The important step in conceiving a child through IVF is to culture the fertilized egg, which is commonly known as “sac culture”. There is no essential difference between five days of sac culture and six days of sac culture, and the key is to decide the timing of the transfer according to the degree of differentiation of the blastocysts and the maturity of the blastocysts. Under normal circumstances, in about five days, the fertilized egg can be differentiated into an early embryo, embryo transfer can be carried out, the development rate of the embryo will have some fluctuations. If the embryo is not fully developed and mature by the fifth day, it may need to be kept in the sac for six days, all of which is fine, and the doctor will choose an opportunity for embryo transfer based on the development as well as maturity of the embryo. As long as the transfer is carried out on time under the guidance of the doctor, the maturity and survival rate of the embryos can be guaranteed, and there is no essential difference, and there is no essential impact on the success rate.