This is a protocol for a Cochrane Review (Intervention). The objectives are as follows:

Primary
1. To assess the effects on maternal, fetal and neonatal outcomes of antenatal progesterone administered to women with a singleton pregnancy and an increased risk of spontaneous preterm birth when compared with either placebo or no intervention and also comparing different modes of administration.

2. To assess the effects on maternal, fetal and neonatal outcomes of antenatal progesterone administered to women with a multiple pregnancy and an increased risk of spontaneous preterm birth when compared with either placebo or no intervention and also comparing different modes of administration.

Secondary
A secondary objective of the review is to determine whether the effects are influenced by the following:
(1) treatment commencing before 24 weeks' gestation;
(2) factors responsible for increased risk of preterm labour:
(a) previous preterm birth;
(b) cervical incompetence.