This multicenter double-blinded placebo-controlled randomised trial aims to evaluate the effectiveness of vitamin D supplementation prior to IVF on the live birth rate in women with PCOS. Women with PCOS scheduled for IVF will be enrolled. Eligible participants will be randomised 1:1 to receive oral capsules of 4000IU vitamin D per day or placebo for around 12 weeks until the day of triggering.

Polycystic ovary syndrome (PCOS) is one of the leading causes of female infertility, affecting around 5% of women with a child-bearing age in China. Vitamin D insufficiency is common in women with PCOS and is associated with lower live birth rates in these women. However, evidence regarding the effectiveness of vitamin D treatment in women with PCOS is inconclusive. This multicenter double-blinded placebo-controlled randomised trial aims to evaluate the effectiveness of vitamin D supplementation prior to IVF on the live birth rate in women with PCOS.

The investigators plan to enroll women with PCOS scheduled for IVF. Eligible participants will be randomised 1:1 to receive oral capsules of 4000IU vitamin D per day or placebo for around 12 weeks until the day of triggering. All IVF procedures will be carried out following a routine method in each center. Women will be followed up until six months after randomisation for those who fail to get pregnant after the completion of the first embryo transfer or delivery for those who get pregnant after the first embryo transfer. The primary outcome is live birth after the first embryo transfer. Primary analysis will be by intention-to-treat analysis. The investigators plan to recruit 860 women to demonstrate women treated with vitamin D have a higher live birth rate than those treated with placebo (48% versus 38%), accounting for 10% loss to follow-up with a significance level at 0.05 and a power of 80%.

Inclusion criteria

1. Women aged 20 to 42 years old;
2. Diagnosed with PCOS (Rotterdam Criteria);
3. Scheduled for IVF;
4. Written informed consent.

Exclusion criteria

1. Women who had three or more failed IVF cycles;
2. Women scheduled for preimplantation genetic testing;
3. Known Vitamin D allergy;
4. Women with a history of chronic absorption syndrome or bile dysplasia, or parathyroid dysfunction, or kidney stones, or blood Calcium ion concentration greater than 2.6mmol/L (normal value 2.25 - 2.75mmol/L (9 to 11mg/dl), or hyperphosphataemia (1.61mmol/L), or metabolic-related bone disease, or chronic diseases that may cause bone abnormalities (liver and kidney insufficiency);
5. Women receiving treatments for tuberculosis, convulsions, and epilepsy because medications treating these diseases may affect the metabolism of vitamin D.
6. Women undergoing an IVF treatment with donor oocytes.

All data relevant to the study will be generated in the article or uploaded as supplementary information. Deidentified participant data will be available from Dr. Dan Zhang (zhangdan@zju.edu.cn) on a reasonable request. Protocols and statistical analysis plans will be included as supplementary information.

Participants and most researchers will be unaware of the actual grouping. Only two researchers who are not involved in participant management or data analysis will be aware of the grouping.

Vitamin D and placebo pills with identical package, size, colour and appearance will be used in this trial. In general, there should be no need to unblind the allocation. If urgent unbinding to participants is necessary, the allocation will be disclosed to the treating physician.