By: 17 September 2013

The length of a human pregnancy can vary naturally by as much as five weeks, according to new research.

Normally, women are given a date for the likely delivery of their baby that is calculated as 280 days after the onset of their last menstrual period. Yet only four percent of women deliver at 280 days and only 70 percent deliver within 10 days of their estimated due date, even when the date is calculated with the help of ultrasound.

Now, for the first time, researchers in the USA have been able to pinpoint the precise point at which a woman ovulates and a fertilised embryo implants in the womb during a naturally conceived pregnancy, and they’ve been able to follow the pregnancy through to delivery. They did this by daily analysis of urine samples, looking at levels of three hormones connected with the onset of pregnancy: hCG, estrone-3-glucoronide and pregnanediol-3-glucoronide. The day of ovulation was identified by the drop in the ratio between the hormones oestrogen and progesterone. “Since the embryo secretes hCG, and mothers generally have little to no hCG in their urine when they are not pregnant, we used the earliest increase in hCG to indicate implantation,” explained Dr Jukic. Using this information, they have been able to calculate the length of 125 pregnancies.

“We found that the average time from ovulation to birth was 268 days – 38 weeks and two days,” said Dr Jukic. “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. 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.”

The study recognised other factors which appeared to influence prgnancy duration such as maternal age, maternal birth weight, previous long pregnancies.

“This last finding suggests that individual women tend to be consistent about when they deliver,” said Dr Jukic.

The authors warn that it is too early to make clinical recommendations based on their study, published in the journal Human Reproduction, and that further research needs to be carried out.