A new study has found that the length of a human pregnancy can vary naturally by as much as five weeks.

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 follow the pregnancy through to delivery.

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," Dr Anne Marie Jukic, a postdoctoral fellow in the Epidemiology Branch at the National Institute of Environmental Health Sciences (Durham, USA), part of the National Institutes for Health, said.

"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," she said.

The research is published online in Europe`s leading reproductive medicine journal Human Reproduction.