Pregnancy length varies even more than we thought...

Research published in Human Reproduction this week has added further insight around so-called post-term pregnancy by showing that variability around pregnancy length is even greater than has previously been thought. The findings also add weight to the idea that, on the whole, it is better not to induce labour simply because a pregnancy has lasted for a certain amount of time.

In a freely available article entitled Length of human pregnancy and contributors to its natural variation, Jukic et al (2013) describe research in which they used existing historical data collected in a research study together with information from some of the women who participated. They identified the point at which ovulation took place from urine collected during the pregnancies of 125 women who conceived naturally, and then measured the length of those pregnancies, discovering that the average time from ovulation to birth was 268 days, which is 38 weeks and two days. (Note: in studies like this one, pregnancy is measured from conception rather than the last menstrual period, because this is more accurate, but it does mean that those of us more used to LMP-related measurements have to add that extra two weeks back in to make sense of the numbers).

Even after excluding preterm births, the length of pregnancies varied by as much as 37 days. Factors which appeared to have an impact on the length of pregnancy in this study include maternal age (with older women having longer pregnancies), previous longer-than-average pregnancy and maternal weight at birth. In a related press release, lead researcher Anne Marie Jukic said,

“We were a bit surprised by this finding. 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.”

I was certainly fascinated to read their paper, whose abstract can be seen below, because the findings of these researchers are aligned with the findings of my own research which explored holistic midwives’ knowledge in relation to “post-term” pregnancy.

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.

They support the notions that it can be completely normal and safe for pregnancy to last longer than is currently deemed ideal, and that it may be preferable to give a due date as a window or range rather than a set point. This is something that I talk about in my book, Inducing Labour: making informed decisions, which looks at lots of evidence relating to post-term pregnancy and other situations in which women are offered induction. You can find more on my work on this area on my post-term pregnancy and induction of labour resources page. But let’s give Jukic et al the final word:

“The length of human gestation varies considerably among healthy pregnancies, even when ovulation is accurately measured. This variability is greater than suggested by the clinical assignment of a single ‘due date’. The duration of previous pregnancies may provide a useful measure of a woman’s ‘natural’ length of pregnancy and may help in predicting an individual woman’s due date.” (Jukic et al 2013: 7).

STUDY QUESTION How variable is the length of human pregnancy, and are early hormonal events related to gestational length?

SUMMARY ANSWER Among natural conceptions where the date of conception (ovulation) is known, the variation in pregnancy length spanned 37 days, even after excluding women with complications or preterm births.

WHAT IS KNOWN ALREADY Previous studies of length of gestation have either estimated gestational age by last menstrual period (LMP) or ultrasound (both imperfect measures) or included pregnancies conceived through assisted reproductive technology.

STUDY DESIGN, SIZE, DURATION The Early Pregnancy Study was a prospective cohort study (1982–85) that followed 130 singleton pregnancies from unassisted conception to birth, with detailed hormonal measurements through the conception cycle; 125 of these pregnancies were included in this analysis.

PARTICIPANTS/MATERIALS, SETTING, METHODS We calculated the length of gestation beginning at conception (ovulation) in 125 naturally conceived, singleton live births. Ovulation, implantation and corpus luteum (CL) rescue pattern were identified with urinary hormone measurements. We accounted for events that artificially shorten the natural length of gestation (Cesarean delivery or labor induction, i.e. ‘censoring’) using Kaplan–Meier curves and proportional hazards models. We examined hormonal and other factors in relation to length of gestation. We did not have ultrasound information to compare with our gold standard measure.

MAIN RESULTS AND THE ROLE OF CHANCE The median time from ovulation to birth was 268 days (38 weeks, 2 days). Even after excluding six preterm births, the gestational length range was 37 days. The coefficient of variation was higher when measured by LMP (4.9%) than by ovulation (3.7%), reflecting the variability of time of ovulation. Conceptions that took longer to implant also took longer from implantation to delivery (P = 0.02). CL rescue pattern (reflecting ovarian response to implantation) was predictive (P = 0.006): pregnancies with a rapid progesterone rise were longer than those with delayed rise (a 12-day difference in the median gestational length). Mothers with longer gestations were older (P = 0.02), had longer pregnancies in other births (P < 0.0001) and were heavier at birth (P = 0.01). We did not see an association between the length of gestation and several factors that have been associated with gestational length in previous studies: body mass index, alcohol intake, parity or offspring sex.

LIMITATIONS, REASONS FOR CAUTION The sample size was small and some exposures were rare, reducing power to detect weak associations.

WIDER IMPLICATIONS OF THE FINDINGS Human gestational length varies considerably even when measured exactly (from ovulation). An individual woman’s deliveries tend to occur at similar gestational ages. Events in the first 2 weeks after conception are predictive of subsequent pregnancy length, and may suggest pathways underlying the timing of delivery.

STUDY FUNDING/COMPETING INTEREST This research was supported by the Intramural Research Program of the NIH, National Institute of Environmental Health Sciences. None of the authors has any conflict of interest to declare.

Jukic AM, Baird DD, Weinberg CR et al (2013). Length of human pregnancy and contributors to its natural variation. Human Reproduction 2013 Aug 6. [Epub ahead of print]