The preventive strategy of screening and treating BV should not be confused with the management of women with symptomatic BV. These women should be treated with oral metronidazole after the first trimester, as vaginal metronidazole and clindamycin preparations appear to be less effective during pregnancy.25
<huc>Q</huc> OBG Management: The Practice Bulletin also stated that, “Screening for risk of preterm labor by means other than historic risk factors is not beneficial in the general obstetric population.” Please outline this set of historic risk factors.
<huc>A</huc> NORWITZ: Risk factors for preterm birth resulting from spontaneous preterm labor, which excludes indicated (iatrogenic) PTD for severe preeclampsia, a prior high vertical (“classical”) cesarean, or chorioamnionitis, are as follows:
- Demographic characteristics such as African-American race, poor socioeconomic status, low pre-pregnancy weight, extremes of maternal age, and absent or inadequate prenatal care.
- Behavioral factors such as cigarette smoking, substance abuse, and high personal stress or a strenuous work environment.
- Aspects of obstetric history such as prior PTD, multiple gestation, uterine anomalies, anemia, polyhydramnios, vaginal bleeding, cervical incompetence, and BV.
Several scoring systems have been developed to predict a woman’s likelihood of delivering preterm. However, reliance on risk factors alone will fail to identify more than 50% of pregnancies that deliver at less than 37 weeks.26,27
The most important risk factor is a history of one or more preterm deliveries. If the prior PTD was due to spontaneous preterm labor, a screening strategy comprised of serial cervical examinations and/or fFN testing should be initiated in the mid- to late second trimester.
Sonography has shown a strong correlation between cervical length and PTD.
If the prior PTD is suggestive of cervical incompetence, it may be appropriate to discuss other management options. These include prophylactic cervical cerclage or serial measurements of cervical length using transvaginal sonography and placement of an emergent cerclage, if indicated.28 (The generally accepted definition of cervical incompetence is the inability to support a pregnancy to term due to a structural or functional defect of the cervix. It is characterized by acute, painless dilatation of the cervix, usually in the middle trimester, culminating in prolapse and/or rupture of the membranes, which leads to preterm—and often pre-viable—delivery.) A history of in utero diethylstilbestrol (DES) exposure or multiple gestation in the absence of a history of cervical incompetence is not generally accepted as a sufficient indication for elective cerclage.28
<huc>Q</huc> OBG Management: What about fFN testing and the use of ultrasound to determine cervical length? The ACOG Practice Bulletin recommended that either modality or “a combination of both may be useful in determining high risk for preterm labor,” adding that the clinical utility of both modalities may rest primarily with their negative predictive value. How do you use cervical ultrasound and fFN screening in your practice?
<huc>A</huc>NORWITZ: In women at risk for preterm birth, serial digital evaluation of the cervix starting in the mid- to late second trimester is useful if the examination remains normal. However, an abnormal cervical finding (shortening, dilatation, or both) is associated with PTD in only 4% of low-risk women and in just 12% to 20% of high-risk women.29 Real-time sonographic evaluation of the cervix, on the other hand, has demonstrated a strong and reproducible inverse correlation between cervical length and PTD.30,31 If the cervical length is lower than the 10th percentile for gestational age, the pregnancy is at a 6-fold increased risk of delivery prior to 35 weeks.30 A cervical length of 15 mm or less at 23 weeks occurs in less than 2% of low-risk women, but is predictive of delivery prior to 28 weeks and 32 weeks in 60% and 90% of cases, respectively.31
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.
The latest Practice Bulletin concludes that: “Despite the usefulness of cervical length determination by ultrasonography as a predictor of preterm labor, routine use is not recommended because of the lack of proven treatments affecting outcome.”2,32 Therefore, perinatologists and sonographers should not include cervical-length measurements in routine prenatal ultrasounds. However, in carefully selected women at increased risk for PTD, serial measurements of cervical length may help modify the risk estimate for preterm birth. This is especially true in women with a history suggestive of cervical incompetence in whom cervical cerclage is being considered. That said, it remains unclear whether placement of a cervical cerclage in women with a shortened cervix can prevent preterm birth or improve perinatal outcome.28
Cervical-length measurement as a screening test for preterm birth was accepted relatively quickly in clinical practice. This is likely because of the ready availability of transvaginal ultrasound in most obstetric suites and labor and delivery (L&D) units, and because of a high level of comfort and expertise with its use. The possibility that a shortened cervix may represent cervical incompetence and that placement of a cervical cerclage may serve to avert PTD altogether or at least delay delivery to a more favorable gestational age, also may be a factor in that acceptance.