Objective:To study the endometrial thickness on the day of M3 and on the day of HCG administration and relationship between pregnancy and the thickness change.Methods:120 unexplained infertility patients were divided into pregnancy group and non-pregnancy group according to the treatment outcome

The thickness of endometrium on the day of menses day3(M3)and on the day of HCG administration and the increase in HMG stimulating cycles were analysed. Resufts:The thickness of endometrium in the pregnancy and non- pregnancy group on respectively the day of M3 were 5.42±1.05mm(pregnancy group)and 5.43±1.08mm(non-pregnancy group)

on the day of HCG administration were 10.76±l.68 mm(pregnancy group)and 9.25±I.75mm(non-pregnancy group)respectively

The chang in the two groups between this two test points were5.26±l.67mm and 3.89±l.40mm respectively.Conclusion:The endometrial thickness in the pregnancy group on the day of HCG administration is thicker than that in non-pregnant group. The functional layer plays a more important role in pregnancy.