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If you've delivered a baby in the past three decades, your doctor likely strapped a device to your belly to monitor your baby's heart rate.

One of the most common obstetric procedures in the United States, electronic fetal heart monitoring has saved lives, reducing the number of babies who die in labor. But it hasn't lowered the frequency of cerebral palsy, a neuro-developmental disorder often caused by birth trauma.

A new study by maternal-fetal medicine specialists at Intermountain Medical Center and the University of Utah shows it's an imperfect gauge of a baby's health — one that can lead to needless, expensive interventions such as cesarean section and forceps deliveries.

"We aren't recommending abandoning monitoring. We just need to do it better," said Marc Jackson, an OB-GYN at Intermountain and the study's lead author. "We're trying to build a better road map for labor."

Fetal heart rate monitoring has been the standard of maternal care since the '80s. It was used in 84 percent of 27 million deliveries between 1997 and 2003, according to federal health data.

Doctors look for irregular patterns, such as a high or low baseline heart rate and sudden drops, accelerations or swings. Certain patterns trip alarms.

Currently, a classification system developed in 2008 by the Eunice Kennedy Shriver National Institute of Child Health and Human Development and the American College of Obstetricians and Gynecologists guides physicians in deciding when and how to intervene.

Category I heart rate patterns are considered normal, and, as a rule, do not indicate fetal stress. Category III patterns are rare and "ominous" and usually "mean the baby ought to be delivered expeditiously," said Jackson.

Category II patterns, on the other hand, are considered indeterminate, and their significance, uncertain.

It's this murky class into which a good share of deliveries fall and that poses a quandary for doctors, said Jackson.

In an attempt to add clarity, Jackson and his colleagues examined the fetal heart-rate patterns of more than 48,000 babies delivered at Intermountain hospitals during 28 months from 2007 to 2009.

The results of the study are published in the October issue of the journal Obstetrics and Gynecology. Researchers measured how long babies spent in each category and traced their neonatal outcomes.

Overall, heart patterns were labeled category I for nearly 78 percent of the births and as category II for 22 percent. Category III ratings were very rare, less than 1 percent.

But when researchers zeroed in on the final two hours of labor, the numbers changed. Category I rates fell to 61 percent while category II rates rose to 39 percent. Category III barely budged."That tells us that babies are much less likely to have reassuring patterns during the final hours of labor," said Jackson.

Researchers also found that a category II baby's outcome hinges on the amount of time spent in that class.

The threshold appears to be 50 percent, said Jackson. Infants that spent less than half of their delivery in category II had the same outcomes as category I babies.

There is, of course, no way of predicting how well or how long a labor will go until it's done.

"But it's clear that category II is a big mosh up of babies that are doing OK and babies that are not doing OK," Jackson said. "The job before us now is to figure out what's a good 'cat 2' or bad 'cat 2.' "

This will entail refining the class. Dozens of combinations of heart patterns now fall into category II, and Jackson estimates that only a handful raise concerns.

Calla M. Holmgren, another of the study's authors, is researching ways of combining heart patterns with the mother's health and pregnancy risk factors, such as whether she smokes or has hypertension.

It's this model that certified nurse midwives like Rebecca McInnis at the Utah Birth Center advocate. The center caters mostly to women with low-risk pregnancies and uses monitoring intermittently.

"We're grateful for the technology," she said, noting it's a safeguard to identify women who require transport to a hospital. "But the more care we can give that's scientific[ally]-based, the better."