Healthy Pregnancy Length Can Also Vary by Five weeks

About 70 percent of women deliver within ten days of their due-date while just four percent deliver exactly after 280 days from the onset of their last menstrual period. A new study has found that human pregnancy duration varies greatly in healthy women, by as many as five weeks.

In the current study, researchers calculated the length of 125 pregnancies. They determined the point where the ovulation began and the fertilized embryo got implanted in the womb.

"We found that the average time from ovulation to birth was 268 days - 38 weeks and two days. "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," said Dr Anne Marie Jukic,  at the National Institute of Environmental Health Sciences (Durham, USA), part of the National Institutes for Health,"

"We were a bit surprised by this finding. 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," Jukic said in a press release.

The study was based on data from North Carolina Early Pregnancy Study, which was collected between 1982 and 1985. The study followed 130 natural pregnancies from conception to birth. All women took daily urine samples and maintained a journal about their lifestyle.

Researchers checked the presence of three hormones in the urine samples- hCG (human chorionic gonadotropin), estrone-3-glucoronide and pregnanediol-3-glucoronide. These hormones are connected with human pregnancy. A change in the ratio of estrogen and progesterone marked the beginning of ovulation. Implantation of the embryo was the day when there was a consistent rise in the levels of hCG.

The team contacted the women to learn about the kind of delivery they had- cesarean or natural and whether they had used any drugs.

The results showed that embryos that took longer to implant also had longer gestation from implantation to delivery. They also found that women who showed late rise in progesterone levels had shorter pregnancy- about 12 days short when compared to a typical pregnancy.

Other factors that affected pregnancy length included age, weight of mother when she was born and history of previous pregnancies.

The study is published in the journal Human Reproduction.