Abstract: Abstract Objective: to investigate the causes of abortion during IVF-ET in order to increasing the pregnancy rate and preventing the occurance of abortion. Methods: 203 patients including 150 patients who had early stage abortion (early abortion group) and 53 patients who had late stage abortion (late abortion group ) were enrolled. The age, BMI, COH and clinical pregnancy results between the two groups were analysed in order to find the causes of spontaneous abortion during IVF-ET treatment. Results: The clinical pregnancy rate in our center is 38.3% while the 16.9% including early abortion rate 12.6 % and late abortion rate 4.4%,.The β-hCG level of 14 days after ET is quite low in the early abortion group while the BMI, Multiple pregnancy and fetal reduction method in late abortion group is high. Conclusion: early abortion is correlated with embryo quality and more than 60% is due to the chromosome abnormalities. Low level of hCG after 14 days of ET will suggest the high risk of early abortion. Additionally increasing BMI and large dose of Gn during COH can infect the receptivity of the endometrium and the embryo implantation. Multiple pregnancy and fetal reduction method can cause prematue membrane rupture and is one of the reason causing late stage abortion. More over fetal malformation is another reason for pregnancy induction.

Key words: abortion, aetiology, IVF-ET