Sasan 2017.

Methods	Randomised controlled clinical trial.
Participants	142 women who were referred to the obstetrical clinic in Besat Hospital of Sanandaj City, Kurdistan Province, Iran, who were receiving prenatal care and had a history of pre‐eclampsia in previous pregnancies.
Interventions	The participants were randomly placed into 2 groups: Group 1 (n = 70) received 50,000 IU pearl vitamin D3 once every 2 weeks; and Group 2 (n = 72) received placebo. Vitamin D or placebo was given until the 36th week of pregnancy. Health worker cadre: not specified.
Outcomes	Maternal: level of vitamin D, pre‐eclampsia. Laboratory method used for assessment of vitamin D concentrations: level of vitamin D was determined through Liebermann–Burchard method.
Notes	By total dose of supplementary vitamin D during pregnancy: more than 56,000 and less than 20,0000 IU of vitamin D; by start of supplementation: 10 weeks of pregnancy, or more; by pre‐gestational BMI (kg/m2): mixed; by supplementation scheme/regimen: once every 2 weeks; by skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown; by latitude: north of the Tropic of Cancer; by season at the start of pregnancy: unknown. Source of funding: unknown/unreported. Dates of the study: not reported dates, Iran. Declarations of interest among primary researchers (or state where this information is not reported by the trial authors): the authors announce that there are no conflicts of interest between different individuals and organisations involved in the study.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	140 pockets of drug and placebo were randomly (by using table of random numbers).
Allocation concealment (selection bias)	Unclear risk	140 pockets of drug and placebo were randomly offered and both study staff and participants did not know about administration of the treatments.
Blinding of participants and personnel (performance bias) All outcomes	Low risk	Both study staff and participants did not know about administration of the treatments.
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Both study staff and participants did not know about administration of the treatments.
Incomplete outcome data (attrition bias) All outcomes	Low risk	Data reported for all participants.
Selective reporting (reporting bias)	Unclear risk	Although participant's level of 25‐hydroxy vitamin D was used to determine study eligibility, serum levels of vitamin D were not reported.
Other bias	Low risk	The study appears to be free of other sources of bias.