Villar 1990.

Methods	Double‐blind, randomised trial
Participants	Johns Hopkins Hospital, Baltimore, 1985‐1988 Pregnant women 17 years or younger; no underlying medical disorders; most were nulliparous with known last menstrual period and singleton pregnancy
Interventions	2 g elemental calcium as 500 mg calcium carbonate tablets, vs placebo tablets. All women were prescribed prenatal vitamin tablets containing 200 mg calcium and 100 mg magnesium per day.
Outcomes	Preterm labour; preterm delivery < 37 weeks (calcium 7.4 vs placebo 21.1%); delivery 30‐37 weeks; idiopathic prematurity; spontaneous prematurity; low birthweight (< 2500 g) (calcium 9.6% vs placebo 21.1%); postdates > 42 weeks (calcium 7.4 vs placebo 5.3%); impaired fetal growth (3.2 vs 3.2%); premature rupture of membranes (2.1 vs 1.0%); Apgar score < 8 at 5 minutes (4.4 vs 10.5%)
Notes	Source of funding: grants from the National Dairy Board and the National Dairy Council. CoI: not stated.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Computer‐generated list of random numbers
Allocation concealment (selection bias)	Low risk	Opaque envelopes with bottle numbers; project co‐ordinator responsible for assigning treatment. Identical tablets and containers were prepared at The Johns Hopkins Hospital pharmacy.
Blinding of participants and personnel (performance bias) All outcomes	Low risk	Double‐blind
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Double‐blind
Incomplete outcome data (attrition bias) All outcomes	Low risk	< 10% attrition
Selective reporting (reporting bias)	Low risk	Expected outcomes reported
Other bias	Unclear risk	Baseline characteristics similar except for maternal weight (higher in placebo group, P < 0.01)