Previous Preterm Birth a Predictor of a Subsequent Small Baby

Nancy A. Melville

February 23, 2012

February 23, 2012 (Dallas, Texas) — Women who have had a previous preterm delivery are at increased risk of delivering a baby that is small for gestational age in subsequent pregnancies, even if those pregnancies are full term, according to a study presented here at the Society for Maternal-Fetal Medicine 32nd Annual Meeting.

Although many investigators have looked at the potential risk for a subsequent preterm birth after a previous one, research is lacking on the risk of having a small baby in a full-term pregnancy after a previous preterm delivery.

"Most previous research focuses on recurrent preterm delivery; however, it has been observed that in recurrent preterm delivery, the risk of the baby being small for gestational age at birth is increased," said lead author Jen Jen Chang, PhD, MPH, assistant professor in the Department of Epidemiology at Saint Louis University School of Public Health in Missouri.

"But little to no previous research has examined the risk of small for gestational-age birth in a full-term second pregnancy following a spontaneous preterm birth in a first pregnancy," she told Medscape Medical News.

The potential risk is a concern because babies born small for their gestational age are at higher risk for developmental problems, morbidity, and mortality, Dr. Chang noted.

In pursuing the issue, Dr. Chang and her colleagues evaluated a database of Missouri births from 1989 to 2005, which included 197,556 white and black women who had delivered their first 2 vertex, singleton pregnancies between 20 and 44 weeks of gestation.

Women with conditions such as chronic hypertension, preeclampsia, renal disease, and diabetes mellitus were not included in the study.

In women who had a full-term second pregnancy, the odds of having an infant who was small for gestational age were greater among those who delivered preterm in their first pregnancy, after controlling for factors such as pregnancy interval, maternal age, race, prepregnancy body mass index, cigarette use during pregnancy, and Medicaid status.

For women who delivered their first child at or before 28 weeks of pregnancy, the odds of having a subsequent baby delivered at full term in the lowest 10th percentile of population birthweight were 71% greater. For those who delivered between 29 and 32 weeks in their first pregnancy, the odds were 90% greater; and for those who delivered between 33 and 36 weeks in their first pregnancy, the odds were 69% greater.

In looking at the odds of having an even smaller baby at full term — in the lowest 5th percentile of population birthweight — the researchers found that for women who delivered their first child at or before 28 weeks of gestation, the odds were 126% higher than for women who had a previous infant with a birthweight appropriate for gestational age; for those who previously delivered at 29 to 32 weeks, the odds were 106% higher; and for those who delivered at 33 to 36 weeks, the odds were 81% higher.

In women who had a recurrent preterm delivery, the odds of having a baby in the lowest 5th or 10th percentile of size for gestational age in the second pregnancy were not significantly higher.

Dr. Chang speculated that the mechanisms behind the risk for small for gestational age babies in subsequent full-term pregnancies might be related to the placenta.

"Placenta biology may be the link to explain the observed findings in our study," she said. "But more research is needed to help us understand the causal mechanism in the association we observed in our study."

A better understanding of the risk presented by previous preterm birth on subsequent fetal growth could be essential for pregnancy management, Dr. Chang added.

"If our findings are confirmed by future research, increased surveillance may be warranted to monitor fetal growth among pregnant women with a history of preterm delivery," she said.

Experts say the findings shed light on how previous pregnancies can influence subsequent births.

"It was an interesting presentation," said R. Phillips Heine, MD, associate professor in the division of maternal–fetal medicine, Department of Obstetrics and Gynecology, at Duke University Medical Center in Chapel Hill, North Carolina.

"The findings suggest that fetal growth restriction and prematurity may have some overlapping mechanisms, and placental dysfunction would certainly be one possibility," said Dr. Heine, who moderated the session.

He agreed that such findings could have clinical implications for pregnant women.

"If confirmed, [the relation] would mean that increased surveillance may be required in this patient population."

The authors have disclosed no relevant financial relationships.

Society for Maternal-Fetal Medicine (SMFM) 32nd Annual Meeting: Abstract 33. Presented February 9, 2012.

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