Babies born late at night or in the wee hours of the morning may have a slightly higher risk of rare brain problems compared to infants delivered during the day, suggests a new study.

Each year, more than 10,000 babies are born in the U.S. with a condition known as neonatal encephalopathy. More than half of these babies will either die or grow up to have problems such as cerebral palsy and epilepsy.

Previous studies have already linked nighttime births with greater risks of poor outcomes, including death. Decreased staffing and fatigue among doctors are often blamed.

However, it has remained unclear whether the timing of the birth might play a role in neonatal encephalopathy, lead researcher Dr. Yvonne W. Wu of the University of California School of Medicine, San Francisco, told Reuters Health in an email.

Wu and her colleagues studied nearly 2 million term babies born in California during a 14-year period to see whether the hour, day and month of delivery might impact the risk of the complication.

Overall, they found that more than 2,000 babies, or about 1.1 per 1,000 births, had brain problems. Sixteen percent of these babies died before reaching 1 month of age.

Babies born at night between the hours of 10 PM and 4 AM had a 22 percent higher risk than babies who were born during the day, report the researchers in the American Journal of Obstetrics and Gynecology.

Also at a higher risk were boys and children of mothers who were over the age of 35, who had not received prenatal care, or who were giving birth for the first-time.

Being born during the weekend, or during particular months of the year, did not change the risk of neonatal encephalopathy.

There are rare cases in which the cause of neonatal encephalopathy is known, such as when the mother has severe bleeding of the placenta or a ruptured uterus, said Wu. “But in most cases, the underlying cause is unclear.”

Consequently, the researchers can’t point to any specific means of prevention. But they do note some strategies that might help immediately after a baby is born with the condition.

“Cooling the baby’s body temperature appears to protect the brain, and may lead to better long-term neurologic outcomes,” Wu said.

While it is possible that differing levels of health care quality during the day and night explain some of the differences, the team cautions that their study does not prove that being born at night causes neonatal encephalopathy. The babies could share something else that places them at higher risk than their daytime-born peers.

Regardless, the condition is extremely rare. “It is very common for babies to be delivered at night,” said Wu. “And the vast majority of babies born at night will have no complications.”

SOURCE: American Journal of Obstetrics & Gynecology, online November 12, 2010.