition, Wilcox and colleagues 3 performed daily urinary hCG assays beginning around the expected time of implantation (day 20 of gestation). Before this time, β-chorionic gonadotropin (hCG) assays can detect preclinical pregnancies. Embryos implant 6 days after conception, but are not generally recognized clinically until 5–6 weeks after the last menstrual period.