A new study has added further weight to the idea that we ought to be individualising women’s ‘due dates’ rather than using a universal calculation. Published in the European Journal of Obstetrics & Gynecology and Reproductive Biology, a paper by Ng and Steer (2016) has shown that the prediction of a due date (which has long been known by those who care to look closely at the evidence to be a notoriously difficult thing) needs to take more factors into account than are currently considered.
Using information from over half a million pregnancies experienced by women who gave birth in London (which means it might not apply to everybody in the rest of the world, but it’s a good-sized database including lots of different kinds of women), Ng and Steer (2016) looked at whether there was a relationship between the lengths of the women’s first and second pregnancies. So their results are based upon the experiences of women who had more than one baby, but that doesn’t mean that their findings might not be useful more generally. Indeed, there is so little research into this area that it’s really exciting to see something which questions the non-evidence-based practice of calculating and then relying on due dates based on universally used but arbitrary notions of when babies are due.
So what do their results show? Well, a couple of findings stand out:
1. Generally, shorter-than-average and longer-than-average pregnancies tend to recur. So if a woman had a long pregnancy last time, the chances are she will again. Same if it was shorter. This won’t be news to people who work with pregnant women, or probably to most women in fact, but it’s always gratifying when research confirms what we already know from experience. This also confirms something that midwives in my own research on this area talked about a lot; in this area, it’s more valuable to look at what is normal for the individual than for the population as a whole.
2. There is some evidence that the length of subsequent pregnancies in the same woman follows a mathematical phenomenon known as regression to the mean. (Don’t click away if you don’t know what that means … you don’t need to!) The fact that this data shows regression to the mean simply means that, if a woman’s first pregnancy is longer or shorter than average, the gestational length of the second pregnancy is likely to be closer to 280 days than the first pregnancy was. So a woman who had a forty-two week pregnancy the first time is more likely to have one that is a bit shorter the second time around, though her pregnancy is still likely to last nearer forty-two weeks than thirty-eight. Same with a woman who went into labour a week before her due date. The second time, the chances are that she’ll give birth (as long as we don’t interfere) somewhere between that time and her due date.
None of this means that we can predict exactly what will happen in any woman or pregnancy, of course. These data simply show us what is more or less likely, and the trends that we are looking at only appear if you look at lots of women. If you are an individual woman reading this, then by all means do feel free to deviate from the population trend. And if you are a birth professional, then by all means use this to further explain to women that our predictions really are only that.
If you’d like to explore this topic further, then I would love to welcome you to my next online course in which we discuss this area in depth: post-term pregnancy: exploring evidence, inspiring confidence!
Objective: To assess the relationship between gestational lengths of the first and second pregnancies in the same women.
Study design: Observational study.
Methods: We used information from a dataset of over 500,000 pregnancies from 15 maternity units in the North West Thames, London. Data on the gestational length in days of the first pregnancy and the gestational length in days of the second pregnancy were correlated using regression models. First and second pregnancies were ascribed to the same women by identical maternal date of birth, ethnicity and maternal height (to within ±3 cm).
Results: There is a statistically significant cubic relationship between the gestational lengths of the first birth and the second birth (R 0.102, p < 0.001). The gestational length of the second pregnancy is likely to be closer to 280 days than the first pregnancy. In the 20% of women who had an interpregnancy interval of less than one year, the next pregnancy was one day shorter for every three months less than 12.
Conclusions: Although the gestation of second pregnancies exhibits regression towards the mean of 280 days, there is still a clinically important tendency for both preterm and postdates pregnancies to recur. Prediction of an estimated delivery date for second pregnancies should take into account both the length of the first pregnancy and the interpregnancy interval if it is less than 12 months.
Ng KYB and Steer PJ (2016). Prediction of an estimated delivery date should take into account both the length of a previous pregnancy and the interpregnancy interval. European Journal of Obstetrics & Gynecology and Reproductive Biology 201: 101-07.