Conventional wisdom states that human pregnancies last for nine months.
This is, as any mother knows, untrue. The “nine months” estimate is fairly rough, as due dates are calculated for 280 days after the onset of a pregnant mother’s last menstrual period (exactly 40 weeks).
But this is, again, an estimate. In practice, pregnancies can last anywhere from 37 weeks to 42 weeks. A baby isn’t necessarily premature if born on the 37-week end of that spectrum, either. A new study shows that due dates vary naturally from one woman to the next.
Researchers at the National Institute of Environmental Health Sciences (NIEHS) published their findings in the journal Human Reproduction.
They studied 125 pregnancies in an attempt to determine how due dates respond to a variety of factors.
For the first time, researchers were able to find the exact time at which a woman’s fertilized embryo implants in the womb during a natural pregnancy. This allowed the team to accurately gauge the real length of each pregnancy.
“We found that the average time from ovulation to birth was 268 days—38 weeks and two days,” said Dr. Anne Marie Jukic, a postdoctoral fellow in the Epidemiology Branch at the NIEHS. “However, even after we had excluded six pre-term births, we found that the length of the pregnancies varied by as much as 37 days.”
“We were a bit surprised by this finding,” Jukic continued. “We know that length of gestation varies among women, but some part of that variation has always been attributed to errors in the assignment of gestational age. Our measure of length of gestation does not include these sources of error, and yet there is still five weeks of variability. It’s fascinating.”
In the past, many scientists believed that variances in pregnancy durations were abnormalities.
The new research could potentially change the way that doctors advise their pregnant patients, although the research team notes that their results will need to be replicated before physicians use it to make clinical recommendations.
“The length of human gestation varies considerably among healthy pregnancies, even when ovulation is accurately measured,” the study’s authors wrote.
“This variability is greater than suggested by the clinical assignment of a single ‘due date.’ The duration of previous pregnancies may provide a useful measure of a woman’s ‘natural’ length of pregnancy and may help in predicting an individual woman’s due date.”
“We also found that events in the first two 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.”
Eventually, doctors use urine analyses and other tools to make recommendations specific to their patients. Jukic notes that individual women seem to be consistent with their own due dates, so biology appears to be a major factor in pregnancy length.
“I am intrigued by the observation that events that occur very early in pregnancy, weeks before a woman even knows she is pregnant, are related to the timing of birth, which occurs months later,” Jukic said. “I think this suggests that events in early pregnancy may provide a novel pathway for investigating birth outcomes.”