Endometrial Effects of Sildenafil in Frozen-Thawed Cycles in Women With Thin Endometrium

This is a Randomised-controlled trial .The investigators recruited 100 women who are known to have thin endometrium in previous IVF cycles (failed/cancelled cycles). They are randomised into group A (50 women) : preparation of the endometrium with Estradiol valerate 2mg/day (every 8 hours)(white tablets of cycloprogenova,Payer,Germany) from the first day of the cycle till 12th day and The investigators add placebo from the first day of the cycle till the day of starting progesterone (It will be stopped 3 days before embryo transfer).Group B (50 women): The investigators add Sildenafil citrate (Viagra,pfizer,United states) to the estradiol valerate. Sildenafil 50 mg will be used daily from the first day of the cycle till the day of starting progesterone (It will be stopped 3 days before embryo transfer). The embryos will be transferred depending on their age on day 3 or day 5. At the transfer date ,the investigators will assess the endometrial thickness, endometrial volume, uterine artery doppler indices (RI,PI) as well as 3D endometrial vascular indices (VI,FI,VFI) .