Length of Pregnancy Research

I am beginning to get the anecdotal sense that more and more women are delivering their babies past their due dates. I’ve been sensing this from my own experiences at births, but also from the experiences of my friends and social circle. Interestingly, however, science is now also beginning to support the anecdotal observation that pregnancy is lasting longer.

There are several implications to a longer pregnancy, but an increase in interventions  is what stands out to me. Rupturing membranes, sweeping the cervix, inductions, and sometimes, c-section. These are only a few examples of interventions that we perform, because previous literature would tell us that going too far past 40 weeks can be harmful to your baby’s health … but is that actually true?

I’ve been lucky to be a part of an interdisciplinary maternal health email distribution that recently discussed this topic. Family doctors, obstetricians, and midwives all shared their stories of working with clients who went past due. Most with good outcomes, and a few with poor ones. But, the interesting part is that most agreed that past-due pregnancies are increasingly common.

I think it’s important that we gain a better understanding, through research, of how the length of pregnancy has changed. Many of the decisions we make as health care providers are based on the research evidence; however, a lot of this type of research was done in the mid-1900s. If we only have outdated research to draw on, how can we provide the best care for present-day clients? More importantly, how could pregnancy not have changed over the last few decades? As our comfortable lifestyles and birth practices change, so too will our bodies and gestational practices. We are having babies later in life. We have access to every health care precautionary technology we could want (for more on that, see my previous blog about the medicalization of birth).

A friend shared this great article with me that summarizes research on present-day length of pregnancy. Hope you find it as interesting as I did. Take a read, and be sure to chat with your midwife, family doctor, or obstetrician about the length of due dates and how it might impact your pregnancy.


The length of a pregnancy naturally varies between different women by as much as five weeks, scientists have discovered for the first time.

Although it has always been known that human pregnancies usually last between 37 and 42 weeks, the variation has previously been attributed to imperfect methods of estimating due dates.

A woman’s due date is calculated as 280 days after the onset of her last menstruation and is only ever considered a best guess. Only four per cent of women deliver on their due date and only 70 per cent within 10 days of it.

However researchers from the National Institute of Environmental Health Sciences (NIEHS) were able to pinpoint the moment women became pregnant by taking daily urine samples – enabling them to isolate the role that natural variation plays in pregnancy length.

In a group of 125 women, they found the average time from ovulation to birth was 268 days – 38 weeks and two days – and that the length of pregnancies varied by as much as 37 days, according to the research, published today in the journal Human Reproduction.

“We were a bit surprised by this finding,” said Dr Anne Marie Jukic, a postdoctoral at the Epidemiology Branch at the NIEHS. “We know that length of gestation varies among women, but some part of that variation has always been attributed to errors in the assignment of gestational age. Our measure of length of gestation does not include these sources of error, and yet there is still five weeks of variability. It’s fascinating.”

The length of a pregnancy could be affected by a number of factors. Older women tended to deliver later, while women who had themselves been heavier at birth also had longer pregnancies.

The Royal College of Midwives (RCM) said the research reaffirmed professionals’ experience “that every woman is different” and urged practitioners to be cautious intervening too early in pregnancy, but said that due dates were still useful.

Mervi Jokinen, practice and standards professional advisor at the RCM said: “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.”


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