Induction is rising and birth weight is falling … but so is perinatal mortality

A new paper in the journal Obstetrics and Gynecology calls attention to the rising rate of induction and the falling birth weight which appears to be a consequence. Decreased term and postterm birthweight in the United States: impact of labor induction implies that this trend is worrisome:

From 1992 through 2003, mean BW fell by 37 g, mean GA by 3 days, and macrosomia rates by 25%. Rates of induction nearly doubled from 14% to 27%. Our ecological state-level analysis showed that the increased rate of induction was significantly associated with reduced mean BW (r = –0.54; 95% confidence interval [CI], –0.71 to –0.29), mean GA (r = –0.44; 95% CI, –0.65 to –0.17), and rate of macrosomia (r = –0.55; 95% CI, –0.74 to –0.32)…

Increasing use of induction is a likely cause of the observed recent declines in BW and GA. The impact of these trends on infant and long-term health warrants attention and investigation.

The following graph illustrates the trend of decreased birth weight.

The decrease in birth weight appears dramatic because of the scale of the y-axis, which displays weights from 3400 gm to 3500 gm. Indeed the fall in birth weight over the entire study was less than 50 gm from 3492.3 gm to 3455.3 gm.

As the authors note:

A reduction of 40 g in BW or 3 days in GA may not matter for an individual infant, but represents a substantial change for a population.

But that change is not necessarily bad. for example, as the authors point out, the rate of macrosomia has dropped by 25% in the same time span. Nonetheless, the authors are worried:

Recent systematic reviews and metaanalyses … concluded that labor induction may reduce perinatal mortality but without increasing the risk of cesarean delivery. As observed in this study, increasing and earlier use of labor induction appears to have shortened the duration of gestation and thus reduced both mean BW and rates of macrosomia. Although several studies have reported increased risks of some causes of neonatal morbidity and maternal complications with increasing GA at term, more and more infants are being delivered at early term gestation (37-38 weeks), up from 19% in 1992 to 29% in 2003. Earlier term birth is associated with increased risk of sudden infant death syndrome, and we have recently documented increases in several adverse birth outcomes among early term births, including increased risks of infant mortality and some types of neonatal morbidity. Thus the impact of these recent trends requires further investigation, including large randomized trials, to ensure that the rise in induction is doing more good than harm.

It’s rather surprising then that the authors did not investigate the trend in perinatal mortality during the same period. The main purpose of labor induction is to reduce stillbirth, which will be reflected in the perinatal death rate (death from 28 weeks of pregnancy to 7 days of life). All their data comes from CDC databases and perinatal mortality is available from the same source.

As the following graph shows, perinatal mortality did indeed drop by 21%.

Correlation, of course, is not causation, and it is possible that perinatal mortality has been steadily dropping for other reasons. And as the authors of the paper point out, there are risks associated with delivery at earlier gestation. Nonetheless, the existing evidence suggests that the increasing induction rate has not led to an increase in perinatal mortality. The primary reason for induction is to reduce perinatal mortality and that is exactly what seems to have happened.