September 18, 2001
A short cervix early in the second trimester of pregnancy appears to be a warning sign of impending premature birth among women who have previously given birth prematurely, according to a study by the National Institute of Child Health and Human Development's (NICHD) Maternal Fetal Medicine Units (MFMU) Network. The study's first author is John Owen, M.D., of the University of Alabama at Birmingham. The study findings appear in the September 19 Journal of the American Medical Association.
"This is another piece of the puzzle in helping us to prevent preterm birth," said Duane Alexander, M.D., Director of the NICHD. "We now have a means of predicting preterm birth very early in the pregnancy of some women who have previously given birth prematurely. Now that we have this early warning sign, the next step will be to develop a way to prevent preterm birth from occurring."
The cervix is the lower part, or "neck" of the uterus. Late in pregnancy, the cervix shortens in preparation for birth.
Preterm birth complicates from 8 to 10 percent of all births, said Catherine Spong, M.D., Chief of NICHD's Pregnancy and Perinatology Branch and coordinator of the MFMU. Most deaths of premature infants occur in infants born before the 32nd week of pregnancy. Infants born before the 37th week of pregnancy are at greater risk for life-threatening infections, a serious lung condition known as respiratory distress, and serious damage to the intestines (necrotizing enterocolitis). The cost of caring for premature infants in the United States exceeds $4 billion each year. Additional information on premature (or preterm) births is available from the Centers for Disease Control and Prevention, at:
ftp://ftp.cdc.gov/pub/Publications/mmwr/wk/mm4809.pdf (PDF - 300 KB)
http://www.cdc.gov/nchs/data/nvsr/nvsr49/nvsr49_01.pdf (PDF - 6.14 MB)
According to Dr. Owen, other researchers have attempted to find whether cervical shortening could predict the likelihood of preterm birth. However, these studies measured the cervix when women were past their 20th week of pregnancy. The current study sought to find if cervical shortening much earlier in the pregnancy, beginning at the 16th week, was predictive of spontaneous preterm birth.
Dr. Owen and his colleagues at 9 of the Maternal Fetal Medicine Units Network sites recruited pregnant women who had spontaneous preterm births before the 32nd week of earlier pregnancies. The cervixes of the women were first examined by ultrasonography between 16 weeks of pregnancy and 18 weeks, six days of pregnancy. The women then received a maximum of 4 ultrasound evaluations before their 23rd week of pregnancy. In all, 183 women completed the study.
The researchers found that women whose cervixes had shortened to less than 25 millimeters in length by the time of the first ultrasound evaluation were 3.3 times more likely to give birth prematurely than were the other women in the study. Women whose cervixes had not shortened until later ultrasound evaluations were also more likely to give birth prematurely.
"Cervical length assessed by endovaginal sonography between 16 weeks' and 18 weeks six days' gestation, augmented by serial evaluations, predicts spontaneous preterm birth before 35 weeks' gestation in high risk women," the study authors wrote.
The researchers cautioned that the ultrasound technique they used to assess cervical length should not be tried without the supervision of a sonologist who has experience with the technique.
Dr. Spong noted that because cervical shortening in early pregnancy is predictive of preterm birth, they can now try to find means to prevent it. Worthy of further study, she said, is investigating whether a surgical technique known as cervical cerclage would decrease the risk of spontaneous preterm birth. This technique involves surgically stitching the cervix with the goal of keeping it from shortening further during pregnancy.
Dr. Owen pointed out, however, that it is not yet known whether cervical cerclage will prevent preterm birth in this group of women. For this reason, he said, women with shortened cervixes and a history of preterm birth should not undergo cerclage unless they are taking part in a clinical study to determine if cervical cerclage is effective.
The NICHD is part of the National Institutes of Health, the biomedical research arm of the federal government. The Institute sponsors research on development before and after birth; maternal, child, and family health; reproductive biology and population issues; and medical rehabilitation. NICHD publications, as well as information about the Institute, are available from the NICHD website, http://www.nichd.nih.gov, or from the NICHD Information Resource Center, 1-800-370-2943; E-mail NICHDInformationResourceCenter@mail.nih.gov