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Date: February 28, 1996 For Release: 6 pm E.S.T Contact: Robert Bock (301) 496-5133
Researchers funded by the National Institute of Child Health and Human Development (NICHD) have identified a significant risk factor for premature birth, which, in turn, is a major cause of infant mortality.
Specifically, a pregnant woman who has been determined by ultrasound to have a short cervix during pregnancy is more likely to give birth prematurely than is a woman with a longer cervix, according to a study by the NICHD's Maternal Fetal Medicine Network. Established in 1986, the Maternal-Fetal Medicine Units Network was formed to permit more effective evaluation of existing and new prenatal and perinatal health treatments through clinical trials.
The study, the largest of its kind to date, appears in the February 29 issue of The New England Journal of Medicine.
The finding provides a relatively easy method for evaluating the chances of a woman giving birth prematurely, the authors wrote. In addition, it may prove particularly useful for selecting women who are candidates for clinical trials of cerclage, the surgical practice of preventing early labor by placing a single suture through the cervix. Premature birth is a major public health problem. Roughly 11 percent of U.S. infants are born prematurely (before the 36th week of pregnancy), according to the National Center for Health Statistics. Of these infants, 4.7 percent will die in infancy, accounting for 58.6 percent of all infant deaths. A method for identifying and preventing premature labor would significantly reduce the infant mortality rate.
"The length of the cervix is directly correlated with the duration of pregnancy: the shorter the cervix, the greater the likelihood of preterm delivery," they wrote.
For the study, the researchers made use of transvaginal ultrasonography, in which the ultrasound probe is placed at the vaginal opening. This method was used to evaluate 2915 women recruited nation wide from the 10 centers of NICHD's Maternal and Fetal Medicine Units, at approximately the 24th week of pregnancy. The women were again scheduled to undergo the transvaginal ultrasound examination at the 28th week, but because some either gave birth, withdrew from the study, or failed to appear for the examination at the scheduled interval, only 2531 actually did so.
The cervix length of each woman was recorded and ranked serially. Based on the length of the cervix each woman was assigned to a particular grouping called a percentile. After the women had all given birth, the researchers calculated their risks of giving birth prematurely, based on the women's ultrasound readings at the time of each evaluation.
Women whose cervical length was above the 75 percentile were the least likely to give birth prematurely. Using these women as a reference point, the researchers calculated the relative risk of giving birth prematurely for the women in the remaining percentiles. "Relative risk" refers to the chances of women giving birth prematurely, as compared to women above a certain percentile. For example, women below the 75 percentile at 24 weeks had a relative risk of 1.98, meaning they were almost twice as likely as the women above the 75 percentile to give birth prematurely. The researchers calculated the risks of giving birth prematurely for the remaining percentiles as follows:
(table did not convert to ASCII text, call NICHD for and original copy to view the table)
The researchers conceded that a shortened cervix may not be a predictor of early labor, but due instead to uterine contractions. Such contractions, which shorten the cervix, could have resulted from the beginnings of labor. The NICHD-supported researchers discounted this possibility, however, as the women enrolled in the study gave no other indications of beginning labor at the time they were examined.
Although previous studies have suggested a link between cervical length and prematurity, the current study is the largest of its kind to do so.
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