Siew 2018.

Study characteristics
Methods	2‐arm active‐controlled randomised trial
Participants	141 women were randomised in a hospital setting in Singapore between January 2014 and February 2017. The population comprised women aged 21 to 45 years with a single intrauterine pregnancy, gestational age of 6‐10 weeks, and presented with pregnancy‐related vaginal bleeding. Exclusion criteria comprised women that had previously used a progestogen in the current pregnancy, women with inevitable miscarriage, planning to terminate their pregnancy, women who conceived using assisted reproductive technology, women known to have recurrent miscarriages, and women who have pre‐existing luteal phase deficiency or other forms of diagnosed progesterone deficiency.
Interventions	200 mg oral micronized progesterone twice daily versus 10 mg oral dydrogesterone twice daily.
Outcomes	The study recorded the following outcomes relevant for this review: miscarriage (≤ 16 weeks)*, adverse drug events. *different time point to that specified in the protocol (miscarriage defined as < 24 weeks).
Notes	Contact with study authors for additional information: yes. Additional data from authors: no. Funded by the Khoo Student Research Award and Pitch for Grant Award. The authors declared no conflicts of interest.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Quote: ''Participants were randomised in blocks of size 4 using a pre‐determined randomisation schedule generated by a biostatistician.''
Allocation concealment (selection bias)	Unclear risk	Small block randomisation.
Blinding of participants and personnel (performance bias) All outcomes	High risk	Study participants and caregivers were not blinded to treatment allocations.
Blinding of outcome assessment (detection bias) All outcomes	Low risk	This was an open‐label study. It is highly unlikely that the outcome was influenced by the lack of blinding.
Incomplete outcome data (attrition bias) All outcomes	Low risk	Attrition bias was < 10% and balanced across study arms.
Selective reporting (reporting bias)	Unclear risk	This study report matches the study protocol (ChiCTR‐IOR‐17011593) that was registered retrospectively.
Other bias	Low risk	This study was funded by the Khoo Student Research Award and Pitch for Grant Award.

hCG: Human chorionic gonadotropin; IM: intramuscular.