Stillbirth Rates Remain Steady Despite 39-Week Rule

Marcia Frellick

November 10, 2015

Two studies published in Obstetrics and Gynecology find no correlation between efforts to limit nonmedically indicated deliveries before 39 weeks and the number of stillbirths.

There have been multiple efforts in the past decade by professional societies, regulatory bodies, and quality collaboratives to lower rates of early term deliveries that are not medically necessary (beginning of week 37 to end of week 38). However there is some concern that 39-week delivery policies could potentially increase term stillbirths, especially if policies are misused for higher-risk pregnancies.

Therefore, the researchers used National Health Statistics data to determine whether those efforts, which lengthened pregnancies overall, were having unintended consequences.

Sarah E. Little, MD, MPH, from the Division of Maternal-Fetal Medicine at Brigham and Women's Hospital in Boston, Massachusetts, and colleagues, studied early-term deliveries as a percentage of total term delivery and calculated term stillbirth rates for each state, both overall and for low- and high-risk women, using birth certificate and fetal death data.

They found a decline in early-term deliveries across the United States, going from 1.12 million (31.8%) of 3.53 million term, singleton births in the early term in 2005 to 978,000 (28.5%) of 3.43 million in 2011. Reductions varied widely by state "ranging from a 25.5% relative reduction to a 3.9% relative increase," the authors write in their article, published online November 5. However, they found no change in the term stillbirth rate (123/100,000 births in 2005 vs 130/100,000 in 2011; P = .189).

In another study, also published online November 5, Marian F. MacDorman, PhD, from the Maryland Population Research Center, University of Maryland in College Park, and colleagues found that the stillbirth rate was unchanged between 2006 and 2012, although the percentage distribution of live births by gestational age changed considerably in that time: Births at 34 to 38 weeks decreased by 10% to 16%, and births at 39 weeks increased by 17%.

The researchers used fetal death and live birth data files to compute gestational age-specific stillbirth rates at 20 weeks of gestation or greater using two methods: traditional (eg, stillbirths at 38 weeks of gestation/live births and stillbirths at 38 weeks) and prospective (stillbirths at 38 weeks of gestation/number of women still pregnant at 38 weeks). They assessed changes in rates and in the percentage distribution of stillbirths and live births.

"To decrease the stillbirth rate, research is needed to identify women early in pregnancy at the highest risk for stillbirth for more careful monitoring and potential intervention. Still, the lack of change in the prospective stillbirth rate from 2006 to 2012 suggests that preventing nonmedically indicated deliveries before 39 weeks of gestation did not increase the stillbirth rate at the national level," Dr MacDorman and colleagues conclude.

Editorial Notes Limitations

An accompanying editorial by Jennifer L. Bailit, MD, and Justin Lappen, MD, from the Department of Obstetrics and Gynecology at MetroHealth Center, Cleveland, Ohio, said that although these data appear to support what physicians are currently doing, the studies have several limitations.

Among them, they say, is the way stillbirth risk is measured. The study by Dr Little and colleagues cannot account for the effect that gestational age distribution had on risk for stillbirth.

"The prospective fetal mortality rate is the preferred metric for measuring stillbirth risk since the denominator (all ongoing pregnancies) captures the population of pregnant women at risk for the outcome of stillbirth," they write. "Notably, the prospective fetal mortality rate increases each week of gestation starting at approximately 35–36 weeks. Therefore, assessing changes in the prospective fetal mortality rate, and not the rate of occurrence by week of gestation, better illustrates the effect of shifting the timing of delivery later in gestation."

Also, using death certificates can be imprecise because although gestational age at delivery can be determined, gestational age at time of death is impossible to glean from administrative data. A fetus who died at 36 weeks, but was delivered at 38 weeks, could not have been helped by an early delivery at 37 weeks, they note.

In addition, the studies can provide evidence only of association, not causation. In both studies, researchers cannot account for trends such as improvements in ultrasound screening and detection of growth restriction, which would lead to appropriate medically indicated late preterm birth.

The studies do emphasize that physicians and administrators must use 39-week rules only for the low-risk populations for which they were intended, Dr Bailit and Dr Lappen say. Clinicians should treat higher-risk patients with condition-specific guidelines.

"The studies by Little et al and MacDorman et al add to the evidence to be considered when balancing the fetal, maternal, and neonatal risks regarding the timing of delivery," they write, "but these data should not be the final word on whether the 39-week rule has unintended consequences."

Obstet Gynecol. 2015;126:1131-1132, 1138-1150. Little abstract, MacDorman abstract

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