to investigate the value of adding sildenafil citrate during endometrial preparation in patients undergoing frozen-thawed embryo transfer and its effect on endometrial thickness and pregnancy rates.

Patients who met these conditions entered the study and 110 patients will be divided into two groups 55 patients in each group based on randomized tables.

Group 1: will be given estradiol in form of Cycloprogynova white pills starting dose is one pill t.i.d may be increased to two pills t.i.d according to endometrial thickness in day 9 measured by vaginal ultrasound.

Group 2: will be given sildenafil citrate tablets (50 mg) daily in addition to the above treatment protocol from the first day of the cycle until the day of starting progesterone. It was discontinued 48- 72 hours prior to the embryo transfer.

On the ninth day of the menstrual cycle, the endometrial thickness will be estimated by transvaginal ultrasonography (arranged in a series every other day). The evaluations will be performed by a single investigator.

If the endometrial thickness was more than 8mm, 400 mg of progesterone vaginal suppository and Duphastone tabs twice daily will be administered.

If the endometrial thickness is less than 8 mm estrogen dosage will be increased to two pills t.i.d and patients would be arranged for follow up every other day for endometrial thickness estimation by transvaginal ultrasound till endometrial thickness is more than 8 mm , then 400 mg of progesterone vaginal suppository and Duphastone tabs twice daily will be administered.

Frozen thawed embryo transfer will be carried out after 5 days (blastocyst stage). Estrogen and progesterone will be continued until two weeks after the embryos are transferred. In cases whom BHCG would be positive, estrogen and progesterone will be continued until the 11th week of pregnancy.

Inclusion Criteria:

1. Age under 40 years
2. Subject has high quality frozen embryos

Exclusion Criteria:

1-A history of endocrine diseases. 2. A history of any previous surgery that could compromise the integrity of endometrium.

3. Cardiovascular, renal and liver diseases. 4. Hypotension (blood pressure less than 90/50). 5. A history of stroke or myocardial infarction. 6. A history of previous implantation failure.