primary outcome:
Vitamin supplementation for preventing miscarriage
Total fetal loss, defined as the combined numbers of early miscarriage (spontaneous pregnancy loss less than 12 weeks' gestation), late miscarriage (spontaneous pregnancy loss greater than or equal to 12 and less than 24 weeks), and stillbirth (pregnancy loss at greater than or equal to 24 weeks).Early or late miscarriage.vitamin C with vitamin E (risk ratio (RR) 1.14, 95% confidence interval (CI) 0.92 to 1.40, seven trials, 18,949 women; Analysis 1.1; high‐quality evidence);vitamin C alone (RR 1.28, 95% CI 0.58 to 2.83, two trials, 224 women; Analysis 2.1);vitamin C with multivitamins (RR 1.32, 95% CI 0.63 to 2.77, one trial, 406 women; Analysis 3.1);vitamin C with vitamin E (RR 0.97, 95% CI 0.70 to 1.34, five trials, 15,882 women; Analysis 1.2; moderate‐quality evidence);vitamin C alone (RR 1.17, 95% CI 0.52 to 2.65, two trials, 224 women; Analysis 2.2);vitamin C with multivitamins (RR 1.19, 95% CI 0.79 to 1.79, two trials, 790 women; Analysis 3.2).vitamin A plus iron and folate (RR 1.01, 95% CI 0.61 to 1.66, three trials, 1640 women; Analysis 4.1; low‐quality evidence);vitamin A alone (RR 1.05, 95% CI 0.90 to 1.23, three trials, 52,480 women; Tau² = 0.01, I² = 73% Analysis 5.1);beta‐carotene alone (RR 1.02, 95% CI 0.98 to 1.07, two trials, 51,163 women; Analysis 6.1);vitamin A or beta‐carotene (RR 1.05, 95% CI 0.91 to 1.21, one trial, 17,373 women; Analysis 7.1);vitamin A with or without multivitamin (RR 0.80, 95% CI 0.53 to 1.21, one trial, 1074 women; Analysis 8.1);vitamin A plus iron and folate (RR 0.86, 95% CI 0.46 to 1.62, two trials, 1397 women; Analysis 4.2; low‐quality evidence);vitamin A alone (RR 0.98, 95% CI 0.92 to 1.04, one trial, 39,668 women; Analysis 5.2);beta‐carotene alone (RR 1.00, 95% CI 0.94 to 1.06, one trial, 39,860 women; Analysis 6.2);vitamin A with or without multivitamin (RR 0.76, 95% CI 0.37 to 1.55, one trial, 1075 women; Analysis 8.2);multivitamin without folic acid (RR 0.49, 95% CI 0.34 to 0.70, one trial, 907 women; Analysis 10.1);multivitamin with/without vitamin A (RR 0.60, 95% CI 0.39 to 0.92, one trial, 1074 women; Analysis 15.1);multivitamin plus iron and folic acid compared iron and folate only groups (RR 0.96, 95% CI 0.93 to 1.00, 10 trials, 94,948 women; Analysis 9.1; high‐quality evidence);multivitamins alone or in combination with other vitamins or micronutrients compared with placebo or no multivitamins groups (Analysis 11.1; Analysis 12.1; Analysis 13.1; Analysis 14.1; Analysis 16.1; Analysis 17.1; Analysis 18.1 (random effects; three trials)).multivitamin plus iron and folic acid compared iron and folate only groups (RR 0.98, 95% CI 0.94 to 1.03, 10 trials, 94,948 women; Analysis 9.2; moderate‐quality evidence);multivitamins alone or in combination with other vitamins or micronutrients compared with placebo or no multivitamins groups (Analysis 10.2; Analysis 11.2; Analysis 12.2; Analysis 13.2; Analysis 14.2; Analysis 17.2; Analysis 18.2 (random effects; three trials)).total fetal loss (Analysis 19.1 (random effects; three trials); Analysis 20.1; Analysis 21.1; Analysis 22.1; Analysis 23.1; Analysis 24.1; Analysis 25.1; Analysis 26.1); orearly or late miscarriage (Analysis 19.2 (random effects; three trials); Analysis 20.2; Analysis 21.2; Analysis 22.2; Analysis 23.2; Analysis 24.2; Analysis 25.2; Analysis 26.2);