A jump in the number of babies born at abnormally low birth weights was the main reason why the U.S. infant mortality rate rose in 2002 for the first time in 44 years, according to a federal study released on Monday.

Almost 500 more babies weighing less than 1 lb 10.5 oz, or 750 grams, were born in 2002, compared to 2001, the Centers for Disease Control and Prevention said in a report.

The majority born at this weight die within a year, the Atlanta-based agency said.

The increase was primarily among babies delivered by women aged 20 to 34, the peak childbearing ages, which appears to contradict speculation that the recent rise in U.S. infant mortality was linked to the tendency of many American women to delay motherhood until later in life.

Women who give birth in their 30s and 40s are more likely to have babies born with birth defects or other complications.

CDC researchers said on Monday that multiple births, which accounted for about 3 percent of all births in 2002 but about 25 percent of the overall increase in infant mortality, may have contributed to the rise in low birth-weight infants.

Delivering twins, triplets and other multiple babies carries a higher risk of premature labor and low birth weight, both of which can be fatal to infants.

But the agency said other factors such as the health of expectant mothers also could have played a role. It noted that rates of maternal anemia, diabetes and chronic high blood pressure increased slightly between 2001 and 2002.

The report was published less than a year after government statisticians revealed that infant mortality rose to 7 deaths per 1,000 live births in 2002, up from 6.8 deaths per 1,000 in the previous year.

It was the first time since 1958 that the rate had not declined or remained unchanged.

There were 27,970 infant deaths in 2002, compared to 27,568 in 2001, out of about 4 million births a year. The CDC said on Monday that provisional data for 2003 suggested that the increase in infant mortality may not be continuing.