The length of human pregnancies can vary by up to five weeks, according to new research published Wednesday, with the average pregnancy lasting 38 weeks among study subjects.

The research, published in the online edition of the journal Human Reproduction, found that natural pregnancies where the conception date was known varied in length by 37 days. The median time from conception to birth was 268 days, or 38 weeks and two days.

The researchers write that their findings may change how doctors assign due dates to expectant mothers.

Traditionally, pregnant women are assigned a due date 280 days after the onset of their last menstrual period, the researchers write in an introduction to their study. Only about 4 per cent of women deliver at 280 days and only 70 per cent deliver within 10 days of their estimated due date.

Typically, the variability in gestational lengths is thought to be due to mistakes when estimating gestational age, largely because methods to determine conception date are imprecise. The authors also note “another source of variability – and perhaps the least understood – is normal variation in the pace of fetal maturation and the timing of natural delivery. The possibility of natural variability is plausible, but little discussed in the literature.”

The researchers point out that the natural variability in gestational lengths can only be examined when there is an accurate measure of conception.

Unlike previous research, which estimated conception dates according to the subjects’ last menstrual periods or with ultrasound, this study more accurately pinpointed conception via hormonal measurements from urine samples taken throughout the ovulation cycle. Because natural conception is believed to occur within 24 hours after ovulation, urine was analyzed for the rapid drop in the ratio of estrogen to progesterone that marks the start of ovulation. The implantation date was determined when a spike in the hormone hCG was detected.

The researchers were then able to calculate gestational periods of 125 live births. Their findings excluded six pre-term births, and accounted for factors such as labour induction or caesarean deliveries, which tend to shorten gestational periods.

They found that gestational periods for the 125 births ranged from 247 to 284 days.

The researchers found that gestational periods were longer for embryos that took longer to implant, among older mothers, when mothers had longer pregnancies in other births, and for mothers who themselves were heavier at birth.

Each year of age and every 100 g increase in the mother’s own birth weight added roughly one day to the mother’s pregnancy.

However, the researchers note that they did not find a link between pregnancy length and factors that other studies have found to influence it, such as the mother’s body mass index, alcohol intake or the baby’s gender.

“Variability in the length of human gestation limits the ability to predict delivery date,” they write.

Rather than the traditional 280 days from the last menstrual period, they say, physicians could consider assigning a range of due dates or to describe the due date as a median.