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An ideal pregnancy lasts 40 weeks. And it looks like there's also an ideal time between pregnancies.

The length of time between giving birth to one baby and getting pregnant with the next should be 18 months or more. Women who get pregnant sooner than that are more likely to have a premature baby.

Women who got pregnant within a year of giving birth were twice as likely to have that new baby born prematurely, a study finds, compared with women who waited at least 18 months.

"The most significant [birth] risks are associated with the shortest birth intervals," says Dr. Emily DeFranco, a professor at the University of Cincinnati who led a study published Wednesday in the International Journal of Obstetrics and Gynaecology.

One in every eight babies born in the U.S. arrives before 37 weeks of pregnancy, according to the Centers for Disease Control and Prevention. That means that they miss the last few weeks in the womb, when organs like the brain, lungs and liver finish developing. That is why premature birth is the leading cause of long-term neurological disabilities in children.

DeFranco and her colleagues dug into the archives at the Ohio Department of Health and found 454,716 birth records from mothers who'd given birth at least twice over a period of six years.

They found that mothers with shorter "birth spacing," as it's called, were more likely to give birth prematurely and less likely to deliver after the expected 40 weeks of pregnancy. Mothers with less than a year between one birth and the next pregnancy had lower chances of giving birth to the next child at the expected 40-week mark.

"We knew already that shorter intervals between pregnancies are associated with higher likelihood of preterm birth," DeFranco tells Shots. "But we did not know whether or not short spacing led to a lower likelihood of delivering after the due date of 40 weeks."

The study also found that black women tended to have shorter intervals between births compared with women of other races (which DeFranco says were predominantly white). That finding is consistent with the accepted knowledge that black mothers are at a higher risk of giving birth preterm. There are other risks that weren't looked at in this study, including being a young or older mother; high blood pressure; infection; and having previously had a preterm birth.

Biologically speaking, a new mother can get pregnant within a month of giving birth to her last child, though health professionals usually recommend that couples wait a month or two after a birth before having sex. DeFranco says the biggest factors that determine birth spacing are family planning and access to birth control.

"Many pregnancies are unintended, and so the availability of birth control factors pretty heavily into birth spacing," she says. "A previous study has shown that the majority of pregnancies with a short birth interval are actually unintended pregnancies."

New mothers should be informed about the importance of adequately spacing out their pregnancies, DeFranco says.

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