Why we’ve got it wrong on ‘overdue’ babies

SO WHY do some babies go overdue in the first place? Research has suggested that higher weight gain in pregnancy, older maternal age and a family history of prolonged pregnancy may contribute.

One simple explanation is that some pregnancies may be slightly misdated if the first day of the last period is used to calculate the due date. This method assumes that ovulation, and therefore conception, takes place two weeks later — yet women may ovulate several days after this, or the embryo may take slightly longer to implant in the uterus. In theory, this could mean a woman is labelled overdue when she’s not.

It may also be that the ideal length of pregnancy varies considerably from woman to woman. A 2013 study, published in the journal Human Reproduction, based on hormone testing found that the length of healthy pregnancies varied by as much as 37 days.

The researchers suggested that rather than the conventional due date, a range of dates would be more helpful — while the duration of a woman’s previous pregnancies should be taken into account too, as that seemed to be a strong indicator of when subsequent babies would arrive.

Professor Mark Johnson, an obstetrician at Chelsea and Westminster Hospital, describes how this might work: ‘If a woman’s first baby arrives before 40 weeks, for instance 38 weeks, her second and subsequent babies are either similar or move closer to the median of 40 weeks.

‘The same is true of women who are closer to the other end of the spectrum. Either their subsequent babies will also be late, or they too will move towards the median of 40 weeks.’