Fetal fibronectin (fFN) is a protein marker that plays a role in pregnancy, and its presence can offer important information about the possibility of preterm labor. The fFN test is a diagnostic tool used by healthcare providers to assess the risk of a pregnant person delivering a baby before 37 weeks of gestation, which is defined as a preterm birth. Understanding the function of this protein and the interpretation of the test results allows for informed decisions regarding pregnancy management.
The Biological Role of Fetal Fibronectin
Fetal fibronectin is a glycoprotein produced by fetal cells, found at the interface between the fetal membrane (chorion) and the uterine lining (decidua). This protein functions as a biological adhesive, maintaining the connection between the amniotic sac and the wall of the uterus throughout pregnancy.
fFN is normally detectable in cervicovaginal secretions early in pregnancy (up to about 22 weeks of gestation) and again as the pregnancy approaches term. In a stable pregnancy, the protein becomes undetectable in the fluid between 22 and 35 weeks because the bond between the fetal membranes and the uterus is firmly established. If this connection begins to break down prematurely, fFN is released and can be detected in the cervicovaginal fluid.
Why and When the fFN Test is Performed
The fFN test is primarily performed to determine the likelihood of spontaneous preterm delivery in pregnant people experiencing symptoms of preterm labor. These symptoms can include regular uterine contractions, changes in vaginal discharge, or pelvic pressure. The test helps clinicians decide whether to intervene immediately or to allow the pregnancy to continue with less intensive monitoring.
The test is typically administered between 22 and 34 weeks of gestation, as this is the period when the protein should not be present in the cervicovaginal fluid. The procedure involves a simple, non-invasive swab to collect a fluid sample from the area near the cervix. To prevent contamination and false-positive results, the test must be performed before a digital cervical examination and at least 24 hours after sexual intercourse or the use of vaginal creams.
The results are most reliable for predicting the risk within a short, two-week window. For a symptomatic person, the fFN test is a valuable tool for ruling out imminent preterm birth, helping to avoid unnecessary treatments and hospitalizations. The test is also sometimes used for high-risk individuals who have a history of previous preterm birth, even if they are not currently showing symptoms.
Understanding the Test Results
Fetal fibronectin test results are reported as either negative or positive, with a critical threshold generally set at 50 nanograms per milliliter (ng/mL) of fluid. A negative result (fFN not detected or below 50 ng/mL) is the most powerful outcome for clinical decision-making. A negative result has a very high negative predictive value, meaning there is less than a 1% chance of delivering within the next one to two weeks.
A negative test provides strong reassurance to both the pregnant person and the care team that labor is highly unlikely in the immediate future. This data often allows healthcare providers to safely discharge the person home and avoid the use of medications to stop contractions or accelerate fetal lung development.
Conversely, a positive result (fFN detected at or above 50 ng/mL) indicates that the bond between the fetal membrane and the uterine lining has been disrupted. A positive result signals an increased risk of preterm labor and delivery within the next seven to fourteen days. However, the positive predictive value is only moderate, meaning many people with a positive result will still carry the pregnancy to term.
The positive result serves as a warning sign, requiring immediate attention and further assessment of the pregnancy. The clinical team will use the positive result in conjunction with other factors, such as cervical length measurements, to determine the overall risk.
Clinical Management After a Positive Result
A positive fFN test result triggers medical actions focused on improving newborn outcomes should an early delivery occur. If the positive result is obtained before 34 weeks of gestation, the primary management step is the administration of antenatal corticosteroids. These steroid injections accelerate the maturation of the baby’s lungs, significantly reducing the risk of respiratory distress syndrome after birth.
Tocolytic medications are also considered; these drugs temporarily suppress uterine contractions. Tocolytics are used to gain a short window of time, typically 48 hours, allowing the corticosteroids to take full effect on the fetal lungs. Increased monitoring, often involving hospital admission, is also instituted to observe for signs of labor progression.
If the pregnancy is very early, especially before 32 weeks, a positive fFN may prompt the healthcare team to transfer the patient to a hospital with a specialized neonatal intensive care unit (NICU). While strict bed rest is debated and not universally recommended, activity limitation and close surveillance remain central to the management plan.