Chan 2020.

Study characteristics
Methods	2‐arm placebo‐controlled randomised trial
Participants	406 women were randomised in a hospital setting in Hong Kong from March 2016 to May 2018. The population comprised women aged 18 to 40 years of age, presence of intrauterine gestational sac(s) only if a urine pregnancy test was first positive within the past 2 weeks or presence of intrauterine fetus(es) with positive fetal heart pulsations or presence of intrauterine fetus(es) with crown‐rump length of < 7 mm and no fetal pulsation on pelvic scanning; and absence of fever (temperature 38.5o C). Exclusion criteria comprised women that history of recurrent miscarriage defined as 3 or more consecutive spontaneous miscarriages; history of known parental chromosomal abnormalities; heavy vaginal bleeding or severe abdominal pain requiring surgical intervention; absence of cardiac pulsation in a fetal pole with crown‐rump length of 7 mm on transvaginal scanning; use of hCG or progestogen for threatened miscarriage prior to recruitment; or women with current or suspected breast or genital cancers, hepatic disease or tumours.
Interventions	40 mg dydrogesterone orally, followed by 30 mg dydrogesterone 3 times a day versus placebo.
Outcomes	The study recorded the following outcomes relevant for this review: live birth, miscarriage (< 20 weeks)*, preterm birth, stillbirth, congenital abnormalities and 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. This study was funded by the Health and Medical Research Fund, HKSAR (reference number 12132341). The authors declare that they have no competing interests.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Computer‐generated in 1:1 ratio in blocks of 10.
Allocation concealment (selection bias)	Low risk	Used sequentially numbered, sealed, opaque envelopes.
Blinding of participants and personnel (performance bias) All outcomes	Low risk	Study participants and caregivers were blinded to treatment allocations.
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Assessors were blinded to treatment allocations.
Incomplete outcome data (attrition bias) All outcomes	Low risk	Attrition bias was < 10%.
Selective reporting (reporting bias)	High risk	The study report matches the study protocol that was registered prospectively (NCT02128685). However, results for 1 of the prespecified outcomes, i.e. proportion of heavy vaginal bleeding or severe abdominal pain requiring surgical intervention (< 20 weeks), were not reported.
Other bias	Low risk	This study was funded by the Health and Medical Research Fund, HKSAR (reference number 12132341).