Participants were healthy with no known fertility problems. The North Carolina Early Pregnancy Study (1982–1985) enrolled 221 women who discontinued contraception in order to become pregnant ( Wilcox et al., 1988). We also examined possible predictors of gestational length, particularly hormonal events in the earliest stages of pregnancy. We used survivorship methods to account for births that occurred after a medical intervention (induced labor or Caesarean delivery). We explored the length of gestation in a cohort of spontaneously conceived pregnancies, using hormone assays of daily urine samples to identify the day of ovulation. Moreover, none of these studies explored the maternal or pregnancy characteristics that might be associated with length of gestation. All of these studies estimated ovulation by basal body temperature-an inexact measure ( Moghissi, 1976 Lenton et al., 1977 Bauman, 1981 Quagliarello and Arny, 1986). Four previous studies of spontaneously conceived pregnancies have attempted to estimate the length of gestation based on ovulation ( Stewart, 1952 Doering, 1962 Guerrero and Florez, 1969 Saito et al., 1972). Thus, the day of ovulation in a conception cycle can be taken to mark the beginning of a pregnancy. Without an exact measure of gestational age, these are impossible to separate.Īlthough natural conception is not directly observable in humans, there is evidence that conception occurs within 24 h after ovulation (see Winston et al., 1993 Wilcox et al., 1998). Thus, in previous studies, characteristics that have been associated with the length of gestation may have arisen from errors in gestational age estimation or natural length of pregnancy or both. Error and natural variability are indistinguishable without an exact measure of gestational age. The possibility of natural variability is plausible, but little discussed in the literature (see Pemberton et al., 2010). Another source of variability-and perhaps the least understood-is normal variation in the pace of fetal maturation and the timing of natural delivery. Natural conception is unobservable, and all estimates of the start of pregnancy (by LMP or ultrasound) are approximate. The observed variability in the gestational length may be due to errors in gestational age estimation. Only 4% of women deliver at 280 days and only 70% deliver within 10 days of their estimated due date, even when the date is estimated by ultrasound ( Mongelli et al., 1996). Pregnant women are routinely assigned a delivery date of about 280 days after the onset of their last menstrual period (LMP). We did not see an association between the length of gestation and several factors that have been associated with gestational length in previous studies: body mass index, alcohol intake, parity or offspring sex. Mothers with longer gestations were older ( P = 0.02), had longer pregnancies in other births ( P < 0.0001) and were heavier at birth ( P = 0.01). CL rescue pattern (reflecting ovarian response to implantation) was predictive ( P = 0.006): pregnancies with a rapid progesterone rise were longer than those with delayed rise (a 12-day difference in the median gestational length). Conceptions that took longer to implant also took longer from implantation to delivery ( P = 0.02). The coefficient of variation was higher when measured by LMP (4.9%) than by ovulation (3.7%), reflecting the variability of time of ovulation. Even after excluding six preterm births, the gestational length range was 37 days. The median time from ovulation to birth was 268 days (38 weeks, 2 days).
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