Jan. 22, 2010 -- THURSDAY, Jan. 21 (HealthDay News) -- Babies brought to term by healthy, white mothers in the United States now weigh almost 3 ounces less than they did 20 years ago, and no one knows why, a new report shows.
This finding was not explained by more Cesarean deliveries or by pregnancy complications, say the researchers from Harvard Medical School.
"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.
This means that fewer babies are being born after their due date and more babies are being born in the three weeks before their due date, she said.
Although birth weights increased for much of the 20th century, the reasons for the more recent, unexpected decline in average birth weights remain unclear, the researchers said.
The lower birth weight of infants is partially explained by slightly earlier births, she said. "But it wasn't only that. Even when we accounted for these fewer days of shorter length of gestation, birth weight appeared to decline," she said.
The report is published in the February issue of Obstetrics & Gynecology.
For the study, Oken's team collected data on almost 37 million U.S. singleton full-term deliveries between 1990 and 2005.
Earlier studies had found increasing birth weights over the past 50 years due in part to older, heavier women having children and fewer women smoking.
However, in this study the researchers found birth weights declined by an average of 1.83 ounces from 1990 to 2005. The drop in average birth weight picked up speed after 1995, they noted.
Moreover, birth weights dropped among women who were considered at low risk for having small babies.
In fact, mothers who were white, well-educated, married, didn't smoke, received prenatal care and had uncomplicated vaginal deliveries had babies weighing 2.78 ounces less, Oken said.
The reasons for declining birth weight is a mystery, Oken said.
Dr. Peter Bernstein, director of Maternal Fetal Medicine Fellowship Program at Montefiore Medical Center in New York City, said he is actually seeing the opposite trend.
"I was pretty surprised; it has not been my experience over that period of time," he said. "My experience is that babies are getting bigger."
Bernstein admitted that his experience is confined to the Bronx in New York City, where women tend to be more obese and diabetic. "If you look at different populations, you get different results," he said.
However, Bernstein added it's true that babies are being born earlier, which could account for most of the lower birth weights the researchers are reporting.
"The due date used to be 40 weeks; now the average is somewhere in the 39th week," he said.
According to Bernstein, earlier births are due to more Cesarean deliveries. The increase in these deliveries is caused by more complications such as high blood pressure and diabetes, he explained.
However, Cesarean deliveries do not fully explain the decline in birth weights, Oken said.
There are other theories. "Over the second half of the 20th century, birth weight increased," Oken said, "so it is possible that this [recent decline] represents a plateauing of that increase in birth weights that was observed over the last 50 or 70 years, and we are getting back to a steadier state."
In any case, low birth weight is a health concern, the researchers said. Smaller babies face short-term complications, such as increased likelihood of requiring intensive care and a higher risk of death. And they can also be at higher risk for chronic disease as adults, Oken said.
On the other hand, heavier babies are subject to other health problems, such as obesity and its attendant problems, she noted.
For more information on pregnancy, visit the U.S. National Library of Medicine.
SOURCES: Emily Oken, M.D., M.P.H., assistant professor, population medicine, Harvard Medical School, Boston; Peter Bernstein, M.D., director, Maternal Fetal Medicine Fellowship Program, Montefiore Medical Center, New York City; February 2010 Obstetrics & Gynecology