Pregnant women should not take their due date too literally – the time a baby spends in the womb varies naturally by up to five weeks, research shows.
The first study to measure the length of pregnancy accurately has concluded what many mothers-to-be have been told down the years – the baby will come when it’s ready.
It found barely one woman in 20 delivers on her due date, given as 280 days or 40 weeks after conception.
Variation: The length of a woman's pregnancy can vary by up to 37 days, a new study has found
The findings show the Duchess of Cambridge’s experience with baby George was typical.
She was known to be overdue, perhaps by as much as a week, and the study showed around three-quarters of babies arrive within ten days of the expected date.
The average length of pregnancy was just over 38 weeks, in-depth analysis of 125 mothers-to-be found.
But that varied by as much as 37 days, says the study published online in the journal Human Reproduction, meaning terms typically vary between 37 and 42 weeks.
Dr Anne Marie Jukic, a postdoctoral fellow in the Epidemiology Branch at the National Institute of Environmental Health Sciences in the US, said: ‘We found that the average time from ovulation to birth was 268 days – 38 weeks and two days.
‘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.’
About normal: The Duchess of Cambridge is believed to have given birth around a week past her due date
The researchers were able to track precisely the point at which a woman became pregnant using information from daily urine samples collected for another study, and followed the pregnancies from conception through to birth.
Four per cent of women delivered at exactly 280 days and only 70 per cent within ten days of their estimated due date.
Embryos that took longer to implant were also delivered later and certain early hormone changes predicted pregnancies that were shorter by an average of 12 days, the study found. Older women delivered later on average, with each year of age adding roughly one day to their pregnancy, while women who had themselves been heavier at birth had longer gestations.
New guidance: Experts say the findings show that the variation in when a woman gives birth could be greater than previously thought
The report said: ‘The length of human gestation varies considerably among healthy pregnancies, even when ovulation is accurately measured.
‘This variability is greater than suggested by the clinical assignment of a single 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.’
Dr Jukic said: ‘The best that can be said is that natural variability may be greater than we have previously thought, and if that is true, clinicians may want to keep that in mind when trying to decide whether to intervene on a pregnancy.’
Mervi Jokinen, practice and standards professional adviser at the Royal College of Midwives, said: ‘This is interesting and useful research.
‘However, midwives have never regarded the due date as anything other than a rough indication of when a woman may give birth.
‘This study and our knowledge of physiology show that the norm for the duration of pregnancy is 37 to 42 weeks. It reaffirms our experience that every woman is different.
‘As practitioners we should be cautious about intervening too early, as every individual has their own pattern of pregnancy.
‘I do not think that this signals the end of midwives and other health professionals giving a woman a single due date.
‘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.’
Read more: http://www.dailymail.co.uk/health/article-2385740/Nine-month-pregnancy-It-vary-37-days-How-just-20-women-birth-date.html#ixzz2bF98vPtd
Follow us: @MailOnline on Twitter | DailyMail on Facebook
