Women who become pregnant again within 18 months after having a baby are more likely to deliver early, according to a new study.

Past reports have also linked having a short interpregnancy interval with a greater risk of premature birth.

The new study “brings up the importance of adequate birth spacing as a potential modifiable way that women, especially high-risk women, can decrease their chance of having a preterm baby,” Emily DeFranco told Reuters Health.
She worked on the study at Cincinnati Children’s Hospital Medical Center in Ohio.

“Women who are at the highest risk for preterm birth are those who have had a previous premature birth so (for those women it is) especially important to try to optimize their pregnancy timing,” DeFranco said.

The findings suggest a short interpregnancy interval is also linked to the risk of a baby being born a week or two early, though not technically premature. These early term births at 37 and 38 weeks of gestation (full term is 39 to 40 weeks) can have a negative impact on a newborn’s health, say the authors.

For their study, DeFranco and her colleagues analyzed information from Ohio birth records from 2006 to 2011. They were able to find the interpregnancy interval for about 450,000 babies born to mothers who had given birth previously.

About 11% of the births occurred after an interpregnancy interval of 12 to 18 months and about 2% followed an interpregnancy interval of less than 12 months.

The researchers found that 53% of women with interpregnancy intervals of less than 12 months gave birth before the 39th week of pregnancy, compared with about 38% of women who had normal interpregnancy intervals of at least 18 months.

Twenty percent of women with the shortest interpregnancy intervals delivered prematurely - before 37 weeks - compared to 10% of women who waited 12 to 18 months between pregnancies and about 8%of women with a normal interpregnancy interval.

African American mothers were more likely to have short interpregnancy intervals. They were also more likely to have premature births, even when their pregnancies were at least 18 months apart, according to findings published June 4 online in BJOG: An International Journal of Obstetrics and Gynaecology.

DeFranco said researchers aren’t entirely certain why a shorter time between pregnancies might increase the risk of earlier birth, but that it probably has to do with nutritional depletion, which has been associated with a variety of pregnancy complications.

It’s important for women to use birth control and plan for adequate time between pregnancies, she said.

“I do love this paper and I think that it shows that we need to really pay attention to this interpregnancy interval and be aware of trying to help our patients figure out exactly what their plans are,” Dr. Mary Rosser told Reuters Health in an email. Rosser, an obstetrician and gynecologist at Montefiore Medical Center in New York, was not involved in the new study.

“I really feel like there needs to be a ‘birth plan’ where patients are involved in their reproductive health so that they choose a birth control method - and maybe this is discussed during the pregnancy, not with the post-partum visit when it’s almost too late,” she said.

Rosser said there are additional reasons to wait at least 18 months between pregnancies. Waiting could help mothers manage their weight and stay healthy and also benefit their earlier child’s health and wellbeing.

“In the gynecology world we really would like to prolong that interpregnancy interval so that mom has a chance to bond with that child,” she said. “We’re also trying to stress the importance of breastfeeding.”

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SOURCE: BJOG 2014

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Influence of interpregnancy interval on birth timing

Results

Of 454 716 births, 87% followed a normal IPI ≥18 months, 10.7% had IPI 12–18 months and 2.2% with IPI <12 months. The risk of delivery <39 weeks was higher following short IPI <12 months, OR (odds ratio) 2.78 (95% CI 2.64, 2.93). 53.3% of women delivered before the 39th week after IPI <12 months compared with 37.5% of women with normal IPI, P < 0.001. Likewise, birth at ≥40 weeks was decreased (16.9%) following short IPI <12 months compared to normal IPI, 23.2%, ^adjOR 0.67 (95% CI 0.64, 0.71). This resulted in a shift of the frequency distribution curve of birth by week of gestation to the left for pregnancies following a short IPI <12 months and 12–18 months compared to, birth spacing ≥18 months.
Conclusions

While short IPI is a known risk factor for preterm birth, our data show that inadequate birth spacing is associated with decreased gestational age for all births. Pregnancies following short IPIs have a higher frequency of birth at all weeks of gestation prior to 39 and fewer births ≥40 weeks, resulting in overall shortened pregnancy duration.

  EA DeFranco,
  S Ehrlich and
  LJ Muglia

Article first published online: 4 JUN 2014

DOI: 10.1111/1471-0528.12891