Transferring 2 fresh embryos and having 1 viable embryo can be caused by various pathogenic factors such as damage to the embryo at the time of transfer, congenital dysplasia of the embryo, and uterine factors. 1. Damage to embryos at the time of transfer: If one of the two fresh embryos is damaged during the transfer process, the damaged fresh embryo may die, which may lead to the transfer of two fresh embryos, one of which survives. 2. Congenital dysplasia of embryo: If 1 of the 2 fresh embryos is transferred with congenital dysplasia caused by chromosomal abnormality or gene mutation, it may lead to the death of this embryo, and 1 fresh embryo may be viable. 3. Uterine factors: If the uterus develops abnormally, or the endometrium is too thin, it may be difficult for the 2 fresh embryos to grow in the uterine cavity at the same time, which may cause the transfer of 2 fresh embryos, and 1 embryo may be viable.