Table 2.

Characteristics of studies included in the systematic review

Study	Country	Primary target population	Inclusion and exclusion criteria	Women with CL ≤ 25 mm (n)/fetuses or infants (n)	Intervention	Primary outcome measure
Fonseca (2007)64	UK, Chile, Brazil, Greece	Women with short cervix	Inclusion: women with singleton or twin gestation and transvaginal sonographic CL ≤ 15 mm Exclusion: major fetal abnormality, painful regular uterine contractions, history of ruptured membranes or cervical cerclage	24/48	Vaginal progesterone capsule (200 mg/day) or placebo from 24 to 33 + 6 weeks	Spontaneous PTB < 34 weeks
Cetingoz (2011)65	Turkey	Women at high risk of PTB	Inclusion: women with at least one previous spontaneous PTB, uterine malformation or twin gestation Exclusion: in‐place or planned cervical cerclage or serious fetal anomaly	7/14	Vaginal progesterone suppository (100 mg/day) or placebo from 24 to 34 weeks	PTB < 37 weeks
Rode (2011)66	Denmark, Austria	Women with twin gestation	Inclusion: women with a diamniotic twin gestation and chorionicity assessed by ultrasound before 16 weeks Exclusion: higher order multiple pregnancies, age < 18 years, known allergy to progesterone or peanuts as active treatment contained peanut oil, history of hormone‐associated thromboembolic disorders, rupture of membranes, pregnancies treated for or with signs of TTTS, intentional fetal reduction, known major structural or chromosomal fetal abnormality, known or suspected malignancy in genitals or breasts or known liver disease	21/42	Vaginal progesterone pessary (200 mg/day) or placebo from 20 to 23 + 6 up to 33 + 6 weeks	PTB < 34 weeks
Serra (2013)67	Spain	Women with twin gestation	Inclusion: women with dichorionic diamniotic twin gestation Exclusion: monochorionic twin gestation, triplet or higher order multiple gestation, elective cervical cerclage prior to 14 weeks, history of hepatic problem or gestational cholestasis, abnormal liver enzymes, abnormal kidney function, local allergy to micronized natural progesterone or peanuts, recurrent vaginal bleeding, recurrent vaginal infection, fetal anomaly, alcohol or illicit drug consumption or smoking ≥ 10 cigarettes/day	6/12	Vaginal progesterone pessary (200 or 400 mg/day) or placebo from 20 to 34 weeks	PTB < 37 weeks
Brizot (2015)68	Brazil	Women with twin gestation	Inclusion: women with naturally conceived diamniotic twin gestation, no previous PTB and gestational age between 18 + 0 and 21 + 6 weeks Exclusion: major fetal abnormality, allergy to progesterone or peanuts, hepatic dysfunction, porphyria, otosclerosis, malignant disease, severe depressive state, current or previous thromboembolic disease, uterine malformation, prophylactic cerclage or ovular infection	21/42	Vaginal progesterone ovule (200 mg/day) or placebo from 18 to 21 + 6 up to 34 + 6 weeks	Mean gestational age at delivery
El‐Refaie (2016)69	Egypt	Women with twin gestation and short cervix	Inclusion: women with dichorionic twin gestation, gestational age between 20 and 24 weeks, transvaginal sonographic CL between 20 and 25 mm, and without signs or symptoms of preterm labor Exclusion: known allergy or contraindication to progesterone therapy, monochorionic twin gestation, known major fetal structural or chromosomal abnormality, single fetal demise, fetal reduction in current pregnancy, cervical cerclage in current pregnancy, medical conditions that may lead to preterm labor, rupture of membranes or vaginal bleeding	224/448	Vaginal progesterone suppository (400 mg/day) from 20 to 24 up to 37 weeks or no treatment	PTB < 34 weeks

Only the first author of each study is given.

CL, cervical length; PTB, preterm birth; TTTS, twin‐to‐twin transfusion syndrome.