Monitoring fetal oxygen levels during delivery does not appear to lower the rate of unnecessary caesarean sections or improve infant health, a large U.S. government study concludes.

Previous research suggested that measuring oxygen levels in the blood of a fetus helps reduce the number of caesarean deliveries. It was thought the measurements may help doctors to understand the reasons behind abnormal heart rates.

In 2000, the U.S. Food and Drug Administration gave conditional approval for one device, called OxiFirst, which measures fetal oxygen levels during delivery and is designed to be used along with electronic tracking of the fetal heart rate.

But in Thursday's New England Journal of Medicine, researchers concluded the device is ineffective, based on their study of 5,341 first-time mothers at 14 hospitals.

"There's no reason to use it," said lead author Dr. Steven Bloom, chief of obstetrics and gynecology at the University of Texas Southwestern Medical Center. "We didn't find any evidence of harm, but why should we invest valuable health-care dollars in something that doesn't have a proven benefit?"

In an accompanying editorial, Dr. Michael Greene of Massachusetts General Hospital agreed, saying the results mean that use of an expensive technology can be curtailed before it becomes widespread.

In the study, sensors were fed through the cervix after a woman's water broke, and then placed against the fetus' face. The sensor provides a continuous reading of fetal oxygen levels, which are displayed on a monitor.

The monitoring device, which costs about $10,000 US, uses sensors to measure levels of oxygen in the fetus' blood.

For half of the women participating in the study, doctors could read the oxygen levels, while the information was hidden for the rest. Computers recorded the data.

The rates of delivery by C-section, forceps or vacuum delivery were nearly the same in both groups, as was the overall health of the infants. There was also no difference in labour complications between the two groups, the researchers found.

The researchers originally planned to study 10,000 women, but they stopped the study early because the results were so conclusive, Bloom said.

In a statement, the FDA said it was reviewing the results to decide whether to change labels on the monitors or tell hospitals about the results, adding it is unlikely to withdraw approval for OxiFirst based on only one study.

Obstetricians are still looking for a way to measure the health of a fetus during delivery.

The research was funded by the U.S. National Institute of Child Health and Human Development, part of the National Institutes of Health.

With files from the Associated Press