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Evan Horowitz

Get ready. Your baby is coming early.

When you’re pregnant, a lot of conversations involve the same few questions. Are you having a boy or a girl? Have you picked a name? Did you hear about this great book (or great sleeping pillow or great technique to make your unborn baby smarter)?

Above all, though, the question you get asked is: When are you due? The obvious answer is that you’re due on your “due date,” the magic day that your doctor calculated based on a standard 40-week pregnancy. But as it happens, the odds of delivering on your due date are extremely small. Only about 1 in 30 women give birth on their due date. You are more likely to give birth a week early than you are to give birth “on time.”

When do women really give birth?

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The most likely birthdate is seven days early, according to a huge sample of birth information put together for the Globe by Brookings Institute researcher and WhenToExpect.com co-creator Matt Chingos.

These figures include not just “spontaneous” deliveries but also planned C-sections and planned inductions. It’s possible that these procedures, which are commonly performed in the week before the actual due date, account for some of the early births. Nevertheless, other researchers who have screened out planned births and looked just at spontaneous births have found a similar pattern, which suggests that babies still like to show up early, even when they’re uncoaxed.

Do first children arrive just as early?

First children tend to stay put for a little longer. On average, they show up two or three days early. Second and third children arrive five to six days early.

Does it matter if it’s a boy or a girl?

Sex doesn’t seem to matter much. Boys and girls arrive at roughly the same time.

How about younger moms vs. older moms?

The older you are, the more likely you are to give birth early. Teenage moms tend to give birth about three days early, while moms in their 40s trek to the hospital about a week in advance. Part of this is due to birth order, as teenage moms are much more likely to be carrying their first children.

Can these numbers tell me when I’m going to give birth?

Unfortunately, no. At best, they might give you a better sense of the odds and help you win some bets with friends. Still, if there is one clear lesson in the data, it’s this: don’t wait until the last minute. Pack your hospital bag, settle on a name, stock up on sleep, and make sure your friends and family are primed to help. Chances are, your baby is coming sooner than you think.

Evan Horowitz digs through data to find information that illuminates the policy issues facing Massachusetts and the United States. He can be reached at evan.horowitz@globe.com. Follow him on Twitter @GlobeHorowitz
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