Average birth weight decreases in the U.S.

NEW YORK (Reuters Health) - Reversing a trend of nearly five decades, birth weight in the U.S. may be on the decline, according to a new study.

From 1990 to 2005, birth weight decreased by 52 grams (1.83 oz) on average. The drop - from 3441 to 3389 grams - leaves the vast majority of babies in the safe range, and the overall health consequences of this development are unclear.

"It is important to study trends in low birth weight over time because an increasing proportion of the smallest babies could lead to increased resource requirements to address health concerns," Sara Donahue of Boston University, who worked on the study, told Reuters Health.

Small babies (usually defined as lighter than 2500 grams, or 5.5 pounds) may face problems such as low blood sugar, lower body temperatures, or an increase in red blood cells, which can cause the blood to thicken and clot.

To track trends in birth weights, Donahue and her colleagues examined birth records for nearly 37 million newborns in the US, excluding California.

To make sure that the recent weight drop wasn't related to changes in maternal age or life style, the researchers also analyzed data for half a million young mothers who were deemed to be at low risk of pregnancy complications. For children of these women, it turned out that birth weight decreased even more, going down 79 grams (2.79 oz).

The number of women giving birth to small newborns increased by one percent among the low-risk mothers, but remained stable overall.

By contrast, the number of women giving birth to large newborns (heavier than 4000 grams, or 8 pounds, 13 ounces) fell by 2.2 percent for the low-risk group and by 1.4 percent overall.

Large babies may be difficult to deliver, resulting in harm to the infant or requirement for cesarean section.

The average length of pregnancy decreased during the study period by 2.4 days (a bit less for the low-risk group), which could also have an effect on birth weight.

"Although the consequences of the modest differences over time in birth weight for gestational age that we observed here are uncertain, any underlying reasons for such a decline may themselves directly influence child health," the authors write in the journal Obstetrics & Gynecology.

So far, the reasons for the weight decline are unknown, and none of the factors examined by the researchers -- including maternal age, smoking, and hypertension -- could explain the new trend.

SOURCE: Obstetrics & Gynecology, February 2010.