Obstetric-care providers have been far more skeptical about fFN testing, and its introduction into clinical practice has been more protracted. An elevated level of fFN (≥50 ng/mL) in cervicovaginal secretions, which probably reflects separation of the fetal membranes from the maternal decidua,33 is associated with premature delivery. However, in a low-risk population, the positive predictive value of a positive fFN test at 22 to 24 weeks for spontaneous PTD prior to 28 weeks and 37 weeks is only 13% and 36%, respectively.34 As such, the value of this test lies primarily in its negative predictive value; 99% of patients with a negative fFN test will not deliver within 7 days.35 ACOG currently recommends the use of this test only in a very specific subgroup of women (particularly, in symptomatic women with intact membranes, cervical dilatation <3 cm, and a gestational age of 24-0/7 to 34-6/7 weeks).2,6
Cervicovaginal swabs for fFN measurement should be taken prior to bimanual examination. For this reason, the clinician should consider collecting a specimen at the time of initial speculum examination in all women being evaluated for preterm labor, regardless of the initial index of suspicion. There is no charge for discarded specimens.
The ACOG bulletin also states that, in order for the fFN test to be “clinically useful, the results must be available from a laboratory within a time frame that allows for clinical decision-making (ideally within 24 hours).”2 The introduction of a rapid fFN test and its approval by the FDA in September 1998 have greatly improved the utility of determining fFN levels in cervicovaginal secretions. The test itself takes 26 minutes to complete, and most laboratories can get a result back to the clinician within 1 to 2 hours.
<huc>Q</huc> OBG Management: What findings on cervical ultrasonography are reassuring for you, and which are nonreassuring?
<huc>A</huc> NORWITZ: The most important measurement on cervical ultrasonography is residual cervical length. Both transvaginal and transperineal sonography are reliable and reproducible ways to assess the length of the cervix,2,36 although transvaginal sonography is considered by most practitioners to be the gold standard. Mean cervical length changes with gestational age,30-32 but a cervical length of 2.5 cm or less at 22 to 24 weeks in a pregnancy at high risk for PTD should be considered abnormal and requires further evaluation.
Funneling (or beaking) at the internal os also is concerning as it may indicate an intrinsically weak cervicoisthmic junction suggestive of cervical incompetence, but the data are less consistent. Some studies have found the presence of funneling to be an independent risk factor for preterm birth (independent of cervical length),37,38 whereas other studies have been unable to confirm this observation.30,39 It also has been suggested that a “cervical stress test” be performed by applying transfundal pressure and watching for funneling at the internal os, and several studies have shown that a positive test is predictive of PTD.40,41 Whether such testing should be performed in all women at risk of preterm birth remains unclear.
There are several factors to consider when assessing cervical length and dilatation. These include the orientation of the transducer, the potential distortion of the cervix by the transducer, and the fact that a full bladder may artificially lengthen the cervix and obscure dilatation of the internal os.42 Careful attention to maternal position also is essential.43
All cervical abnormalities should be reported to the patient. Deciding whether to repeat the cervical ultrasound in 1 to 2 weeks versus placing a cervical cerclage versus bed rest should be individualized, and will depend on such factors as gestational age, a priori risk of PTD, and patient preference. I typically review in detail the risks and potential benefits of each management option with the patient, and will recommend cervical cerclage if the pregnancy is at high risk of PTD, if the gestational age is less than 24 weeks, and if there is evidence of progressive cervical shortening with a residual length (with or without funneling) of less than 2 cm.
Whether cervical length and fFN are additive in their ability to predict preterm delivery in women at high risk remains controversial.
Although I have chosen 2 cm as a cutoff for recommending cervical cerclage, the optimal cutoff value remains controversial, ranging from 1.5 to 3 cm.38 Whether the cutoff value should differ for women with multiple gestations or women who have had prior cervical surgery is unclear.
<huc>Q</huc> OBG Management: When you use ultrasonography in combination with fFN screening, how do you make decisions based on the combination of results? How do you proceed in the face of discordant findings?