Term Pregnancy

Abstract & Commentary

By John C. Hobbins, MD, Professor, Department of Obstetrics and Gynecology, University of Colorado Health Sciences Center, Denver, is Associate Editor for OB/GYN Clinical Alert.

Dr. Hobbins reports no financial relationship to this field of study.

Synopsis: Two recent studies have shown that despite a significant trend toward increased early-term delivery rates and early-term delivery inductions over the last two decades in the United States, there is a significantly higher infant mortality rate for these babies, compared with those delivered at full term.

Sources: Reddy UM, et al. Term pregnancy. A period of heterogeneous risk for infant mortality. Obstet Gynecol 2011;117:1279-1287. Murthy K, et al. Trends in induction of labor at early-term gestation. Am J Obstet Gynecol 2011;204:435.e1-6.

The concept that "term" pregnancy applies to any pregnancy extending past 36 completed weeks has been recently strongly challenged. In fact, in some hospitals, protocols have been drafted to discourage doing elective or repeat Cesarean sections prior to 39 weeks, instead of a previously accepted threshold of 38 to 38½ weeks. Two recently published articles1,2 have emerged that shed an interesting light on the timing of term deliveries — one involving perinatal death and the other focusing on trends in induction of labor at term. Although both articles deal with ethnic differences, the overall trends are enough to get our attention.

Reddy et al reviewed data from the National Center for Health Statistics from 1995 to 2006 and subdivided term pregnancy data into "early-term" (37/0 – 38/6 weeks) and "full-term" (39/0 – 41/6 weeks) categories.1 The authors analyzed complete data sets on 46,329,018 singleton pregnancies where gestational ages were available — 25.3% were early term and 54.2% were full term. They found a steady rise in early-term deliveries from 21.8% in 1991 to 28.9% in 2006. Meanwhile, the rate of full-term deliveries decreased from 81.3% to 54.2%. The biggest increase in early-term deliveries occurred in non-Hispanic black mothers, rising from 31.4% to 38.3% during those years.

As suspected, early-term infants have higher mortality rates than full-term infants. For example, in 2006, the rate of early-term mortality was 3.9/1000 compared to 2.6/1000 for full-term deliveries. The good news is that during the study period there was an overall decline in infant mortality rates in both categories. Additionally, non-black Hispanics had a drop in early-term infant mortality by 34.4% and non-Hispanic whites by 22.4%. Unfortunately, the bad news is that during this time period the mortality rate for non-Hispanic blacks rose by 15.8%.

The authors concluded that there was an overall improvement during the span of 11 years in infant and neonatal mortality for early-term pregnancies. However, it was clear that those delivered < 39 weeks fared worse in all ethnic groups than full-term babies delivered at equal or greater than 39 weeks.

The second study reviewed the same data source from the National Center for Health Statistics from 1991 and 2006.2 The authors were interested in trends in induction of labor for patients with early-term pregnancies, using the same definitions (37/0 – 38/7 weeks vs 39/0 – 41/6 weeks). They excluded patients with worsening intra-partum conditions such as premature rupture of the membranes or pregnancy-induced hypertension — in other words, those with seemingly valid reasons for induction. Patients with diabetes and chronic hypertension were included in the analysis, but broken out separately in the results.

The authors found that of 39,150,722 patients who were eligible, only 4.9% had early-term inductions (ETI). During the study period, the ETI rate rose from 2% in 1991 to 8% in 2006. Interestingly, those without chronic hypertension or diabetes had greater increases in induction rates (from 1.9% to 7.8%, representing a 317% increase) than those with diabetes (8% to 16.6% or a 108% rise) or chronic hypertension (13% to 27%, or a 103% rise). The largest increase in inductions across the board was in non-Hispanic white women.

Commentary

The differences in mortality rates between early- and full-term deliveries may even be underestimated since the analysis included patients in the "full-term" category whose pregnancies had extended past 41 weeks. These patients may have an inherently higher perinatal mortality rate and could blunt the difference between groups.

With the exception of non-Hispanic black women, infant and neonatal mortality rates have improved since the early 1990s, which probably has to do with better neonatal care. However, mortality rates are significantly higher for those neonates delivered between 37 and 39 weeks than for those delivered later. Given the now documented increase in ETIs, especially in non-Hispanic white patients, it is hoped that this will not translate into a large increase in later neonatal/childhood morbidity. The messages from both studies are not new, but these two studies have huge numbers and the most concerning trend is the alarming increase in ETIs in the United States. One wonders how many of these were for questionable reasons, including patient or provider convenience. The above data certainly lend credence to a more conservative approach to the timing of delivery.

References

  1. Reddy UM, et al. Term pregnancy. A period of heterogeneous risk for infant mortality. Obstet Gynecol 2011;117: 1279-1287
  2. Murthy K, et al. Trends in induction of labor at early-term gestation. Am J Obstet Gynecol 2011;204:435.e1-6.