It has been reported that low-dose aspirin and Heparin would lead to an increased Pregnancy rate in patients undergoing IVF-ET. Low-dose aspirin may improve uterine and ovarian perfusion and that aspirin might enhance endometrial receptivity and ovarian responsiveness as well, which could result in better implantation and pregnancy rates after IVF or ICSI treatment. This study assesses potential effects of low-dose aspirin (100 mg daily) on pregnancy rate following frozen embryo transfer.

In the present randomized double-blind prospective study, the patients will be randomly assigned to the study and control groups. With the onset of endometrial preparation and estrogen treatment, the study group will receive 100 mg of oral aspirin and the control group will be given placebo. When endometrial thickness will be found between 8-14 mm, therapy with progesterone (100 mg IM daily) will be started and embryo transfer will be performed 48 to 72 hours later. Doppler ultrasonography also is done to calculate Resistive Index (RI) and Pulsatility Index (PI), the day after progesterone administration.

βHCG will be assessed 14 days after ET. If a pregnancy will be achieved, the patients in study and control groups will be instructed to continue the aspirin or placebo through 5 weeks and if the pregnancy test result will be negative, the treatment with aspirin or placebo will be stopped. The patients will be followed until 20 weeks of gestation and the results of treatment cycle will be compared in two groups.

Inclusion Criteria:

• Patients with "long or antagonist" protocol
• Women who did not achieve a pregnancy following a fresh embryo transfer or women whom their embryos had not been transferred due to OHSS
• The women who had frozen embryos available for another transfer no contraindications for aspirin

Exclusion Criteria:

• The patients with history of recurrent abortion