Table 3  Randomised, placebo-controlled studies: Progesterone for the prevention of preterm birth in asymptomatic women with singleton pregnancies and a short cervix.

Author	Year	Number of patients Screening	Number of patients (progesterone vs. control)	Inclusion criteria	Progesterone type	Dose and interval	Period of use (WG)	Primary outcome	Reduction in preterm birth
WG: weeks of gestation; PB: preterm birth, 17-OHPC: 17α-hydroxyprogesterone caproate a history of PB (55% in active treatment vs. 57% in placebo group), previous surgery on the cervix (4 vs. 8%), uterine anomalies (20 vs. 19%) or prenatal exposure to diethylstilbestrol (8 vs. 11%)
A) 17-OHPC
Winer et al. 68	2015	–	51 vs. 54	High PB risk a , cervical length < 25 mm at 20 – 31 WG	17α-OHPC i. m.	500 mg/week	20 – 31 to 36	Interval (days) until birth: 76 ± 5 days vs. 72 ± 5 days (p = 0.480)	No
B) Vaginal progesterone
Fonseca et al. 31	2007	24 620	125 vs. 125	Cervical length < 15 mm (n = 24 twin pregnancies) at 20 – 25 WG	Vaginal pessaries	200 mg/day	24 to 34	PB < 34 WG: 19.2 vs. 34.4% (p = 0.020)	Yes
Hassan et al. 34	2011	32 091	235 vs. 223	Cervical length 10 – 20 mm at 20 – 24 WG	Vaginal gel	90 mg/day	20 – 24 to 36	PB < 32 WG: 8.9 vs. 16.1% (p = 0.020)	Yes