Table 1. Characteristics of the included studies
Study	Country	Period of enrollment	Eligibility criteria	Controlled ovarian stimulation	Intervention	Control	Study size*	Age*	BMI*
Chakravarty et al., 2005a	India	Jan-2002 to Jun-2003	Women aged 25–42 years undergoing IVF/ICSI treatment (fresh cycles) with normal endometrial thickness (7–2mm) on the day of embryo transfer and no demonstrable endometrial disease	Long GnRH agonist using rFSH 150-300 IU/Day, triggering with hCG 10,000 IU IM	Dydrogesterone 20mg/day	VP capsule 600mg/day	N=430 (79 vs. 351)	<35 years: 47% vs. 49%;35-40 years: 33% vs. 40%; >40 years: 20% vs. 11%	NR
Ganesh et al., 2011	India	NR	Normoprolactinemic euthyroid women aged 23-42 years with a history of tubal factor infertility, male factor infertility, idiopathic infertility, endometriosis-related infertility, and ovulatory disorders	Short GnRH agonist cycle, using rFSH 150-300 IU/Day, triggering with hCG 10,000 IU IM	Dydrogesterone 20mg/day	VP capsule 600mg/day	N=881 (422 vs. 459)	NR	NR
Patki & Pawar, 2007	India	Jan-2004 to Dec-2005	NR	Long GnRH agonist cycle, no other details	Dydrogesterone 30mg/day	VP capsule 600mg/day	N=675 (366 vs. 309)	NR	NR
Rashidi et al., 2016	Iran	Jan-2015 to May-2016	Patients undergoing FET with embryos left from past fresh or frozen cycles, canceled previous cycles (poor endometrium or ovarian hyper stimulation syndrome) or candidates for embryo donation.	NR	Dydrogesterone 40mg/day	VP capsule 800mg/day	N=120 (60 vs. 60)	31.7±6.5 vs. 33.3±5.7	65.3±7.0 64.8±9.7
Saharkhiz et al., 2016	Iran	Apr-2014 to Jan-2015	Women aged 20-40 years, BMI between 18-30 kg/m2, no visible endometrial pathology	Long GnRH agonist cycle or GnRH antagonist cycle, no other details	Dydrogesterone 40mg/day	VP capsule 800mg/day	N=210 (96 vs. 114)	30.6±5.4 vs. 31.0±5.2	26.0±3.5 vs. 26.2±3.8
Salehpour et al., 2013	Iran	May-2012 to Dec-2012	Euthyroid normoprolactinemic women <40 years with male factor infertility	Short GnRH agonist cycle, using rFSH 150-300 IU/Day, triggering with hCG 10,000 IU IM	Dydrogesterone 40mg/day	VP capsule 800mg/day	N=80 (40 vs. 40)	29.4±5.3 vs. 31.8±6.1	24.2±3.0 vs. 24.2±3.9
Tournaye et al., 2017	38 sites in Austria, Belgium, Germany, Finland, Israel, Russia and Spain	Aug-2013 to Mar-2016	Euthyroid normoprolactinemic women aged 18-42 years, BMI ≥18 to ≤30 kg/m2undergoing IVF (fresh cycles), FSH ≤15 IU/L and estradiol <80 pg/mL in early follicular phase (Day 2–4), normal transvaginal ultrasound, with <3 unsuccessful IVF attempts or history of ≤3miscarriages	NR	Dydrogesterone 30mg/day	VP capsules 600mg/day	N=1031 (520 vs. 511)	32.5±4.5 vs. 32.5±4.4	23.3±3.1 vs. 23.2±3.2
Zarei et al., 2016	Iran	Dec-2014 to Mar-2015	Patients aged 20-40 years undergoing FET, with unexplained infertility, tubal factor infertility, mild male factor infertility, premature ovarian failure, polycystic ovarian syndrome (PCOS), endometriosis stage I or II, and normal uterine cavity	Oral estradiol 6mg/day, until endometrial thickness reached 8-14mm; after that, IM progesterone injections 100 mg/day for 3 days.	Dydrogesterone 20mg/day	VP capsules 800mg/day	N=222 (110 vs. 112)	32.9±5.1 vs. 33.5±5.2	NR
Zargar et al., 2016	Iran	Apr-2014 to Mar-2015	Women aged <40 years, with infertility lasting for <5 years, with regular menstrual cycles, normal hormone levels, and normal transvaginal ultrasound	Triggering with hCG 10,000 IU IM; no more details	Dydrogesterone 30mg/day	VP capsules 800mg/day	N=412 (212 vs. 200)	30.0±5.0 vs. 31.9±4.8	NR
NR = not reported; ART = assisted reproductive technology; FET = frozen embryo transfer; VP = vaginal progesterone; * = study vs. control NOTES: One study reported a conflict of interest (Tournaye et al., 2017); Two studies reported funding sources (Tournaye et al., 2017; Zarei et al., 2017); All studies obtained approval from ethics committees, and one study did not provide informed consent (Chakravarty et al., 2005a); All were parallel studies; Two studies reported an mean of three embryos transferred (Ganesh et al., 2011; Salehpour et al., 2013), and one study reported an mean of two embryos transferred (Rashidi et al., 2016).