The aim of the present study is to investigate serum estradiol and progesterone levels in women conceiving after natural, estradiol + progesterone or gonadotropin stimulated frozen embryo transfer (FET) cycles.

Enrolled women will be randomized based on their ovulation pattern. Ovulatory women will be randomized to either natural cycle or estradiol + progesterone substituted cycle treatment for FET. Anovulatory women will be randomized to either estradiol + progesterone substituted cycle or gonadotropin stimulated cycle treatment for FET.

Included women will undergo blood testing every two weeks for serum estradiol and progesterone levels until gestational age 9+6. Routine vaginal ultrasounds will be performed as well as additional pregnancy ultrasounds.

Secondary obstetric outcomes will be investigated using the womens medical journals.

All the treatments are considered standard treatments for FET.

Inclusion Criteria:

• Age > 18 years < 40 years
• BMI < 35 kg/m2
• Normal wet smear within the past three years
• Thawed blastocysts (day 5 or 6) after either IVF or ICSI treatment

Exclusion Criteria:

• Age < 18 years
• BMI > 35 kg/m2
• Oocyte donation
• HIV/ hepatitis
• Undiagnosed vaginal bleeding
• Uterine malformations
• Persisting ovarian cysts
• Tumors in Hypothalamus, pituitary, thyroid or adrenal
• Previous breast cancer
• BRCA1/2
• Unregulated thyroid disease
• Cardiovascular disease
• Breast feeding
• Present or previous chemotherapy/radiation therapy
• Present or previous malignant disease
• Smoking
• Alcohol/drug abuse