A study published in Acta Obstetricia et Gynecologica Scandinavica might be helpful to women who are frustrated by the discrepancy between their own due date (as calculated by the date of their last period) and that calculated through ultrasound scanning.
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.
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. So they 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.
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 calulated 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.
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 I am up for doing at least some of that work, and if you would care to join me then you can find a lot of resources on my web site and if you’re a midwife or birth worker you may also like to join one of my online courses to help you increase your knowledge and build your own confidence in discussing this area.