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Pregnant women may be better off with a range of delivery dates rather than a set date due to variability in gestation times, researchers found.

A cohort of pregnant mothers who were followed from conception to birth showed that, despite a median gestational period of 268 days, length of pregnancy varied by 37 days even after excluding mothers who gave birth preterm, according to , and colleagues.

Factors affecting length of pregnancy included length of time to conception, rate of progesterone rise, the mother's age, birth weight, and length of pregnancy in previous births, they wrote online in Human Reproduction.

The authors noted that despite an estimated pregnancy period of 280 days from the mother's last menstrual period routinely assigned to mothers, only 4% of pregnancies fall on that date, while 70% occur within 10 days of that estimate "."

The authors studied the length of gestation through a cohort of 125 spontaneously conceived pregnancies to examine potential predictors of pregnancy length, such as hormonal levels at the early stages of pregnancy.

Although the study population was small, it showed that "even with the best dating possible, due dates are just an estimate," noted Loralei Thornburg, MD, of the University of Rochester Medical Center in New York, who was not involved in the study.

"Babies aren't like eggs, there isn't a specific date, but instead a range of when babies become mature," Thornburg told 麻豆传媒, adding that the main takeaway from the study was that events early in a pregnancy affect delivery timing.

Participants collected first morning urine specimens over 6 months or through the eighth week after their last menstrual period, and were followed to determine delivery date. Urine was analyzed for estrone-3-glucuronide, pregnanediol-3-glucuronide, and human chorionic gonadotropin (hCG). Day of ovulation was established through a rapid drop in the ratio of estrogen to progesterone.

Mothers were contacted to provide information on labor induction or Caesarean delivery without labor.

Additional data collected and tested as a gestational length predictor included age, pre-pregnancy BMI, height, smoking status, alcohol use, and parity, as well as early pregnancy bleeding, time from ovulation to implantation, level of hCG on the day of implantation, rate of rise of hCG over the first 7 days after implantation, and pattern of corpus luteum rescue -- a process "by which hCG from the embryo prevents the regression of the corpus luteum and stimulates its continued production of progesterone."

Participants also reported pregnancy-related medical conditions, neonatal medical conditions, the mother's birthweight, and the recalled pregnancy length for her prior live singleton births before and after the study birth.

The mothers had a median age of 29 at baseline, were predominantly white (95%), predominantly nonsmokers (94%), and most had a college degree (71%). The majority were parous at enrollment (54%) and had normal BMI (80%).

Length of gestation ranged from 247 days to 284 days after adjusting for preterm births and mothers with pregnancy-related medical conditions.

The median time from ovulation to birth was 268 days, while the mean was 267.

Later birth was associated with older age adding approximately 1 day for each year of age (P=0.003) and heavier weight at birth adding 1 day for each 100 g heavier the mother's birth weight was (P=0.01). Adult BMI or height were not associated with a longer gestation.

Pregnancies with a corpus luteum rescue pattern showing a late progesterone rise were associated with a median 12-day shorter pregnancy than those with an early rise (P=0.006). Additionally, embryos that "took longer to implant also took longer from implant to delivery (P=0.04)."

None of the other confounders, such as alcohol use and parity, were associated with length of pregnancy in the cohort.

They concluded that "events in the first 2 weeks after conception were strongly predictive of the total length of pregnancy, suggesting that the trajectory for the timing of delivery may be set in early pregnancy."

"A more evidence-based approach [to assigning a birth date] might be to assign a range of due dates (perhaps the interquartile range, in which half of women will deliver) or to describe the due date as a median," the authors offered as alternatives to the 280-day gestation length traditionally explained to expecting mothers.

Thornburg noted that the study did not address what may occur in patients who deliver early and who may have additional hormonal and early pregnancy markers, as well as maternal and fetal indications that may induce an early delivery.

"Doctors have been telling women for a long time that 'due dates' don't stand for expected due date, but estimated due date and are simply that -- estimates," she cautioned, adding that "a lot of the birth is up to the baby, and the timing of delivery is going to depend on both maternal and fetal well-being and risk factors in the pregnancy."

They cautioned that their study may be limited by self-reported data from years after the fact, lack of multiple comparisons, potential effects of environment or lifestyle that would affect pregnancy, and lack of generalizability of data.

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