The average pregnancy lasts 280 days — but average is the key word there. That whole 9-months-equals-healthy-baby-thing? Apparently it’s not exact. Rather, a new study published in Human Reproduction shows that pregnancy time can vary by over a month and still produce a healthy baby.
While scientists have always known that the length of time it takes to carry a fetus to term can vary, in the past it’s been assumed that pinpointing the moment of conception will give you a pretty close delivery date estimate. But upon realizing 70% of women don’t deliver on their presumed due date, researchers at the National Institute of Environmental Health Sciences decided to reexamine data from a thirty-year-old study in North Carolina.
The original study consisted of data from 1982 to 1985, collected from 125 women over a period of time starting before conception and continuing to birth. By measuring hormone levels in each woman’s urine, the researchers calculated the exact date of conception. Then, after weeding out outlier cases that differed for other medical reasons, they discovered that healthy pregnancy duration could range from 247 to 284 days — a 37 day difference. While it should be noted that this is a fairly small sample size, the range of gestational length is still quite impressive.
In general, the study also found that older women and those who were heavier babies themselves had longer pregnancies. Pregnancy length tended to remain fairly constant for each individual as well — so if a woman took longer to deliver once, she likely would again when carrying another child later in life.
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