The study investigated whether vitamin D supplementation improves the chance of a live birth in women with polycystic ovary syndrome (PCOS) undergoing in vitro fertilization. A multicenter, double-blind, placebo-controlled trial enrolled 876 women from 24 fertility centers in China who were randomized to vitamin D or placebo for up to 90 days before conception. Vitamin D levels increased significantly in the vitamin D group (from 16.5 ng/ml to 32.3 ng/ml), while they remained low in the placebo group (from 16.1 ng/ml to 18.2 ng/ml). The primary outcome was a live birth after the first embryo transfer, which occurred in 52.0% of women in the vitamin D group and 50.2% of women in the placebo group. The difference between groups was not statistically significant (adjusted hazard ratio 1.03). Severe ovarian hyperstimulation syndrome occurred in three women in the vitamin D group and six in the placebo group. The study concluded that vitamin D supplementation before IVF in women with PCOS did not increase birth rates.