Luteal Support in Frozen-Thawed Embryo Transfer Cycles

In natural pregnancy, successful implantation and pregnancy necessitates well-functioning corpus luteum. Luteal phase support (LPS) is critical in assisted conception cycles particularly for replacement frozen embryo transfer cycles owing to lack of corpus luteum. 3 different LPS options have been suggested: * Progesterone in oil (intramuscular injections) is rapidly absorbed after i.m. injection and high progesterone plasma concentrations are reached after approximately 2h. Despite satisfactory plasma levels, the possible side-effects of pain and swelling are limiting the use. * Vaginal application of progesterone is widely accepted with satisfactory endometrial levels and patient convenience. However, increased vaginal discharge and possible vaginal irritation are the most common side effects. * Oral administration of synthetic progesterone (dydrogesterone) would offer a convenient way of progesterone administration. Better bioavailability than natural progesterone and less cost makes dydrogesterone preferable. Also, good quality evidence revealed satisfactory outcomes in fresh embryo transfer cycles. However, there is lack of good quality evidence for dydrogesterone as a LPS in frozen-thawed cycles. The investigator's aim is to compare 3 widely used LPS in frozen-thawed embryo transfer cycles. Following allocation of the subjects, hormone replacement will be scheduled for the endometrial preparation. Briefly; On day 2 of menstrual bleeding, endometrial preparation with oral estradiol (E2) (2 mg twice daily) will be initiated. Approximately 10 days after initiation of E2 administration, the subject will undergo a transvaginal ultrasound examination to assess endometrial development and serum progesterone (P) level will be checked. Once the subject achieved a trilaminar endometrium with a thickness of \>7 mm with serum P levels \<1.5 ng/ml, she will start the progesterone treatment to which she will be randomized to one of the 3 arms. After 3 days of P treatment along with E2 pills, cleavage embryo transfer will be performed. If the subject will receive blastocyst, 5 days of P treatment will be scheduled. If these criteria were not met at the first evaluation, she will allowed up to 10 additional days (20 days total) of estrogen stimulation.