9 Months May Not Be 'Normal' Pregnancy, After All Study finds pregnancy can naturally fluctuate by up to 5 weeks By Ruth Brown, Newser Staff Posted Aug 7, 2013 10:14 AM CDT 20 comments Comments (Shutterstock) (Newser) – A new study throws the whole concept of a "due date" into question, finding the length of pregnancy can naturally vary by as much as five weeks. The study of 125 women expands upon what the BBC reports are some fairly broad stats: Only 4% of women actually deliver when predicted, and 30% do not give birth within 10 days of their due dates. As LiveScience explains, that variation was previously attributed to inaccurate calculations of the baby's age. But the US National Institute of Environmental Health Sciences study used urine samples to determine precise ovulation and implantation dates, and, after excluding premature births, found the natural gestation period varied by up to 37 days. (The average ovulation-to-birth time was 268 days.) The study also found there was a link between gestation length and the age of a mother, the weight she was at birth, and the time it took for embryos to implant. "The emphasis on a single due date may make the length of pregnancy seem more predictable than it really is," says a study researcher. The upshot? Perhaps a "delivered by" date would be better than a "due date," suggests a spokesperson for the UK's Royal College of Obstetricians and Gynecologists.