One of the biggest choices an expectant mother faces is how to handle the pain of childbirth. More than 60 per­cent of American women choose relief in the form of an epi­dural, a combination of local anesthetic and narcotic administered into the epidural space surrounding the spinal cord. Although most doctors believe that the injections are safe, a new study suggests that they may increase the risk that a mother will develop a fever during labor, which could, in rare instances, pose risks to her baby.

Epidurals have long been controversial. Some studies have suggested that women who ask for them are more likely to have emergency cesarean sections, but a 2011 review reported that epidurals do not increase C-section risk compared with other forms of pain relief. The same study did find, however, that epidurals make it more likely that doctors will have to deliver with the help of forceps or a vacuum.

Now mothers have new findings to factor into their decisions. In a study published in February in the journal Pediatrics, researchers at the Harvard School of Public Health and Harvard Medical School followed 3,209 women with low-risk pregnancies who were giving birth to their first child. Of those receiving epidurals, nearly one out of five developed a fever of at least 100.4 degrees Fahrenheit during labor compared with only 2.4 percent of those receiving other drugs or no pain relief. The higher the mother’s fever, the more likely the baby was to have low Apgar scores after birth—an indicator of overall newborn health—as well as low mus­cle tone and breathing difficulties. And the 8.6 percent of women receiving epidurals who developed a fever of greater than 101 degrees F were more than six times as likely as non­febrile moms to have babies who had new­born seizures, although the overall seizure risk was only 1.3 percent. No one knows why epi­durals appear to be associated with fevers, but senior author Ellice Lieberman, a biologist and obstetrician at the Harvard School of Public Health, believes that the drugs might be invoking an inflammatory response.

The big remaining question is whether these fevers might have more permanent health effects. “We don’t really know,” Lie­berman says, but most effects “seem to be transient.” Nevertheless, because a fever takes an average of six hours to develop after an epidural has been administered, women who want to minimize their risk could con­sider asking for pain relief only when baby seems well on its way.