While a pregnancy is supposed to last around 40 weeks, anyone who has ever given birth early or gone overdue will tell you this can be wide off the mark. Now scientists have shown that the length of a pregnancy can vary naturally by up to five weeks.

According to a team from the US, a woman's due date is usually calculated as 280 days after the onset of her last period. However, just 4% of women actually give birth on their due date and while some may be close to their date, almost one in three do not give birth within 10 days of their calculated day of delivery.

This applies even when ultrasounds are used to help with the dating of a pregnancy.

However, the scientists have been able to pinpoint the exact time when a woman ovulates and the fertilised embryo implants in her womb during a natural pregnancy. These pregnancies could then be monitored until a baby was born. This marks the first time this has been done.

With this information, the scientists were able to calculate the length of 125 pregnancies. With all of these pregnancies, the women had stopped taking contraception in an attempt to get pregnant. All were considered healthy with no known fertility problems.

"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," they explained.

They said that they were ‘surprised' by the findings.

"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," they commented.

Meanwhile, the study also found that embryos that took longer to implant tended to lead to longer pregnancies, while pregnancies that displayed a late rise in progesterone were shorter by an average of almost two weeks compared to pregnancies with an early rise of progesterone.

"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. I think this suggests that events in early pregnancy may provide a novel pathway for investigating birth outcomes," commented Dr Anne Marie Jukic of the US National Institutes of Health.

The study also noted that older women tended to have longer pregnancies. In fact, each additional year in age added around one day to the pregnancy.

Meanwhile, women who had high birth weights when they were born had longer pregnancies, as had women who had previously experienced a longer pregnancy. According to the scientists, this indicates that individual women ‘tend to be consistent about when they deliver'.

The scientists concluded that the length of human pregnancies ‘vary considerably even when ovulation is accurately measured'.

"I think the best that can be said is that natural variability may be greater than we have previously thought. 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," they concluded.

Details of these findings are published in the journal Human Reproduction.

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