14/08/2014 16:52 BST | Updated 22/05/2015 10:12 BST

Baby Due Dates Vary By As Much As Five Weeks

Baby due dates vary by as much as five weeks

Don't set your hopes too high on your baby's due date when you get pregnant – because you might be wrong by anything up to five weeks.

A new study suggests that pregnancies naturally vary by as much as 37 days.

Until now, due dates have been a source of guessing games at baby showers. Just look at how we waited and waited for baby Prince George to arrive!

But a study published in the journal Human Reproduction suggests that even with incredibly accurate information, choosing a due date is largely a guessing game - something that surprised the researchers.

"We found that the average time from ovulation to birth was 268 days – 38 weeks and two days," said Dr. Anne Marie Jukic of the National Institutes for Health.

"Even after we had excluded six pre-term births, we found that the length of the pregnancies varied by as much as 37 days."

Pregnant women usually get a due date that's 280 days after the first day of their most recent menstrual period.

Using that method, four per cent of women deliver on their due dates.

To get more accurate information, the researchers looked at the results of urine samples collected in the North Carolina Early Pregnancy Study.

Three hormones connected with the onset of pregnancy: hCG (human chorionic gonadotropin), estrone-3-glucoronide and pregnanediol-3-glucoronide were identified in the urine, with the date of ovulation marked by the drop in the ratio between the hormones oestrogen and progesterone.

"Since the embryo secretes hCG, and mothers generally have little to no hCG in their urine when they are not pregnant, we used the earliest increase in hCG to indicate implantation," said Dr. Jukic.

Researchers also found that the length of pregnancy seemed to be affected by things that occur early on in pregnancy: embryos that took longer to implant, for example, tended to take longer from implantation to delivery.

And pregnancies that showed a late progesterone rise were significantly shorter.

They also identified other factors such as maternal age, birth weight of the mother, and the length of previous pregnancies all appear to factor into the length of pregnancy.