WASHINGTON (AP) - Obstetricians are about to get the first fetal monitor to measure the oxygen level inside an unborn baby's blood during labor. This breakthrough device promises to help doctors determine which babies really are in distress and may need to be delivered by Caesarean section.

The Food and Drug Administration approved the OxiFirst system Monday, calling it the first major technological development in fetal monitoring in some 30 years.

Doctors have long used devices called pulse oximeters that measure oxygen saturation in the blood of children and adults. But the proper oxygen level of a fetus was something of a mystery, and there have been no devices that could try to measure it.

During labor, monitors that measure fetal heart rate and the mother's uterine contractions are routinely used.

But just because a fetal heart rate looks a little slow doesn't always mean a baby is in trouble, explained Dr. Diane Mitchell, an FDA gynecologist in charge of the OxiFirst approval. Doctors routinely struggle to determine when a so-called "non-reassuring" fetal heart rate is due to lack of oxygen and thus requires an immediate C-section, or when it's not a problem and the mother should continue trying to deliver normally.

The OxiFirst, like other pulse oximeters, works by measuring light waves passing through the skin. But it was developed to use different wavelengths necessary to capture oxygen levels in a fetus; it turns out that normal fetal oxygen saturation is lower than the level needed even by a newborn.

Once the mother's water breaks during labor, the sensor is threaded up into the uterus and placed against the fetus' temple or cheek. A cable connects it to a monitor where doctors read the oxygen levels.

Manufacturer Mallinckrodt/Nellcor Inc. studied 1,000 women, half of whom had standard monitoring during labor and the other half had OxiFirst added to standard monitoring.

The rate of C-sections because of that "nonreassuring fetal heart rate" was 10 percent among women who got standard monitoring - but only 5 percent among women given the extra OxiFirst monitoring.

But that doesn't prove OxiFirst prevents C-sections, the FDA cautioned. For reasons scientists cannot explain, Caesareans credited to the mother's "difficult labor" actually increased in the OxiFirst group. Thus, when overall C-sections were tallied, roughly 26 percent of women had Caesareans regardless of OxiFirst.

To clear up the confusion, Mallinckrodt has agreed to study all C-sections in hospitals that buy the new monitors, Mitchell said.

That could be a lot - up to one-fourth of the nation's 4 million annual births involve that puzzling fetal heart rate at some point during labor.

Pleasanton, Calif.-based Mallinckrodt will begin taking hospital orders for the $11,000 OxiFirst monitors Thursday.

Copyright 2000 The Associated Press. All rights reserved.