NCT02128685.

Trial name or title	A randomised double‐blind controlled trial of the use of dydrogesterone in women with threatened miscarriage in the first trimester: study protocol for a randomised controlled trial
Methods	Double‐blind, RCT
Participants	A total of 400 women presenting with first‐trimester threatened miscarriage will be enrolled
Interventions	Intervention group: Dydrogesterone 40 mg orally, followed by 10 mg orally 3 times/d Control group: placebo
Outcomes	Primary outcome: percentage of miscarriage before 20 weeks of gestation
Starting date	Registered on 29 April 2014
Contact information	Department of Obstetrics and Gynaecology, Queen Mary Hospital, 102 Pokfulam Road, Hong Kong Special Administrative Region, China. dcmanka@gmail.com
Notes