The proportion of babies born weighing more than 4 kilograms (a little under 9 pounds) has increased significantly over the past decade in Sweden, North America, and Europe.

According to a new report, increases in mothers’ weight, and decreases in their smoking habits, are behind the rising trend in large babies in Sweden.

High birth weight brings with it an increased likelihood of complications for mothers and their newborns, the authors note in the medical journal Obstetrics and Gynecology.

Pamela J. Surkan from Harvard School of Public Health in Boston, Massachusetts, and colleagues used the Swedish Birth Registry to examine whether a change in rate of large-baby births over time could be explained by a change in maternal risk factors.

Between 1992 and 2001, the proportion of large infants increased from 3.3 percent to 3.9 percent, the authors report, and the proportion of infants weighing at least 4.5 kg increased from 3.7 percent to 4.6 percent.

During the same interval, the prevalence of overweight moms increased from 25 percent to 36 percent, while the rate of daily smoking decreased from 23 percent to 11 percent.

Overweight women were twice as likely and obese women were more than three times as likely as women of normal weight to have large infants, the researchers note.

“This study indicates yet another reason why physicians should encourage women of child bearing age to maintain a normal weight,” Surkan told Reuters Health.

SOURCE: Obstetrics and Gynecology, October 2004.