The aim of this study is determining the affects of serum vitamin D levels to implantation and clinical pregnancy rates in women undergoing elective vitrified/thawed embryo transfer. Two groups will be identified according to the serum vitamin D levels on eFET transfer day and later on implantation, clinical pregnancy and ongoing pregnancy rates will be compared between those groups

In the last decade there have been growing evidence about Vitamin D receptors and their role in female genital tract. But these studies are yet not well designed and the results should be interpreted cautiously. Although some of these studies advocating the Vitamin D deficiency as an negative circumstance on pregnancy rates some of the others don't agree. According to contradictory results understanding the actual role of those receptors needs more observation.

Latest retrospective analyse postulated that, vitamin D deficiency in fresh single embryo transfer cycles may reduce pregnancy rates mediated by endometrium or ovarian reserve and stimulation characteristics.

For this reason the investigators aimed to explore the effect of serum Vitamin D levels on pregnancy rates in infertile population undergoing frozen embryo transfer.

Inclusion Criteria:

• 18 years to 35 years age, a frozen ET with 1 Day 5 (blastocyst stage) embryo
• Infertile women undergoing ICSI (intracytoplasmic sperm injection) cycle
• Factors affecting fertility are Tubal
• Unexplained or ovulatory disturbances,

Exclusion Criteria:

• Need to do Preimplantation Genetic Diagnosis
• Presence of endometriosis and adenomyosis
• Severe male factor infertility

Infertile women undergoing a frozen ET with 1 Day 5 (blastocyst stage) embryo