Nine months long is what a pregnancy is said to be. But hang on…. for some moms-to-be the duration can extend by as much as five weeks.
According to the findings of a new study, the length of a pregnancy can naturally vary between different women by as many as five weeks, the variation previously attributed to imperfect methods of estimating due dates.
By using urine samples researchers have been successfully able to pinpoint the exact moment when a woman ovulates and a fertilized embryo gets implanted in the womb for a naturally conceived pregnancy and follow her through to delivery.
Using the findings of the urine analysis, the researchers calculated the length of 125 pregnancies.
All participants were non-smokers, not obese with no known fertility problem. None of them were taking contraceptives.
The women were required to maintain daily diaries and collect daily first-morning urine samples for six months or until the end of the eighth week in case they became pregnant.
The samples were tested to check levels of three hormones linked to the onset of pregnancy - human chorionic gonadotropin (hCG), estrone-3-glucoronide and pregnanediol-3-glucoronide.
The day of ovulation is characterized by the drop in the ratio between the hormones oestrogen and progesterone, while embryo implantation is marked by a rise in levels of hCG.
"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.
The findings of the study are reported in the journal Human Reproduction.