Why are babies shrinking?

For the past twenty years, there’s been a rapid secular trend toward lower birth weight in the U.S.

Between 1990 and 2005, the birth weight of full-term babies in the U.S. declined nearly two ounces to an average of 7 pounds, 7.54 ounces, a reversal of a trend that had seen birth weights climb steadily since the 1950s, according to the study. Babies were also born 2.5 days earlier on average in 2005 than in 1990, the study said.

Two of our kids would be in this sample – the twins are excluded as multiple births. So I’m interested.

The lower-birth-weight trend couldn't be explained by common factors such as how much weight mothers gained during pregnancy, whether the delivery was induced or by cesarean section, the amount of prenatal care, or maternal-health issues such as smoking and hypertension, researchers said. Only babies born at between 37 weeks and 41 weeks of gestation, which doctors consider full term, were studied. Researchers repeated their analysis in a sample of low-risk womenhealthy, educated Caucasians in their mid-to-late 20sand found that the decrease in birth weight was even more pronounced, suggesting that the trend wasn't the result of changes in the population of mothers.

My candidate is pregnancy tests. Women now know with much greater accuracy their gestational age, which affects the composition of the sample here. Most of the decline in birth weight happened between 2000 and 2005 (between Sophie and Goodwin!) in the U.S. sample. That coincides with a real rise in early ultrasound and super-early pregnancy detection home kits.

If I’m right, the decline is just an artifact of reducing the noise between 40 and 42 weeks. Fetal weight gain in the last few weeks is around a quarter pound a week. I don’t think reducing uncertainty about high-gestational-age births would account for the whole decline, but it could explain a large fraction of it.