Women may want to take more time between pregnancies, as new research suggests those who have short intervals between their pregnancies are more likely to experience a decrease in the length of the following one, even putting babies at risk of preterm birth in some cases.

The research, which used birth records from the Ohio Department of Health, is published in the journal BJOG: An International Journal of Obstetrics and Gynaecology.

According to the Centers for Disease Control and Prevention (CDC), preterm birth is defined as one that lasts less than 37 weeks. Because important growth and development occurs throughout pregnancy, particularly in the final weeks, some premature babies may face lifelong problems, including intellectual disabilities, cerebral palsy, breathing and respiratory problems, visual problems, hearing loss, and feeding and digestive problems.

To further investigate how a short inter-pregnancy interval (IPI) might affect pregnancy duration, the researchers analyzed 454,716 live births from women with two or more pregnancies during a 6-year period and assessed the influence of short IPI on length of the following pregnancy.

For the study, IPI was defined as the time from the immediate preceding birth to conception of the next pregnancy.

The women were separated into two groups: those with IPIs less than 12 months and those with IPIs between 12-18 months. The team then compared these groups with women who had an “optimal IPI” of 18 months or more.

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Women with an optimal IPI over 18 months were less likely to give birth before 39 weeks, compared with women with shorter IPIs.

The researchers found that, overall, women with shorter IPIs were more likely to give birth before 39 weeks of pregnancy, compared with women who had an optimal IPI over 18 months.

In detail, of the women with an IPI less than 12 months, 53.3% delivered before 39 weeks, whereas only 37.5% of women with an optimal IPI did so.

Additionally, pregnancies that lasted more than 40 weeks occurred less frequently in women with a short IPI of less than 12 months (16.9%), compared with women with a normal IPI (23.1%).

Regarding preterm births – those less than 37 weeks long – the rate was higher in women whose IPI was less than 12 months. Compared with women who had an optimal IPI, these women were more than twice as likely to give birth to a preterm baby.

Commenting on their findings, co-author Emily DeFranco, assistant professor at the University of Cincinnati College of Medicine in Ohio and the Center for Prevention of Preterm Birth at Cincinnati Children’s Hospital Medical Center, says:

Short inter-pregnancy interval is a known risk factor for preterm birth, however, this new research shows that inadequate birth spacing is associated with shorter overall pregnancy duration.

This study has potential clinical impact on reducing the overall rate of preterm birth across the world through counselling women on the importance of adequate birth spacing, especially focusing on women known to be at inherently high risk for preterm birth.”

As part of the study, the researchers also analyzed differences in racial groups and found that black mothers tended to have shorter IPIs, compared with non-black mothers.

And the rate of preterm births was also higher in black mothers who had a short IPI less than 12 months, compared with non-black mothers, at 26.4% versus 8.7%, respectively.

Even among all women who conceived after an optimal IPI, black women still exhibited a higher rate of overall preterm births, compared with non-black mothers. The researchers say this suggests maternal ethnicity is a predictor of preterm birth, regardless of optimal birth spacing.

“We know that inadequate birth spacing is associated with more adverse pregnancy outcomes, including preterm birth, in many countries like the US,” says Deputy Editor-in-Chief of BJOG John Thorp.

He continues:

This large population-based study further strengthens this and puts more emphasis on the importance of optimal birth spacing, of 18 months or more, especially among women with additional risk factors for preterm birth.”

The authors say women should be advised on the importance of leaving optimal space between pregnancies, particularly as it could help reduce preterm birth rates.