<huc>A</huc> NORWITZ: In my practice, it is unusual for women to be screened with both cervical ultrasound and fFN testing. I use cervical ultrasound more often in the mid- to late second trimester in asymptomatic women with a history of preterm birth suggestive of cervical incompetence, and fFN more often in symptomatic women presenting to the outpatient clinic or L&D unit remote from term. However, there are a few exceptions. These include women with higher-order multiple gestations (triplets and up), in whom the risk of preterm birth remote from term is extremely high, and symptomatic women at 22 to 28 weeks, when the bimanual examination suggests cause for concern and ultrasound confirms substantial cervical shortening.
Whether cervical length and fFN are additive in their ability to predict PTD in women at high risk, or whether they are simply 2 separate methods of assessing the same pathophysiologic process, remains controversial. Recent data suggest that these tests are indeed additive. High-risk women at 22 to 24 weeks with a residual cervical length of less than 2.5 cm and a positive fFN screening test have a 65% risk of delivering at less than 35 weeks, even if they are asymptomatic at presentation.2,44
A negative fFN test excludes imminent delivery, with less than 1% of such women delivering within 14 days of presentation.
Which of the 2 tests is more reliable in any given patient also is not clear. This becomes important when the testing is discordant. In a woman with an abnormal cervical examination remote from term, a negative fFN test is reassuring because the data suggest that she is highly unlikely to deliver within the next 2 weeks.2 How to interpret a positive fFN test in an asymptomatic woman with normal cervical length, however, is not clear. As the bulletin states: “The clinical implications of a positive test have not been evaluated fully.”2
My approach to such patients is to increase antenatal surveillance, but not to modify their care in any other way on the basis of a single positive fFN, i.e., no corticosteroids, no tocolysis, no bed rest. I typically will not repeat the fFN test, although some practitioners would recommend repeating it in 1 to 2 weeks if the patient remains undelivered. Although a subsequent negative fFN test cannot “remove” the implications of the previous positive test, some evidence suggests that 2 negative test results following a positive test reduce the risk of spontaneous preterm birth back to baseline.45
<huc>Q</huc> OBG Management: The bulletin stated that fFN may be useful in “avoiding unnecessary intervention” in symptomatic women by virtue of its negative predictive value. What has been your experience in this regard?
<huc>A</huc> NORWITZ: The first question to ask is how best to define a “symptomatic” woman. ACOG says the following symptoms and signs suggestive of preterm labor deserve further evaluation:
- Uterine contractions (with or without pain)
- Intermittent lower abdominal pain, dull backache, pelvic pressure
- Vaginal bleeding during the second or third trimester
- Menstrual-like intestinal cramping (with or without diarrhea)
- Change in vaginal discharge (amount, color, consistency)
- Vague sense of discomfort characterized as “not feeling right”
This question pertains specifically to symptomatic women presenting to the out-patient clinic or to the L&D unit. Of all women at 24-0/7 to 34-6/7 weeks with symptoms or signs suggestive of preterm labor, about 80% will be fFN-negative, i.e., fFN <50 ng/mL in cervicovaginal secretions. A negative fFN test effectively excludes imminent delivery, with less than 1% (1 in 125) of such women delivering within 14 days of presentation.
A positive fFN test, on the other hand, will predict delivery within the next 14 days in only 16% (1 in 6) of symptomatic women. As such, the value of the fFN test lies primarily in its negative predictive value (124 of 125 women with a negative fFN test will not deliver within the next 14 days).33-35,45 Indeed, a negative fFN test in symptomatic women has been shown to reduce admissions for preterm labor, length of stay, and use of tocolytic agents,46 as well as to reduce unnecessary transfers to a tertiary care center.47 These benefits translate into substantial cost savings46,47 and likely minimize adverse events in pregnant women by avoiding unnecessary interventions.
<huc>Q</huc> OBG Management: What is your screening modality of choice for symptomatic women and why?
<huc>A</huc> NORWITZ: Two key elements should be considered when evaluating a woman who presents with 1 or more symptoms or signs suggestive of preterm labor: the gestational age and the best estimate of the patient’s a priori risk of PTD. The latter requires knowledge about the presence or absence of risk factors for preterm birth (especially a history of prior PTD), uterine contractility, cervical examination (including dilatation, effacement, and station), presence or absence of ruptured membranes, and fetal well-being.