The study, led by researchers at The Ohio State University Medical Center with grants from the National Institutes of Health, also found that other screening tools for preterm birth may not be as effective as originally believed.
The study, published in this week's (1/24) issue of The New England Journal of Medicine, reinforces the results of earlier clinical studies that found ambulatory monitors were no better at detecting premature birth than were many other diagnostic tools or surveillance programs.
In the multi-center study led by Dr. Jay Iams, director of OSU Medical Center's Division of Maternal-Fetal Medicine, researchers found that while uterine contraction frequency is significantly related to preterm birth, the measurement of such data is not clinically useful in predicting early birth.
"Technically, uterine monitoring sounds like a concept that should have merit, but studies aren't supportive," said Iams who notes that home monitoring has been around since mid-1980s and has been the subject of considerable debate.
When introduced to the market, home monitors were heralded as an early warning system to help get women believed to be in early labor to hospitals where they could be given medications to stop the contractions and allow the pregnancy to continue normally. As it was then, premature delivery is a leading cause of disability and death in newborns.
Despite studies casting doubt on their effectiveness, many doctors continue to prescribe the monitors for high-risk women. Patients wear the monitors daily to detect uterine contractions, often a sign of early labor. The device relays information via phone line to nurses at a central monitoring office who contact the patient's physician if there is a problem. Used daily for up to 10 weeks during a pregnancy, the devices can cost as much as $100 per day.
The NIH study involved 306 women with and without risk factors for preterm birth who were monitored from 24 to 36 weeks gestation.
During the course of the study, uterine activity was recorded from the portable monitors for at least two hours each day. Participants also received periodic monitoring for preterm birth using digital and transvaginal ultrasound assessment of the cervix and collection of fetal fibronectin, a protein that has been shown to predict preterm delivery.
None of the tests demonstrated superiority over the others for predicting preterm delivery nor did any have good sensitivity or provide a positive predictive value.
"We found relatively small differences in contraction frequency between women who subsequently delivered a preterm infant and those who gave birth at term," said Iams.
Transvaginal ultrasound was the most sensitive and consistent predictor of preterm birth before 35 weeks. However, this test had a positive predictive value below 40 percent, even in women with increased risk for preterm delivery.
Iams said the results of this and previous studies indicate more work lies ahead to find accurate methods to detect uterine contractions.
In a 1998 study, doctors from Kaiser Permanente Medical Center in Santa Clara, Calif., compared daily monitoring with weekly phone contact with nurses. The study showed that neither the monitoring nor the daily calls were better than weekly calls in preventing preterm birth.
The most recent study was conducted at 11 sites in the United States.
Journal
New England Journal of Medicine