Risk screening for spontaneous preterm labour.

Prevention of viable spontaneous preterm birth before 34 weeks' gestation through screening is one of the key aims of antenatal care as birth below this threshold of prematurity has implications for child, mother and society. If women can be identified to be at high risk of spontaneous preterm birth in early pregnancy, they can be targeted for more intensive antenatal surveillance and prophylactic interventions (primary prevention). However, the disease mechanisms behind preterm birth are not well understood. Consequently, tests for its prediction and treatments for its prevention are not well developed. Additionally, no randomised controlled trial focusing on prevention of spontaneous preterm birth related perinatal morbidity and mortality through a screening programme exists. This chapter describes a generic framework for combining screening information with therapeutic effect to delineate its role in a screening programme. We use test-treatment combination of previous history of preterm birth and progestational agents as an example. A decision-making framework is built using: (1) evidence for post-test probabilities; (2) evidence for therapeutic effectiveness; and (3) integration of the two evidences to estimate the effect of the test-treatment combination with numbers needed to treat (NNTs). The NNT to prevent one case of spontaneous preterm birth before 34 weeks' gestation with progesterone is seven in women with a previous history; NNT is 41 in women without a previous history; and it is 28 when previous history was not used to guide a decision about prevention. The proposed framework makes decisions about screening and prevention explicit.