To date, several studies have been done to explore the relationship between Vitamin D level and IVF outcome; however, the results are controversial. Sample sizes in these studies mentioned above were relatively small. In addition, there are no prospective cohort studies in this field. Thus, here the investigators carry this prospective observational cohort study to demonstrate the relationship between vitamin D level and IVF outcome.

Vitamin D level is measured before the commence of IVF cycle. According to Vitamin D level, patients are divided into 3 groups: A, Deficient, <20 ng/mL; B, Insufficient, 20-29.9 ng/mL; C, Replete, >30 ng/mL.

Ongoing pregnancy rate are compared among these three groups after controlling age, number of embryos transferred, ect.

In addition, Vitamin D level is also meassured before embryo transfer in FET cycles with PGT treatment. The effect of Vitamin D on IVF outcome is detected in patients transferred with Euploid embryos.

Inclusion Criteria:

1. age between 21-40 years old
2. basal FSH hormone level less than 10 mIU/mL
3. the first cycle of IVF treatment with standard long protocol
4. both ovarian present and normal uterus.

Exclusion Criteria:

1. Important data missing
2. Gametes donation cycles
3. Anyone that does not meet the inclusion criteria

Patients whose IVF cycles were cancelled due to ovarian hyperstimulation syndrome were also excluded);.