Normally, women are given a date for the likely delivery of their baby that is calculated as 280 days after the onset of their last menstrual period, researchers said. (Agencies)
Yet only four percent of women deliver at 280 days and only 70 percent deliver within 10 days of their estimated due date, even when the date is calculated with the help of ultrasound, they said.
Now, for the first time, researchers in the US have been able to pinpoint the precise point at which a woman ovulates and a fertilized embryo implants in the womb during a naturally conceived pregnancy, and follow the pregnancy through to delivery.
Using this information, they have been able to calculate the length of 125 pregnancies.
"We found that the average time from ovulation to birth was 268 days - 38 weeks and two days," said Dr Anne Marie Jukic from the National Institute of Environmental Health Sciences, US.
"However, even after we had excluded six pre-term births, we found that the length of the pregnancies varied by as much as 37 days.
"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," she said.
Researchers took information from daily urine samples collected by women taking part in an earlier study, which followed 130 singleton pregnancies from unassisted conception through to birth.
The women had discontinued contraception in order to become pregnant; they were healthy, with no known fertility problems and they were also less likely to smoke or be obese.
The women completed daily diaries and collected daily first-morning urine samples for six months or until the end of the eighth week if they became pregnant.
The urine samples were analyzed for three hormones connected with the onset of pregnancy: human chorionic gonadotropin (hCG), estrone-3-glucoronide and pregnanediol-3-glucoronide.
The day of ovulation was identified by the drop in the ratio between the hormones estrogen and progesterone. Embryo implantation was identified as the first day of a sustained rise in levels of hCG.
"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'" researchers said.
Normally, women are given a date for the likely delivery of their baby that is calculated as 280 days after the onset of their last menstrual period, researchers said.