pregnantThe length of pregnancy can naturally vary by up to five weeks, scientists at the US National Institute of Environmental Health Sciences have claimed.

The researchers studied 125 women and followed them from conception to birth, and found that a number of factors were involved in determining the gestation period. Writing in the journal Human Reproduction, the scientists claim the age of the mother and the mother’s weight as a baby were key indicators of gestation length.

Additionally, the scientists argued that due dates for expectant mothers may be of little use, as only 4 per cent delivered when expected and only 70 per cent within 10 days of the predicted date.

Length of gestation varies
The scientists found the average gestation period to be 268 days – or just over 38 weeks – and that after excluding six premature births from the study, the natural gestation period could vary by as much as 37 days. However, the scientists also believe there is a consistent link between when women deliver on subsequent pregnancies – meaning the first pregnancy could be used as a guide for future pregnancies.

Researcher Dr Anne Marie Jukic said: “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.”

“I think the best that can be said is that natural variability may be greater than we have previously thought and, if that is true, clinicians may want to keep that in mind when trying to decide whether to intervene on a pregnancy.”