Between 1990 and 2005, the birth weight of full-term babies in the U. S. reduced to nearly two ounces to an average of 7 pounds, 7.54 ounces, a reversal of a trend that had witnessed a steep climb in birth since the 1950s, according to the study.

"We [also] found that the length of gestation is shortened by about 2.5 days even in term births", said lead researcher Dr. Emily Oken, an assistant professor of population medicine.

The finding, published Thursday in the Journal of Obstetrics and Gynaecology, claims potentially afflicting public-health implications, if the trend continues. In addition, low-birth-weight babies are highly prone for a host of health diseases.

This finding was not explained by more Caesarean deliveries or by pregnancy complications, say the researchers from Harvard Medical School.

The study initiated by Oken's team involved data on almost 37 million U. S. singleton full-term deliveries between 1990 and 2005.

However, the observation showed a 1% increase in the number of the lowest-weight babies and suggested the birth-weight decline didn't stop in 2005, which surprised the researchers.

Potential factors as better control of gestational diabetes—when a mother develops diabetes during pregnancy—and more physical activity during pregnancy were not analysed in the study, said Dr. Kramer.