The length of pregnancies varies by up to five weeks

A gestation of 40 weeks is the exception rather than the rule

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A pregnancy length varies from woman-to-woman by as much as five weeks, research has found.

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Pregnancies are measured based on the last date of a woman’s period. Add 280 days, and that is how doctors estimate a due date.

Only 4% of women deliver on their “due date”, 70% within 10 days of it. Scientists thought that this variation was down – in part – to imperfect methods of pinpointing when the pregnancy started.

A new study, from the National Institute of Environmental Sciences (NIEHS) – the first of its kind – looked at 125 women’s gestation from their exact point of conception. It was based on data collected from the North Carolina Early Pregnancy Study from 1982 and 1986.

Researchers were able to find the exact date of conception by analysing these women’s daily urine samples from when they were trying to get pregnant.

It then measured each pregnancy’s length. It found that factors such as women’s weight, age and the time to implantation had an impact on the gestation period.

The findings were published in Human Reproduction and challenged the idea that a due date is helpful. 

The average pregnancy was 268 days, which is 38 weeks and two days, from ovulation (40 to 41 weeks from a woman’s last menstrual period). After excluding preterm babies, the length of pregnancies ranged from 35 to 40 weeks from conception (38 to 43 weeks from last menstrual period).

“We were a bit surprised by this finding,” said Dr Anne Marie Jukic, author on the study.

“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.

“The emphasis on a single due date may make the length of pregnancy seem more predictable than it really is. Providing women with a range of due dates may be a better way to communicate the length of pregnancy.”

The researchers advised that giving women a due date, based on 40 weeks of pregnancy, left them feeling “anxious” when they passed it.

It also found that older women, and those who had been heavy at birth, had longer pregnancies. Early pregnancy also gave clues about when women were likely to go into labour: embryos that took longer to implant took longer to deliver and embryos that showed a late rise in progesterone had a shorter gestation period.

“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,” Anne Marie said. “I think this suggests that events in early pregnancy may provide a novel pathway for investigating birth outcomes.”

It is unlikely that the NHS will change the way women are advised on their due without further research. The Royal College of Midwives (RCM) said that due dates are still useful. “I do not think that this signals the end of midwives and other health professionals giving a woman a single due date,” Mervi Jokinen, practice and standards professional advisor at the RCM said.

“It should be explained to the woman that the due date is always an estimate and, as this research and our experience shows, this can vary widely.”

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