40 weeks is not always the natural length of a pregnancy, researchers report today.
It is widely believed that first pregnancies tend to continue past the due date, as was the case for the Duchess of Cambridge who recently delivered her first baby several days over the "standard" 40 weeks.
But this is not always the case, say researchers.
Previous studies on the typical length of gestation have used the date of last menstrual period or ultrasound scans. Both of these are imperfect measures, say Dr Anne Marie Jukic of the National Institute of Environmental Health Sciences, Durham, USA, and colleagues.
Instead, the team measured hormone levels in urine during125 singleton pregnancies to accurately date ovulation, conception and implantation of the foetus. They excluded pregnancies that ended artificially early due to caesarean or induction.
"The median time from ovulation to birth was 268 days (38 weeks, two days)," they write in the Human Reproduction journal. This equates to 40 weeks and two days, counted from the last period.
"Even after excluding six preterm births, the gestational length range was 37 days (five weeks, two days)," they add.
Pregnancies tended to last longer for older women, those who had previously had a longer pregnancy, and those who were themselves heavier at birth. Factors that have previously been linked to longer pregnancies – body mass index, alcohol intake, number of previous births or sex of the baby – were not linked to longer pregnancies in this study.
The authors write: "Human gestational length varies considerably even when measured exactly (from ovulation). An individual woman’s deliveries tend to occur at similar gestational ages."
Mervi Jokinen, of the UK Royal College of Midwives, said: “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."
She added: "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."
Jukic, A. M. et al. Length of human pregnancy and contributors to its natural variation. Human Reproduction 7 August 2013 doi:10.1093/humrep/det297
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