The review found no eligible studies for this question.
Vitamin C and vitamin E showed no clear difference in total fetal loss compared with placebo (high certainty).
Vitamin C and vitamin E showed no clear difference in early or late miscarriage compared with placebo (moderate certainty).
Vitamin C and vitamin E showed no clear difference in stillbirth compared with placebo (moderate certainty).
Vitamin D supplementation probably leads to lower pre‐eclampsia compared with placebo or no intervention (moderate certainty).
Vitamin D supplementation probably leads to lower gestational diabetes compared with placebo or no intervention (moderate certainty).
Vitamin D supplementation may leads to lower maternal adverse events: severe postpartum haemorrhage compared with placebo or no intervention (low certainty).
It is very uncertain whether vitamin D supplementation changes maternal adverse event: nephritic syndrome compared with placebo or no intervention (very low certainty).
Vitamin D supplementation showed no clear difference in preterm birth (less than 37 weeks' gestation) compared with placebo or no intervention (low certainty).
Vitamin D supplementation probably leads to lower low birthweight (less than 2500 g) compared with placebo or no intervention (moderate certainty).
Vitamin C supplementation showed no clear difference in major cardiovascular event Physicians compared with placebo (low certainty).
It is very uncertain whether vitamin C supplementation changes cardiovascular mortality Physicians compared with placebo (very low certainty).
It is very uncertain whether vitamin C supplementation changes all‐cause mortality Physicians compared with placebo (very low certainty).
Vitamin C supplementation showed no clear difference in total myocardial infarction (fatal and non‐fatal) Physicians compared with placebo (low certainty).
Vitamin C supplementation showed no clear difference in total stroke (fatal and non‐fatal) Physicians compared with placebo (low certainty).
Vitamin C supplementation showed no clear difference in self‐reported CABG/PTCA Participant self‐reports compared with placebo (low certainty).
Vitamin C supplementation showed no clear difference in self‐reported angina Participant self‐reports compared with placebo (low certainty).