There is mounting evidence that cervical length is inversely related to risk of preterm birth.The real question, however, is: What should be done about transvaginal cervical length: Should we be screening, or not? As recently as 2009, a Cochrane Review did not advocate universal screening for cervical length as a predictor for preterm birth4—despite mounting evidence that cervical length is inversely related to risk of preterm birth, with progressively shorter length (starting at <25 mm) associated with significantly higher risk of preterm birth.8,9 Keeping in mind that the decision to screen depends on your ability to treat the condition for which you are screening, what was needed was proof that intervention works.
2011 brought two studies that recommend screening for cervical length based on a successful reduction in preterm birth with a specific intervention. A large, randomized trial of vaginal progesterone gel for the prevention of preterm birth used universal screening for shortened cervical length (10 to 20 mm) as the criterion for randomization to treatment or placebo. The investigators demonstrated a 45% reduction in preterm birth of less than 33 weeks in the treatment arm.10
An interesting aspect of this study: The reduction in preterm birth was not, in fact, seen in patients who had a history of preterm birth, suggesting that this may be a different patient population that benefits from vaginal progesterone.
On the other hand, a recent meta-analysis concluded that patients who meet the criteria of 1) cervical length less than 25 mm and 2) a history of prior spontaneous preterm birth experience a significant reduction in preterm birth and a reduction in perinatal morbidity and mortality if they have cervical cerclage placed.11
Although these publications lead us to hope that there may be some benefit from preventive intervention for preterm birth, the question of how to screen for, and prevent, spontaneous preterm birth remains somewhat nebulous: It hasn’t been determined which patient population will benefit from which combination of screening and intervention. Larger trials for specific populations are still needed.
WHAT THIS EVIDENCE MEANS FOR PRACTICE
This is what we know, for now:
- Women who have a history of spontaneous preterm birth should have a thorough evaluation of their OB history to determine possible modifiable risk factors (e.g., smoking, short inter-pregnancy interval) and to determine, as definitively as possible, the likely cause of that preterm birth
- Women who have a singleton pregnancy and a history of either spontaneous preterm labor or preterm rupture of membranes can be offered progesterone supplementation as intramuscular 17a-hydroxyprogesterone or a vaginal preparation to reduce their risk of preterm birth
- Women who have an asymptomatic shortening of the cervix, as measured on transvaginal US at 18 to 24 weeks’ gestation, can be offered vaginal progesterone to reduce their risk of preterm birth
- Women who have a history of preterm birth and cervical shortening may see a reduction in their risk of preterm birth from cerclage placement
- The use of screening for cervical length or progesterone supplementation, or both, in a multiple gestation pregnancy are not recommended because their benefit in this population has not been demonstrated.
Until we fully understand the various etiologic pathways of spontaneous preterm birth, we won’t have a one-size-fits-all solution to this major cause of perinatal morbidity and mortality.
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