Who's most likely to get an inaccurate due date?

Almost every woman in the western world is given an estimated due date, and this is usually the date on which her pregnancy will be deemed to have reached 40 weeks in length.

We know that only 4% of babies will be born on their due date, that our methods of calculating this date are crude and that there exists variation in the length of healthy human pregnancy, but these things do not stop us.

Nowadays, even though many women calculate their own date almost as soon as they realise they are pregnant, often using web-based calendars or apps and the dates of their own cycles, the dates that they generate are overridden if they decide to have a dating scan and the sonographer’s measurements indicate that the size of their baby is suggestive of a different expected date of birth.

In Your Own Time was written to help parents and professionals better understand the issues and the evidence relating to the current induction epidemic. Looks at the evidence relating to due dates, ‘post-term’, older and larger women, suspected big babies, maternal race and more.

But due dates generated by ultrasound are based on the notion that there is a link between the size of the baby and the length of pregnancy and, as the authors of this latest study point out, this is not the case.

Instead, there is variance in the growth of babies in utero, just as there is variance in the growth and size of two year-olds, five year-olds or sixty-seven year-olds.

Who does this affect the most?

The authors of a study published in Acta Obstetricia et Gynecologica Scandinavica set out to discover which women were most likely to be affected by this discrepancy, and they did so via an analysis of more than 1.2 million births whose details were recorded on the Swedish Medical Birth Register.

And what they found was fascinating.

The women who had the highest chance of a large negative discrepancy – that is, where the LMP date was earlier than the US date, i.e. the fetus was smaller than expected when dated by ultrasound and so the due date was postponed – were those who had a body mass index of 40 kg/m2 or higher.

“Other factors associated with large negative discrepancies were: diabetes, young maternal age, multiparity, body mass index between 30 and 39.9 kg/m2 or <18.5 kg/m2, a history of gestational diabetes, female fetus, shorter stature (<−1 SD), a history of preeclampsia, smoking or snuff use, and unemployment.” (Kullinger et al 2016)

Large positive discrepancies (+4 to +20 days) were associated with male fetus (OR 1.80, 95% CI 1.77–1.83), age ≥30 years, multiparity, not living with a partner, taller stature (>+1 SD), and unemployment.

I often hear from women with a large positive discrepancy in this area, because these are the women who, having calculated their own due date, are told that their expected date of delivery will be changed to an earlier date.

This can affect them enormously as birth nears, because they are likely to be advised to have their labour induced for ‘post-dates’ at a time when, but their own calculations, their pregnancy has only just reached the forty week point.

Sara Wickham’s bestselling book explains the process of induction of labour and shares information from research studies, debates and women’s, midwives’ and doctors’ experiences to help women and families become more informed and make the decision that is right for them.

Overall, more than one in six women had a discrepancy of more than a week, and this gap could comprise a pretty significant effect on the experience of late pregnancy.

So it would be nice to think that this element of the study – which adds to a growing body of knowledge questioning common practice in this area – would be taken into account and that things will change.

But I don’t see that as very likely.

Our culture has become very attached to due dates, despite their fallibility on a number of levels, and it will take a lot of work to turn that tide. But there’s plenty of room for discussion, and the starting point is to ensure that individual women and families are well informed about this.