Fetal fibronectin (fFN) is a protein produced by fetal cells, found where the fetal membranes meet the mother’s uterine lining. Measuring fFN in the cervicovaginal fluid is a non-invasive way to assess the risk of premature delivery. Healthcare providers use this test to guide medical decisions when a patient shows signs of potential early labor.
The Biological Function of Fetal Fibronectin
Fetal fibronectin is a glycoprotein that acts as a powerful adhesive or biological “glue” at the maternal-fetal junction. This protein firmly binds the chorion, the outer membrane of the amniotic sac, to the decidua, the specialized lining of the uterus during pregnancy. This bond maintains the integrity of the pregnancy and keeps the baby securely implanted.
Early in pregnancy, fFN is normally detectable in cervicovaginal secretions as the membranes settle into the uterine lining. Once attachment is complete, the protein recedes, becoming undetectable between 22 and 35 weeks of gestation. If fFN reappears prematurely during this period, it suggests the adhesive layer is beginning to dissolve or separate. This breakdown signals a heightened risk of spontaneous preterm birth.
Using fFN to Assess Preterm Labor Risk
The fFN test is not a routine screening tool but is used in specific clinical situations to predict short-term delivery risk. It is most often performed when a pregnant woman presents with signs of possible preterm labor, such as contractions, cramping, or changes in vaginal discharge. The test is typically administered between 22 weeks and 34 weeks and six days of gestation.
To collect the sample, a healthcare provider uses a sterile swab to gather secretions from the posterior fornix of the vagina or the outer cervical area during a speculum examination. The specimen is analyzed in a laboratory to determine if fFN is present above the threshold of 50 nanograms per milliliter. To ensure accuracy, the fFN test must be collected before any digital cervical examination or transvaginal ultrasound that manipulates the cervix.
Patients must avoid factors that can cause a false-positive result, including sexual intercourse, douching, or the use of vaginal creams or lubricants within 24 hours preceding the test. These precautions are necessary because blood or semen can contaminate the sample and interfere with the protein measurement. The test provides an objective assessment of risk, complementing the provider’s clinical evaluation.
What Positive and Negative Results Indicate
The results of the fFN test are reported as either negative or positive, and their interpretation differs significantly in clinical value. A negative result, meaning fFN is not detected or is below the 50 ng/mL cutoff, is highly reassuring. This outcome has an exceptionally high negative predictive value, often exceeding 99%, for delivery within the next seven to fourteen days.
This strong predictive power allows clinicians to confidently rule out imminent preterm labor, often enabling safe discharge from the hospital or clinic. Avoiding unnecessary hospital admissions, steroid injections, and medications to halt contractions reduces patient stress and healthcare costs. The patient can then return to routine prenatal care unless new symptoms develop.
Conversely, a positive result indicates fFN protein is present in the cervicovaginal secretions, signaling an elevated risk of delivering prematurely within the next one to two weeks. The positive predictive value is moderate, meaning only a fraction of women with a positive result will actually deliver early. Despite this, a positive fFN result prompts immediate action and closer monitoring by the medical team.
Management following a positive result involves interventions aimed at improving the baby’s outcome should an early delivery occur. This often includes administering antenatal corticosteroids, such as betamethasone, to accelerate the maturation of the baby’s lungs. Tocolytics may also be used temporarily to slow or stop uterine contractions, allowing time for the steroids to take effect. This proactive approach helps providers prepare for the possibility of a preterm birth.