The natural length of pregnancies can vary by as much as five weeks, according to a new study.
Pregnant women typically are given a likely birth date that is 280 days after the onset of their last menstrual period. Only 4% of women deliver at 280 days, however, and just 70% deliver within 10 days of their estimated due date.
The authors of the study said they're the first to find a way to pinpoint the precise point at which a woman ovulates and a fertilised embryo implants in the womb during a naturally conceived pregnancy, and to follow the pregnancy through to delivery.
This is done by analysing women's urine for the presence of three hormones associated with the onset of pregnancy.
Five weeks of variability
Using this information, the researchers were able to calculate the length of 125 pregnancies, according to the study, which was published on August 7 in the journal Human Reproduction.
"We found that the average time from ovulation to birth was 268 days – 38 weeks and two days," Dr Anne Marie Jukic, a postdoctoral fellow in the epidemiology branch at the U.S. National Institute of Environmental Health Sciences, said in a journal news release. "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 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," Jukic said. "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."
It's too early to make any clinical recommendations based on the findings and further research is required, the study authors said.
"I think the best that can be said is that natural variability may be greater than we have previously thought, and if that is true, clinicians may want to keep that in mind when trying to decide whether to intervene in a pregnancy," Jukic said.
The US Department of Health and Human Services outlines how to have a healthy pregnancy.