It's being hailed as the first major technological development in fetal monitoring in 30 years. It's a device designed to help obstetricians determine which babies are in trouble and need to be delivered by Caesarean section.

The U.S. Food and Drug Administration approved the OxiFirst system Monday. It's the first fetal monitor to measure the oxygen level inside an unborn baby's blood during labor.

Before this, doctors couldn't get a proper oxygen level from the fetus. They had access to devices called pulse oximeteres that measure oxygen saturation in the blood of children and adults.

When a woman goes into labour, obstetricians use monitors that measure fetal heart rate and the mother's contractions.

But doctors say that a slow fetal heart rate doesn't necessarily mean the baby is in distress. It's often the obstetrician's call to determine when such a fetal heart rate is due to lack of oxygen, which would require an immediate C-section — or when the baby's not really in trouble, and the mother should continue to deliver normally.

The OxiFirst works by measuring light waves passing through the skin — but it uses different wavelengths necessary to capture oxygen levels in a fetus. Normal fetal oxygen saturation is lower than the level needed even by a newborn.

Here's how it's used:

When the mother's water breaks, the sensor is threaded up into the uterus and placed against the fetus' temple or cheek. That's connected to a monitor where doctors read the oxygen levels.

The FDA, however, warns that there's no proof that OxiFirst prevents C-sections. To find out more, device manufacturer Mallinckrodt/Nellcor Inc. will study all C-sections in hospitals that buy the new monitors.