Although many pregnant women look forward to day their developing fetus becomes a bundle of joy nestled in their arms, none would choose a preterm delivery and its myriad complications, including death and cerebral palsy. A short cervix is a predictor of preterm birth, and cervical length can be assessed with an ultrasound exam. A new study evaluated the advantages of cervical length screening and which women should be screened. The findings were published in the July edition of the journal Obstetrics & Gynecology by researchers at Northwestern University Feinberg School of Medicine, Chicago, Illinois; and the University of Alabama at Birmingham, Birmingham, Alabama.
The authors note that cervical length screening in women with a history of prior preterm birth is recommended by the American College of Obstetricians and Gynecologists; however, it is currently controversial whether universal second-trimester (middle three months of pregnancy) cervical length screening should be used as a strategy among women without prior preterm birth. Proponents of universal screening note that two large randomized trials found that vaginal progesterone reduces the risk of preterm birth in women with a short cervix. The objective of the study was to estimate whether there are demographic or clinical risk factors that are associated with a short cervix and whether these factors can be utilized in an algorithm to optimize the efficiency of a cervical length screening program.
The study group comprised 18,250 women with a single pregnancy without a history of spontaneous preterm birth. The women underwent routine transvaginal second-trimester cervical length screening. Seven risk factors for preterm birth were compared by cervical length status. A statistical analysis was conducted to identify risk factors for a short cervix (cervical length 2.5 cm or less). Different prediction models for a short cervix, based on the number of risk factors present, were developed and test characteristics for cervical length assessment for different risk-based screening approaches were calculated.
The investigators found that among the 18,250 women screened, 164 (0.9%) had a short cervix. Maternal age and conception by in vitro fertilization were not significantly associated with a short cervix. However, African American and Hispanic race, current tobacco use, prior preterm birth, and having a prior cervical excisional procedure (e.g., cervical conization) were independent risk factors for a short cervix. The researchers found that If only women who had any of these factors were offered transvaginal cervical length screening, the specificity increases from 62.8% for universal screening to 96.5% with a risk-based approach. The sensitivity with one variable present to offer transvaginal scanning was 62.8% and with two factors 14%. Sensitivity refers to: if a person has a disease, how often will the test be positive? Sensitivity refers to: if a person does not have the disease, how often will the test be negative?
The authors concluded that limiting cervical length screening to women with at least one of the identified risk factors for a short cervix significantly decreases the number of ultrasound exams for cervical length assessment. However, this scheme results in nearly 40% of women with a short cervix not being diagnosed.
