Samimi 2017.

Methods	Double‐blind randomised and controlled clinical trial.
Participants	80 women aged 18 to 35 years were examined from November 2013 to March 2015 at the Shabihkhani Maternity Hospital in Kashan, Iran.
Interventions	Participants were randomised into 2 groups: Group 1 (n = 40) received a pill of vitamin D3 400 IU/day and Group 2 (n = 40) received a placebo tablet that was similar to vitamin D3, with no active ingredient as a placebo. Both groups received standard treatment with vaginal progesterone (Behvarzan, Iran) at a dose of 400 mg per day. The serum levels of vitamin D3 were evaluated in the tenth and twentieth weeks to prevent any possible poisoning. If so, the patient was excluded from the study. Health worker cadre: all participants in the study received antenatal care and were given folic acid and ferrous sulphate at least 1 month prior to pregnancy, under the supervision of a gynaecologist. They were checked by monitoring serum β‐hCG level levels and abdominal ultrasound until the confirmation of pregnancy, after which the mothers were divided into 2 groups of intervention and control using permuted block randomisation with twenty blocks of size 4. Only the person responsible for the distribution of drugs knew how the women were allocated to the treatment groups.
Outcomes	Maternal: the serum level of vitamin D3, serum level of IL‐23, serum levels of vitamin D3 and IL‐23, spontaneous abortions. Laboratory method used for assessment of vitamin D concentrations: not specified.
Notes	By total dose of supplementary vitamin D during pregnancy: more than 200,000 IU of vitamin D; by start of supplementation: less than 20 weeks of pregnancy; by pre‐gestational BMI (kg/m2): unknown; by supplementation scheme/regimen: daily; 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: this study was funded by a research grant of the Kashan University of Medical Sciences. Dates of the study and location: November 2013 to March 2015, Kashan, Iran. Declarations of interest among primary researchers (or state where this information is not reported by the trial authors): the authors declare that there is no conflict of interests regarding the publication of this paper.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Participants were divided into 2 groups of intervention and control using permuted block randomisation with 20 blocks of size 4.
Allocation concealment (selection bias)	Low risk	Only the person responsible for the distribution of drugs knew how the women were allocated to the treatment groups.
Blinding of participants and personnel (performance bias) All outcomes	Low risk	Only the person responsible for the distribution of drugs knew how the women were allocated to the treatment groups.
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	The trial reported that it was double blinded but they did not specify if those performing the assessments were blinded.
Incomplete outcome data (attrition bias) All outcomes	Low risk	Data were reported for all participants.
Selective reporting (reporting bias)	Unclear risk	There is insufficient information to permit judgement.
Other bias	Low risk	The study appears to be free of other sources of bias.