Pregnancy Length Can Vary Up to 5 Weeks, Study Says

Free diapers, coupons, unsolicited advice from strangers on the bus—when you first become pregnant, you get a lot of things! But one of the most important things you get is your due date, which is key for everything from measuring fetal health to tracking on baby sites which piece of fruit your fetus resembles. Plus, "When are you due?" is always the first question people ask when they spot your bump. But new research from the U.S. National Institute of Environmental Health Sciences says that assigning women a due date may do more harm than good because normal pregnancies can vary by up to five weeks.

The study, published in the journal Human Reproduction, followed 125 women with normal, uncomplicated pregnancies and discovered that there was much wider range of "normal" than previously thought. "We were a bit surprised by this finding," Anne Marie Jukic, Ph.D., lead author of the study, said in a press release. "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." She adds that even though their measure of length of gestation does not include these sources of error, there is still five weeks of variability, a result she calls utterly fascinating.

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The researchers found that many things factor into pregnancy length, including a woman’s age, how long the embryo takes to implant in the uterus, and even the mother's own birth weight. However, pregnancy length stays consistent between pregnancies in the same woman.

Five weeks may not sound like much when you're finishing summer vacation, but when it comes to how long babies need to bake, even one week can make a huge difference to the health of the baby and mother. Both preemies and post-term babies have an increased risk of injury, complications, and death.

Annette Hetzel, a registered nurse who specializes in labor in delivery, explains that it is very rare to see any pregnancy go outside of the 38- to 41-week range. "If labor starts before 37 weeks, they usually try to stop it. If it is after 41 weeks, they usually advise an induction because the risk of fetal demise increases."

That said, Hetzel has seen a wide variety of deliveries in her 20 years of birthing babies. "I remember one lady that for all six babies she had, it was a full 10 months gestation. And the babies weren't gigantic or showing typical signs of being overdue. It was just how her body worked." She adds that intervening in a normal pregnancy—either by using powerful drugs to stall labor or by inducing labor early or performing an unnecessary c-section—can also introduce complications that wouldn't otherwise have happened.

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So is it still the best plan to give women a precise due date?

"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," Jukic said. She adds that the study is too small to make general conclusions from, and more research still needs to be done.

Hetzel is more pragmatic, pointing out that doctors and women need something to measure pregnancies by so they know whether development is progressing normally. "Besides, I think everyone knows that the due date is just a guesstimation. I don't care if you give a range or a specific date, every woman worries about how long she'll be pregnant anyhow. No one wants to be pregnant any longer than they have to!"

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