Preterm birth remains one of the great mysteries and challenges in obstetrics. How do we know which pregnancies are at risk of preterm birth, especially early preterm birth- before 34 weeks pregnancy? A lab test called the Fetal Fibronectin or FFN test has long been performed after 22 weeks of pregnancy to help guide doctors in estimating the risk of preterm delivery. Doctors collect a small swab of vaginal fluid and send it off to the lab. In most hospitals, the result comes back positive or negative (a qualitative test).
Doctors use FFN because of its high “negative predictive value.” In other words, a negative result tells us that preterm birth is not likely. However, a positive result doesn’t tell us much at all. Currently, we use FFN collected at 22-35 weeks pregnancy to predict spontaneous preterm birth. Historically, doctors haven’t used FFN prior to 22 weeks because of the belief that fibronectin is normally present in 17% of vaginal secretions at earlier gestations. A new study in the February 2016 Green Journal puts this long-held assumption to the test.
The authors included almost 900 women at high risk for preterm birth in the study. High risk women were identified by characteristics such as a history of prior preterm birth or a short cervical length. The authors collected FFN in these women at 18 to 21 weeks and again at 22 to 28 weeks of pregnancy. They found that the early FFN test had comparable predictive abilities to the later, traditionally-timed FFN test.
Instead of using the simple qualitative yes-no FFN, they used a quantitative FFN test. This quantitative test gave an exact level of FFN instead of just a positive-negative result. The qualitative test uses a threshold of 50, but the quantitative test allowed the researchers to find exact thresholds for risk levels. Using a quantitative FFN test enabled researchers to stratify women into low, moderate, and high risk groups. A low early FFN, less than 10, had a preterm birth risk of less than 4%, which is less than the general population. More than 60% of the cohort of high risk women fell into this group, meaning that doctors could provide reassurance for the majority of high risk women. An early FFN of 10 to 200 was moderate risk. A high early FFN of greater than 200 had a 40% risk of preterm birth before 34 weeks.
The authors point out that the quantitative test allows for better predictive ability than the qualitative test. In fact, 13% of women with negative qualitative FFNs delivered before 34 weeks.
Many of us have heard of cervical length as another useful predictive tool. Can we toss that test to the side now? Nope. The researchers found that using cervical length measurements improved their predictive abilities. The two measures- cervical length and FFN- work synergistically to provide the best possible prediction of preterm birth.
Miss Hezelgrave, Dr. Shennan and their team of researchers at King’s College London were funded by Hologic.