Fetal Fibronectin (fFN) Calculator
Calculate your preterm birth risk using fetal fibronectin test results combined with gestational age, cervical length, and clinical risk factors. This free fFN calculator uses Bayesian probability analysis with published likelihood ratios to estimate delivery probability within 7 days, 14 days, and before 34 and 37 weeks of gestation for both singleton and twin pregnancies.
This calculator is provided for informational and educational purposes only. It is not intended to replace professional medical advice, diagnosis, or treatment. Always consult with a qualified healthcare professional before making any medical decisions. The results from this calculator should be used as a reference guide only and not as the sole basis for clinical decisions.
| Delivery Timeframe | Negative fFN (below 50 ng/mL) | Your fFN Result | High Risk fFN (200+ ng/mL) |
|---|
| fFN Range (ng/mL) | Risk Category | Preterm Birth Before 34 Weeks | Delivery Within 7 Days |
|---|---|---|---|
| Below 10 | Very Low Risk | ~2.7% | ~1.1% |
| 10 to 49 | Negative (Low Risk) | ~11.0% | ~1.8% |
| 50 to 199 | Positive (Elevated) | ~14.9% | ~5-16% |
| 200 to 499 | High Risk | ~33.9% | ~16-41% |
| 500 or above | Very High Risk | ~47.6% | ~41%+ |
This calculator is provided for informational and educational purposes only. It is not intended to replace professional medical advice, diagnosis, or treatment. Always consult with a qualified healthcare professional before making any medical decisions. The results from this calculator should be used as a reference guide only and not as the sole basis for clinical decisions.
About This Fetal Fibronectin Preterm Birth Risk Calculator
This fetal fibronectin (fFN) calculator is designed for healthcare providers, maternal-fetal medicine specialists, labor and delivery nurses, and pregnant women who want to understand the clinical significance of their fFN test results. The calculator accepts both qualitative fFN results (positive or negative at the 50 ng/mL threshold) and quantitative fFN concentration values to generate comprehensive preterm birth risk estimates for delivery within 7 days, within 14 days, and before 34 and 37 weeks of gestation.
The calculator uses Bayesian probability analysis with published likelihood ratios from large-scale clinical studies and meta-analyses to transform pre-test probability into post-test risk estimates. Pre-test probability is calculated from user-provided clinical parameters including symptom status, gestational age, transvaginal cervical length, pregnancy type (singleton or twin), and prior spontaneous preterm birth history. The likelihood ratios are stratified by quantitative fFN concentration bands (below 10, 10-49, 50-199, 200-499, and 500+ ng/mL) based on data from the Preterm Prediction Study, EQUIPP, PREMET, and other prospective cohort studies.
The results are displayed through multiple visualization formats: an SVG gauge showing the primary 7-day delivery risk, horizontal gradient zone bars mapping the fFN concentration and risk position, a detail grid with key metrics including negative predictive value, a pre-test to post-test probability comparison, and a tabbed section containing a three-way scenario comparison table, risk factor impact analysis, and quantitative fFN reference ranges with associated preterm birth probabilities.
Fetal Fibronectin (fFN) Calculator: Complete Guide to Preterm Birth Risk Assessment and Clinical Interpretation
Preterm birth, defined as delivery before 37 completed weeks of gestation, remains one of the most significant challenges in obstetric care worldwide. Affecting approximately 7 to 12 percent of all pregnancies globally, preterm delivery is the leading cause of neonatal mortality and a major contributor to long-term childhood morbidity. Among the clinical tools developed to predict preterm birth, fetal fibronectin (fFN) testing has emerged as one of the most widely studied and clinically validated biomarkers. This comprehensive guide explains how the fetal fibronectin test works, how to interpret both qualitative and quantitative results, and how clinicians use this information to guide management decisions for women presenting with symptoms of preterm labor.
The fetal fibronectin calculator presented above allows healthcare providers and patients to input their fFN test results alongside other clinical parameters, including gestational age, cervical length, symptom status, and pregnancy type, to generate a comprehensive risk assessment for preterm delivery within 7 days, within 14 days, and before 34 and 37 weeks of gestation. Understanding these results in clinical context is essential for informed decision-making about interventions such as antenatal corticosteroid administration, tocolytic therapy, and maternal transfer to tertiary care facilities.
What Is Fetal Fibronectin and Why Does It Matter
Fetal fibronectin is an extracellular matrix glycoprotein with a molecular weight exceeding 450,000 daltons. It is localized at the maternal-fetal interface of the amniotic membranes, specifically between the chorion and the decidua, where it functions as a biological adhesive or “glue” that helps secure the pregnancy to the uterine wall. The protein is produced by amniocytes and cytotrophoblasts and plays a critical role in maintaining the structural integrity of the choriodecidual junction throughout pregnancy.
Under normal physiological conditions, fetal fibronectin is present in high concentrations in amniotic fluid, reaching approximately 100 micrograms per milliliter during the second trimester and 30 micrograms per milliliter at term. However, in the cervicovaginal secretions of women with intact membranes, fFN levels are typically very low (below 50 nanograms per milliliter) between 22 and 34 weeks of gestation. This low baseline level reflects the intact barrier between the fetal membranes and the cervical canal.
When the choriodecidual junction is disrupted through mechanical forces, inflammatory processes, or infection, fetal fibronectin is released into the cervicovaginal secretions. Elevated levels of fFN, defined as concentrations at or above 50 nanograms per milliliter, are therefore considered a biomarker of disruption at the maternal-fetal interface, signaling an increased risk of spontaneous preterm birth. This principle forms the basis of fFN testing as a clinical tool for predicting preterm delivery.
How the Fetal Fibronectin Test Is Performed
The fetal fibronectin test is performed by collecting a specimen from the posterior vaginal fornix or ectocervix using a sterile Dacron swab. The swab is rotated against the target site for approximately 10 seconds to absorb an adequate sample of cervicovaginal secretions. The collected specimen is then analyzed using an enzyme-linked immunosorbent assay (ELISA) that employs the FDC-6 monoclonal antibody, which specifically recognizes the III-CS domain unique to fetal fibronectin, distinguishing it from other fibronectin family members.
The test can be processed using either a laboratory-based ELISA or a rapid bedside analyzer. The rapid fFN test system provides results within approximately 10 to 20 minutes, making it suitable for use in acute clinical settings such as labor and delivery triage units. Both qualitative results (positive or negative at the 50 nanograms per milliliter threshold) and quantitative measurements (providing the exact concentration in nanograms per milliliter) are available depending on the testing platform used.
The fFN test should only be performed between 22 and 34 weeks of gestation, before any digital cervical examination, and in the absence of factors that may cause false-positive results. The cervical dilation must be less than 3 centimeters, membranes must be intact, and the sample should not be collected within 24 hours of sexual intercourse or vaginal bleeding, as these conditions may contaminate the specimen and reduce accuracy.
Qualitative Versus Quantitative Fetal Fibronectin Testing
The traditional qualitative fFN test uses a binary threshold of 50 nanograms per milliliter. A result below this cutoff is reported as negative, while a result at or above this level is reported as positive. This threshold was established through receiver operating characteristic (ROC) curve analysis and represents a balance between sensitivity and specificity for predicting spontaneous preterm birth before 35 weeks of gestation.
More recently, quantitative fFN testing has gained clinical attention because it provides the exact concentration of fetal fibronectin in the cervicovaginal specimen, allowing for more granular risk stratification. Research has demonstrated that the risk of spontaneous preterm birth increases progressively with rising fFN concentrations from approximately 20 to 300 nanograms per milliliter, with concentrations above 300 nanograms per milliliter not associated with further incremental risk increases. Studies have identified several clinically useful thresholds for quantitative testing.
Diagnostic Performance of the fFN Test
The clinical value of the fetal fibronectin test lies primarily in its high negative predictive value, which is its ability to rule out imminent preterm birth when the result is negative. In symptomatic women presenting with contractions between 24 and 34 weeks, a negative fFN test (below 50 nanograms per milliliter) has a negative predictive value exceeding 99 percent for delivery within 7 days. This means that fewer than 1 in 100 women with a negative fFN test will deliver within the following week, providing substantial reassurance to both clinicians and patients.
The positive predictive value of the test is more variable, ranging from approximately 10 to 46 percent depending on the population studied and the prevalence of preterm birth in that population. In symptomatic women, the positive predictive value for delivery within 14 days is approximately 17 to 19 percent for singletons. While this means that the majority of women with a positive test will not deliver prematurely, the relative risk of preterm delivery is substantially elevated compared with women who test negative.
Likelihood Ratios and Bayesian Risk Assessment
Likelihood ratios provide a more useful clinical metric than sensitivity and specificity alone because they can be applied to individual pre-test probabilities to generate patient-specific post-test probabilities of preterm delivery. The positive likelihood ratio (LR+) indicates how much more likely a positive test result is to occur in a woman who will deliver preterm compared with one who will not, while the negative likelihood ratio (LR-) indicates how much less likely a negative result is in a woman who will deliver preterm.
For the qualitative fFN test in symptomatic women, the pooled positive likelihood ratio for predicting spontaneous preterm birth is approximately 5.4 for delivery before 37 weeks and 5.0 for delivery within 7 to 10 days of testing. The negative likelihood ratio is approximately 0.25 for delivery before 37 weeks and 0.02 to 0.19 for delivery within 7 days. These values indicate that a positive test moderately increases the probability of preterm birth, while a negative test substantially decreases it.
Post-Test Odds = Pre-Test Odds x Likelihood Ratio
Post-Test Probability = Post-Test Odds / (1 + Post-Test Odds)
Pre-Test Probability Estimation: Clinical Risk Factors
The clinical utility of fFN testing depends heavily on the pre-test probability of preterm delivery, which is influenced by multiple factors. Women presenting to labor and delivery triage with symptoms suggestive of preterm labor, such as regular uterine contractions, pelvic pressure, or menstrual-like cramping between 24 and 34 weeks of gestation, have a baseline preterm delivery risk that varies considerably based on their clinical profile.
The most important determinants of pre-test probability include prior history of spontaneous preterm birth (the single strongest predictor, with a recurrence risk of approximately 30 to 50 percent), current gestational age, cervical length on transvaginal ultrasound, frequency and regularity of contractions, the presence of cervical change, the number of fetuses (singleton versus multiple gestation), and the presence of known risk factors such as uterine anomalies, cervical insufficiency, or genitourinary infection.
For the general symptomatic population presenting with contractions, the overall risk of delivery within 7 days without any testing is approximately 5 to 10 percent. Among women with additional risk factors such as a short cervix (below 25 millimeters) or a history of preterm birth, this baseline risk may be substantially higher, reaching 15 to 30 percent. Conversely, women presenting with mild, irregular contractions, a long cervix, and no prior preterm birth history have a very low baseline risk, often below 3 percent.
Combining fFN testing with transvaginal ultrasound cervical length measurement improves predictive accuracy by up to 50 percent compared with either test alone. When both the fFN test is negative and the cervical length exceeds 30 millimeters, the risk of delivery within 7 days approaches zero. Conversely, a positive fFN test combined with a short cervix (below 15 millimeters) identifies the highest-risk group requiring immediate intervention.
Clinical Interpretation by Gestational Age
The clinical significance and management implications of fFN test results vary with gestational age at the time of testing. At earlier gestational ages (24 to 28 weeks), the potential consequences of preterm delivery are more severe, making the information from fFN testing particularly valuable for guiding decisions about antenatal corticosteroid administration, magnesium sulfate for neuroprotection, and maternal transfer to facilities with appropriate neonatal intensive care capabilities.
Between 24 and 28 weeks, a positive fFN test warrants serious clinical attention. The risk of respiratory distress syndrome, intraventricular hemorrhage, necrotizing enterocolitis, and other complications of extreme prematurity is high, and the window for beneficial interventions is narrow. Administration of antenatal corticosteroids (betamethasone or dexamethasone) requires at least 24 to 48 hours to achieve maximal benefit for fetal lung maturation, making early identification of at-risk pregnancies critical.
Between 28 and 32 weeks, while the risks of prematurity remain substantial, survival rates improve significantly with each additional week of gestation. The fFN test result remains highly relevant for guiding steroid administration and decisions about tocolytic therapy. Between 32 and 34 weeks, the benefits of prolonging pregnancy are incremental but still meaningful, and fFN testing helps avoid unnecessary hospitalization and treatment in women at low risk of imminent delivery.
Impact of fFN Results on Clinical Management
The primary clinical utility of fFN testing lies in its ability to guide management decisions and potentially reduce unnecessary interventions. A negative fFN test result in a symptomatic woman can help clinicians avoid unnecessary hospitalization, tocolytic therapy, antenatal corticosteroid administration, and maternal transfer, all of which carry costs, potential side effects, and resource implications. Studies have shown that adding fFN testing to preterm labor triage protocols may reduce hospital admission rates and length of stay for symptomatic women.
When the fFN result is positive, clinicians may consider several interventions depending on the gestational age and clinical context. These include administration of antenatal corticosteroids if not previously given and the gestational age is between 24 and 34 weeks, magnesium sulfate for fetal neuroprotection if delivery appears likely before 32 weeks, tocolytic therapy to delay delivery long enough for corticosteroids to take effect, and transfer to a facility with appropriate neonatal intensive care capabilities if the current facility lacks these resources.
While fFN testing has excellent predictive accuracy, randomized controlled trial evidence for its impact on clinical outcomes is limited. A Cochrane review of six trials involving 546 women found that management guided by fFN knowledge may reduce preterm birth before 37 weeks, but the evidence quality was low. The American College of Obstetricians and Gynecologists notes that although fFN and cervical length may predict preterm birth, there is not yet strong evidence that using these measures to guide management improves outcomes.
Factors Affecting Test Accuracy
Several clinical and procedural factors can affect the accuracy of fFN testing and must be considered when interpreting results. False-positive results may occur due to contamination of the specimen with maternal blood, amniotic fluid, semen (if intercourse occurred within 24 hours), lubricant from vaginal examination, or cervical mucus dislodged during a digital examination. Pre-eclampsia has also been associated with elevated fFN levels independent of preterm labor risk.
The timing of specimen collection relative to digital cervical examination is critical. A digital exam performed before specimen collection can mechanically disrupt the choriodecidual junction and release fFN into the cervicovaginal secretions, producing a false-positive result. For this reason, the fFN specimen must always be collected before any digital cervical examination. Vaginal bleeding at the time of testing should prompt consideration that the result may be unreliable, although some studies suggest that the presence of blood on the specimen does not significantly affect the negative predictive value.
Additionally, the test is not validated for use after cervical cerclage placement, in women with ruptured membranes, in those with cervical dilation of 3 centimeters or greater, or at gestational ages outside the 22 to 34 week window. Testing after 34 weeks is generally not clinically useful because the management implications of threatened preterm labor change significantly as the pregnancy approaches term.
Fetal Fibronectin in Asymptomatic High-Risk Women
While fFN testing is most commonly used in symptomatic women presenting with signs of preterm labor, it has also been studied as a screening tool in asymptomatic women at high risk of preterm delivery. These include women with a history of prior spontaneous preterm birth, those with a short cervix detected on routine ultrasound screening, and women with multiple gestations.
In asymptomatic high-risk women, the test performance characteristics differ from those in symptomatic populations. The sensitivity tends to be lower (approximately 43 to 92 percent), while the specificity remains high (52 to 93 percent). The positive predictive value ranges from 13 to 85 percent, and the negative predictive value ranges from 86 to 99 percent, depending on the population studied and the outcome definition used.
Quantitative fFN testing has shown particular promise in asymptomatic high-risk women. Research has demonstrated that using alternative risk thresholds (below 10 nanograms per milliliter for very low risk and above 200 nanograms per milliliter for very high risk) improves accuracy compared with the standard qualitative threshold of 50 nanograms per milliliter. Women with fFN concentrations below 10 nanograms per milliliter have a very low risk of spontaneous preterm birth before 34 weeks (approximately 2.7 percent), while those with concentrations above 200 nanograms per milliliter face a risk of 33.9 to 37.7 percent.
Fetal Fibronectin in Twin and Multiple Pregnancies
Multiple gestations carry a substantially higher baseline risk of preterm birth compared with singleton pregnancies, with preterm delivery rates reaching approximately 56 percent for twins and even higher for higher-order multiples. The risk of delivery before 37 weeks is approximately 41 percent for twins, while delivery before 34 weeks occurs in about 13 percent and before 32 weeks in approximately 7 percent of twin pregnancies.
The performance of fFN testing in twin pregnancies has been evaluated in several studies. In symptomatic twins, fFN testing demonstrates a sensitivity of approximately 66 to 71 percent, specificity of 64 to 97 percent, positive predictive value of 19 to 67 percent, and negative predictive value of 93 to 97 percent for predicting delivery within 7 to 14 days. The negative predictive value, while still clinically useful, is somewhat lower than in singleton pregnancies due to the higher baseline prevalence of preterm birth in this population.
For asymptomatic twin pregnancies, meta-analyses have found that fFN testing has limited accuracy for predicting preterm birth before 32, 34, and 37 weeks of gestation, with pooled sensitivities ranging from 33 to 39 percent and specificities from 80 to 94 percent. Despite these limitations, fFN testing retains clinical value in the triage of symptomatic twin pregnancies, particularly for its ability to identify women at very low risk of imminent delivery who may be safely managed as outpatients.
Validation Across Diverse Populations
The fetal fibronectin test was originally developed and validated primarily in North American and European populations. Subsequent research has evaluated its performance across diverse ethnic and geographic populations, with generally consistent findings regarding its high negative predictive value. Studies conducted in Asian, African, and Middle Eastern populations have confirmed the clinical utility of fFN testing, though some variation in positive predictive values has been observed due to differences in preterm birth prevalence and healthcare system practices across regions.
Population-level differences in preterm birth rates, which range from approximately 5 percent in some Northern European countries to 18 percent in parts of sub-Saharan Africa, influence the positive predictive value of the test. In populations with higher baseline preterm birth rates, the positive predictive value tends to be higher, while in low-prevalence populations, a positive test is more likely to be a false positive. This underscores the importance of considering local preterm birth epidemiology when interpreting fFN results.
Ethnic differences in preterm birth risk, which may be influenced by genetic factors, socioeconomic determinants, access to prenatal care, and environmental exposures, should also be considered. Some studies have suggested that fFN testing may have varying performance across different ethnic groups, though the overall pattern of high negative predictive value appears to be consistent. Healthcare providers globally should consider using population-specific data when available to refine their interpretation of fFN results.
Regional Variations and Alternative Predictive Tools
While fetal fibronectin testing is widely available in many healthcare systems, its utilization varies considerably across regions. In some settings, transvaginal ultrasound cervical length measurement is used as the primary screening tool for preterm birth risk, with fFN testing reserved for women with intermediate cervical lengths (15 to 30 millimeters) where the risk of preterm delivery is uncertain based on cervical assessment alone. This sequential testing approach can reduce the number of fFN tests performed while maintaining clinical accuracy.
Alternative biomarkers for preterm birth prediction have been studied, including phosphorylated insulin-like growth factor binding protein-1 (phIGFBP-1, marketed as Actim Partus), placental alpha macroglobulin-1 (PAMG-1, marketed as PartoSure), and interleukin-6. Each of these biomarkers has demonstrated some utility in predicting preterm delivery, though fFN remains the most extensively validated and widely used.
Clinical prediction models that combine multiple variables, including fFN level, cervical length, gestational age, obstetric history, and other clinical factors, offer the potential for more accurate risk stratification than any single test. The QUiPP (Quantitative Instrument for the Prediction of Preterm birth) app, developed at King’s College London, is one example of such a model that integrates quantitative fFN concentration with cervical length and other factors to generate individualized risk estimates for preterm delivery at various gestational age thresholds.
Limitations and Considerations
Despite its clinical utility, fFN testing has several important limitations that must be understood. The test does not identify the underlying cause of threatened preterm labor, and a positive result does not indicate whether preterm delivery is due to infection, cervical insufficiency, decidual hemorrhage, uterine overdistension, or other pathological processes. Different etiologies may require different management strategies regardless of the fFN result.
The moderate positive predictive value means that the majority of women with a positive fFN test will ultimately deliver at or near term. This can lead to unnecessary interventions, anxiety, and healthcare utilization if positive results are not interpreted within the broader clinical context. The “rule-out” capability of a negative test is substantially more reliable and clinically actionable than the “rule-in” capability of a positive test.
Cost-effectiveness analyses of fFN testing have produced mixed results. While the test may reduce unnecessary hospitalizations and treatments in women with negative results, the cost of the test itself, combined with the potential for overtreatment of women with positive results, means that the overall economic impact depends heavily on local healthcare costs, preterm birth prevalence, and clinical management protocols.
When to Seek Professional Medical Advice
Pregnant women experiencing any of the following symptoms between 22 and 37 weeks of gestation should contact their healthcare provider promptly: regular uterine contractions occurring every 10 minutes or more frequently, persistent low back pain or pelvic pressure, change in vaginal discharge (increase in amount, change in consistency, or presence of blood-tinged mucus), rupture of membranes (leaking or gushing of fluid from the vagina), or abdominal cramping with or without diarrhea.
It is important to emphasize that fFN testing is only one component of a comprehensive clinical assessment and should never be used in isolation to make management decisions. Clinical judgment, incorporating the full range of available information including patient history, physical examination findings, ultrasound assessment, and laboratory results, remains paramount. Patients should discuss the meaning of their fFN test results with their healthcare provider in the context of their individual clinical situation.
The fetal fibronectin test requires proper specimen collection by trained healthcare professionals and laboratory processing. It is not available as a home test. Women experiencing symptoms of preterm labor should seek evaluation at their healthcare facility rather than attempting self-assessment. The test result must be interpreted by a qualified clinician in the context of the complete clinical picture.
How This Calculator Works
The fetal fibronectin calculator uses published clinical data from multiple large-scale studies and meta-analyses to estimate the risk of preterm delivery based on user inputs. The calculator integrates the fFN test result (either qualitative or quantitative) with additional clinical parameters to generate risk estimates for delivery within 7 days, within 14 days, and before 34 and 37 weeks of gestation.
For quantitative fFN values, the calculator uses risk stratification data from prospective cohort studies and applies Bayesian probability calculations using published likelihood ratios to generate post-test probability estimates. The pre-test probability is estimated based on the user’s input regarding symptom status, gestational age, cervical length (if available), pregnancy type (singleton or twin), and history of prior preterm birth.
The risk estimates provided by this calculator are approximations based on population-level data and should be viewed as educational tools rather than definitive clinical guidance. Individual risk may differ based on clinical factors not captured by the calculator, and all management decisions should be made in consultation with qualified healthcare professionals who can evaluate the complete clinical picture.
Frequently Asked Questions
Conclusion
Fetal fibronectin testing has established itself as a valuable clinical tool in the assessment of preterm birth risk, particularly for its exceptional ability to rule out imminent preterm delivery in symptomatic women. With negative predictive values consistently exceeding 98 percent for delivery within 7 days, a negative fFN test provides substantial reassurance that can guide clinical decisions about interventions, hospitalization, and resource utilization. The evolution from qualitative to quantitative testing has enhanced the ability to stratify risk across the full spectrum of fFN concentrations, enabling more personalized risk assessment.
However, the fFN test is not without limitations. Its moderate positive predictive value means that many women with positive results will not deliver prematurely, and the test should always be interpreted within the broader clinical context, including gestational age, cervical length, symptom severity, obstetric history, and other relevant factors. Integrated approaches that combine fFN testing with cervical length measurement and clinical prediction models offer the most accurate risk stratification currently available.
As research continues to refine our understanding of preterm birth prediction and prevention, the fFN test will likely remain an important component of the clinical toolkit. Ongoing studies are exploring improved risk prediction algorithms, optimal management protocols guided by fFN results, and the cost-effectiveness of various testing strategies across different healthcare settings and populations worldwide.