Nearly a quarter of the estimated four million women who give birth each year in the United States receive an antibiotic during labor in order to protect their infants from developing a serious infection.

But these women may be getting five times the necessary dose of medication, said Ohio State University researchers.

"Physicians may be using too much penicillin," stated researcher David Colombo. "There is a shortage of this type of penicillin -- the kind that is given intravenously. Also, we worry that the overuse of antibiotics overall will lead to the increased resistance of harmful bacteria to these drugs."

Penicillin is the most common antibiotic given to a woman at risk of transferring the bacteria group B Streptococcus (GBS) to her newborn. Penicillin is thought to protect the baby against GBS, which can cause sepsis, a sometimes-fatal blood infection.

The Centers for Disease Control and Prevention recommend that these women get about 3 grams of penicillin (5 million units) when labor begins, followed by 1.5 grams (2.5 million units) every four hours until delivery.

But a dose of about 0.5 grams (1 million units) given initially and again every four hours until delivery may give the same protective effect, said Colombo, an assistant professor of obstetrics and gynecology at Ohio State.

The study was published in the American Journal of Obstetrics and Gynecology.

Fifteen healthy pregnant women in their third trimester of pregnancy, but who were not yet in labor, were given one 0.5-gram dose of intravenous penicillin. The researchers wanted to analyze the concentration of penicillin in each woman's blood, so they took blood samples via a catheter at one, five, 15 and 30 minutes after the initial dose and then once every 30 minutes for a total of four hours.

They found that this lower dosage of penicillin is excreted from the body at the same rate as are the 3- and 1.5-gram doses, suggesting that a lower dosage of penicillin provides levels sufficient enough to protect a newborn from infection caused by GBS.

"The baby is really the one who is being treated," said Colombo. "We can give the 0.5 gram doses of penicillin to mothers in labor and get the same beneficial results as we do with the larger doses."