Background The prophylactic administration of antenatal progesterone has been demonstrated to reduce the rate of preterm birth in women with high-risk singleton, but not twin, pregnancies (Combs, 2011; Fonseca et al., 2007; Lim, 2011; Rouse et al., 2007; Rode, 2011).

Objectives To determine the effects of prophylactic in utero progesterone, given in twin pregnancy for the prevention of preterm birth, on child health and development at three to six years.

Methods We followed up babies of women in the STOPPIT study (Norman, 2009). Developmental outcomes were assessed using two validated parent-completed questionnaires, the Child Development Inventory (CDI) and the Health Utilities Index. Health outcome data were obtained from national records. The primary outcome was defined as the presence of at least one twin with developmental delay, in at least one domain on the CDI.

Results Parent-completed questionnaires were obtained for 42% of twin pairs (mean age 4.58 years). The proportion of twin pairs with developmental delay was similar in both groups.

Congenital anomalies were noted in 16 twin pairs exposed to progesterone in utero vs 14 exposed to placebo (p=0.78). No significant differences were noted in other reported child health measures.

Conclusion Exposure to in utero progesterone, given in twin pregnancy for the prevention of preterm birth, has no significant beneficial or adverse impact on child health and behaviour at three to six years. Long-term follow-up studies of singleton babies exposed to in utero progesterone (OPTIMUMM study) will determine if progesterone has benefits in alternative scenarios.