Risk scoring

Accurate prospective identification of a subgroup of pregnancies at increased risk of preterm labour and delivery permits the rational use of interventions aimed at prolonging gestation. Several epidemiological and obstetric associations are well recognised (Chapter 1) including low socioeconomic status, young age and primiparity, ethnic group and previous preterm delivery or mid-trimester loss (Lumley 1993).

In an effort to provide a useful screening test for preterm delivery risk, several scoring systems have evolved that quantify a variety of epidemiological and pregnancy features, together with digital assessment of the cervix. By applying the scoring system described by Creasy et al. (1980) to a San Fransisco population, Holbrook et al. (1989) found that gestational age at delivery was inversely correlated with risk score. However, in this series in which 15.8% of women were classified as high risk, the positive predictive value (PPV) of a high score was only 22.8%. Other studies have also found risk scoring to perform poorly, particularly in primigravidae since there is no past obstetric performance upon which to assign risk. This is illustrated by the study of Mercer et al. (1996) who constructed a scoring system using over 100 clinical variables obtained at 23–24 weeks' gestation in nearly 3000 women. Women were considered to be at high risk if their risk for spontaneous preterm delivery was > 20%.