Monday, November 10, 2008

New study shows that neonatal mortality increases after 41 weeks ... maybe

A new study published in the American Journal of Obstetrics and Gynecology shows that neonatal mortality rises after 41 complete weeks of pregnancy. It is well known that the stillbirth rate begins to rise at the end pregnancy. The American College of Obstetrics and Gynecology (ACOG) recommends that anyone who reaches 42 complete weeks of pregnancy have labor induced. At 42 completed weeks, the stillbirth rate is approximately double that at 40 weeks, and the stillbirth rate continues to rise thereafter.

This new study demonstrates that even liveborn babies delivered after 41 completed weeks are at increased risk. The study is Increased neonatal mortality among normal-weight births beyond 41 weeks of gestation in California, by Bruckner, et al. This is a very large study:
Of all live births in California over the
test period (n=2,667,938), we removed infants with a gestational age of less than
38w0d or greater than 42w6d. We then excluded infants weighing less than 2500 g or more than 4500 g. These restrictions yielded a study population of 1,815,811 normal-weight term births.

Among normal-weight term births, CDHS registered 499 neonatal deaths, for an overall rate of 2.75 deaths per 10,000 live births. Neonatal mortality appears highest at 41w0d to 41w6d.
The study showed:
Infants born at 41w0d to 42w6d of gestation appear at increased risk of neonatal mortality as compared with those born at 38w0d to 40w6d of gestation (aOR = 1.34; 95% confidence interval [CI], 1.08-1.65). Of note, gestational age of 41w0d to 42w6d confers a greater neonatal mortality risk than any other variable included in the analysis.

We tested whether our findings changed with the addition of macrosomic (ie,greater than 4500 g) infants by including this group in the multivariate analysis. The aOR differed slightly from that of the original test (aOR = 1.32; 95% CI, 1.08-1.63) and remained statistically significant.
This study has several strengths. It is impressively large, giving it greater statistical power. It eliminates babies with IUGR, an entirely separate cause of death that might otherwise skew the results. It also eliminates babies above 4500 gms, making it more likely that neither shoulder dystocia or gestational diabetes are involved in the deaths.

The one thing that bothers me about the study is the authors decision to group births over 41 completed weeks with births over 42 completed weeks. It would have been helpful to see the distribution of deaths. The authors tell us that the risk of neonatal death was greater at 41w0d to 41w6d, than at 42w0d to 42w6d. This puts the conclusion of the study in doubt. If postdates really were the cause of the increased mortality rate, we would expect to see the mortality rate rise as the gestational age increases.

It is possible that the neonatal mortality rate rises as gestational age increases, but that this effect is masked by the deaths of babies delivered between 41w0d to 41w6d for other reasons. Since a postdates induction is not required until 42 weeks, the group delivered between 41w0d to 41w6d contains babies who were delivered early for other signs of compromise such as low amniotic fluid or non-reassuring NST's or biophysical profiles. Until that issue is addressed, however, the authors cannot and should not conclude that gestational age greater than 41 completed weeks, in and of itself, increases the risk of neonatal death.

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