Iran 2010.

Methods	A double‐blind, randomised, placebo‐controlled trial.
Participants	110 healthy pregnant women with a previous preterm delivery who were receiving prenatal care from obstetrics and gynaecology outpatient clinics of Isfahan University of Medical Sciences were recruited for the trial. The healthy pregnant women were 18–35 years, at 12–16 weeks' gestational age at delivery, height > 150 cm, weight > 45 kg, non‐smoker, no complicated pregnancy, but with history of preterm delivery, carrying a singleton fetus, living in Isfahan and willingness to continue current medications for the duration of the study.
Interventions	The treatment group received (50 mg/day Zn as Zn sulphate) produced by a local pharmaceutical company, Alhavi Pharmaceutical Laboratory, Tehran, Iran, from the day of reporting (12–16th weeks of gestation) until delivery, and the control group received placebo. Both groups administered capsules orally before meals once a day. The capsules were distributed monthly during prenatal visit. Compliance with study treatment was established by asking the women about missed doses and by counting unused sachets. The doses used are safe during pregnancy. Intervention: 50 mg/day Zn as Zn sulphate (n = 42). Placebo: (n = 42).
Outcomes	Maternal outcomes Caesarean section; pre‐eclampsia; intrauterine growth restriction. Neonatal outcomes Small‐for‐gestational age; gestational age at birth; preterm birth; low birthweight.
Notes
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Women were randomised according to a pre‐existing list produced by a computer program.
Allocation concealment (selection bias)	Low risk	Both woman and physician who assessed the outcome were not aware of treatment type that the woman was receiving. The masking of the active and placebo treatments was preserved by creating treatments that looked identical. The hospital pharmacist was informed of all randomisation assignments and was responsible for labelling the study drug and maintaining a master list linking the women and their treatment assignments.
Blinding of participants and personnel (performance bias) All outcomes	Low risk	As above.
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	No description.
Incomplete outcome data (attrition bias) All outcomes	High risk	Only 42 out of allocated 55 women in the intervention group and 42 out of 55 women in the control group were analysed (26% lost to follow‐up in each group).
Selective reporting (reporting bias)	Unclear risk	Not enough information to make this judgement. No information on if the protocol had been published prior to the trial.
Other bias	Unclear risk	No significant baseline differences except for higher haemoglobin concentrations in the zinc group (MD 0.5 g/dL).