Many of us are under the impression that pregnancy lasts nine months, but new research shows the actual gestation period can vary by as much as five weeks for women.

Doctors typically try to give a woman a due date that is 280 days after her last menstrual period. But, only 4 percent of women deliver on their due date, and only 70 percent deliver within 10 days of that estimate, according to the researchers.

Dr. Virginia Beckett, spokesperson for the Royal College of Obstetricians and Gynaecologists, admitted to the BBC that not much is known about exactly how long woman would be pregnancy for.

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"This is a very interesting piece of work and knowing when is the right time to deliver is a huge issue," Beckett, who was not involved in the study, said.

Researchers followed 125 single-birth pregnancies between 1982 and 1985 that were conceived naturally. All the subjects were healthy and had no fertility problems. The women were asked to donate daily urine samples until the end of the eighth week of their pregnancy, as well as keep daily diaries.

The team looked for three hormones connected with pregnancy in their urine samples: hCG (human chorionic gonadotropin), estrone-3-glucoronide and pregnanediol-3-glucoronide. Using this data, the researchers found the exact time a woman ovulated and the moment the fertilized embryo implanted itself in the womb. The day of ovulation was determined when there was a drop in the ratio between estrogen and progesterone hormones while implantation was identified as the first day of hCG hormone increases.

The researchers connected back with the women in 2010 to find out when they went into labor, whether it was induced and if they needed a C-section. They found out that the average time of ovulation to birth was 268 days, which equals 38 weeks and 2 days.

People's actual gestation periods varied greatly from the average. After excluding six premature births from the study, researchers saw pregnancy times differed as much as 37 days.

"We were a bit surprised by this finding," Dr. Anne Marie Jukic, a postdoctoral fellow in the Epidemiology Branch at the National Institute of Environmental Health Sciences (Durham, USA) which is part of the National Institutes for Health, said in a press release. "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."

Researchers also saw that older women had longer pregnancy times, with each additional year of age equaling about an extra day of pregnancy compared to younger moms.

Women who had been heavier when they themselves were born also had longer gestation periods. Each 100 g increase in the mother's birth weight corresponded to another day of pregnancy.

If a woman had long pregnancies before or after the study, that too affected overall times. Each one week increase in the average length of all her pregnancies corresponded with a 2.5-day longer time for the pregnancy that was tracked during the study.



Hormonally, the researchers found that mothers who had embryos that took longer to implant also had longer gestation periods. In addition, pregnancies that had a late rise in progesterone levels were shorter by 12 days compared to those that had an early rise.

"I am intrigued by the observation that events that occur very early in pregnancy, weeks before a woman even knows she is pregnant, are related to the timing of birth, which occurs months later," Jukic said. "I think this suggests that events in early pregnancy may provide a novel pathway for investigating birth outcomes."

Beckett added that telling women that they have a "due date" often causes anxiety when that day comes and passes. This study shows that the science itself isn't that exact.

"It would be better to say, 'You will be delivered by this time to take the pressure off," she said.

The study was published in Human Reproduction on Aug. 6.