This is the first study to look at pregnancy this way. The length of pregnancy can vary naturally by as much as five weeks, research suggests. The study of 125 women is the first to calculate gestation by pinpointing the exact time of conception. It found age, time to implantation and a woman's own birth weight were also linked to pregnancy length. An expert said the findings, published in the journal Human Reproduction, challenged whether a "due date" for women was helpful.

Due dates are often calculated as 280 days after the start of the woman's last menstrual period or, more accurately, by ultrasound. Yet only 4% of women deliver when predicted, and only 70% within 10 days of their estimated due date.

The research team at the US National Institute of Environmental Health Sciences measured hormone concentrations in daily urine samples taken from women trying to conceive naturally to determine exactly when ovulation and implantation of the fertilised egg had occurred. They found that the average length from ovulation to birth was 268 days, just over 38 weeks. Once they excluded six premature births, they found that gestation varied naturally by as much as 37 days.

Dr 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." "This is a very interesting piece of work, and knowing when is the right time to deliver is a huge issue," said Dr Virginia Beckett, spokesperson for the Royal College of Obstetricians and Gynaecologists. "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 study also showed that embryos that took longer to implant also took longer from implantation to delivery. Older women were more likely to have longer pregnancies, and there was also a link between gestation and a mother's weight when she was born. The researchers also found that the length of previous or subsequent pregnancies was related to the length of the one being studied, suggesting a consistency about when women deliver. But they said it was too early to make any clinical recommendations.

"I think the best that can be said is that natural variability may be greater than we have previously thought and, if that is true, clinicians may want to keep that in mind when trying to decide whether to intervene on a pregnancy," said Dr Jukic.

Dr Beckett said very little was known about the exact mechanisms that determine when labour begins. This is a very interesting piece of work, and knowing when is the right time to deliver is a huge issue, she added. It supports the suggestion that giving someone a "due date" may not be a great idea and can make women feel anxious when they go over. "It would be better to say, 'You will be delivered by this time,'" she said, "to take the pressure off." How long a healthy pregnancy lasts can vary by as much as five weeks, even when doctors precisely determined the date of conception, a new study suggests. Although the length of a healthy pregnancy is known to be variable, some of this variation was thought to be due to errors in determining the age of the baby, the researchers said. The new study was able to pinpoint the exact day of conception by analyzing urine samples from 125 women who were trying to become pregnant in the early 1980s. Changes in hormone levels in the urine were used to determine the day of ovulation—presumed to be the same day as conception—as well as the day the embryo implanted in the uterus. On average, pregnancies lasted 38 weeks from the day of conception to the day the baby was born, or about 40 to 41 weeks from the day of the women's last menstrual period. The latter measure is more commonly used to determine a woman's due date. But even after excluding babies born preterm, pregnancy length ranged from about 35 to 40 weeks from conception to birth (about 38 to 43 weeks from the last menstrual period). The researchers were surprised to see such variation even with a precise determination of the day of conception, said study researcher Dr. Anne Marie Jukic, a postdoctoral fellow in the Epidemiology Branch at the National Institute of Environmental Health Sciences in Durham, N.C. The findings suggest that giving a woman a precise due date may not be the best way to communicate pregnancy duration. Only about 4 percent of women actually deliver on their due date, which is typically estimated as 280 days after the last menstrual period. "The emphasis on a single due date may make the length of pregnancy seem more predictable than it really is," Jukic said. Providing women with a range of due dates may be a better way to communicate pregnancy length, she added. The length of participants' previous pregnancies was also strongly linked to the length of their current pregnancy, suggesting that this measure might help determine a woman's natural pregnancy length, the researchers said. The study also found that early-pregnancy characteristics may help predict due dates: embryos that took longer to implant were associated with later deliveries, while pregnancies in which embryos showed a late rise in progesterone were about 12 days shorter than those with an early rise. However, some experts criticized the study. Dr. Tomer Singer, a reproductive endocrinologist and infertility specialist at Lenox Hill Hospital in New York, said it did not provide much new information.