A large cohort study of Sweden’s national IVF registry, between 2007 and 2017, concluded that the steady increase in birth rates can be explained by two clinical trends emerging in IVF: the transfer of embryos at the blastocyst stage of their development and the increasing use of rapid freezing of embryos freezing vitrification. An embryo cultured for five or six days after removal from the ovary is called a blastocyst. Vitrification is a rapid freezing technology that turns cells into a frozen state in seconds, the speed of the technology means that no ice crystals are formed that could damage the frozen cells during thawing. Therefore, there is a significant increase in the overall birth rate and this is associated with an increase in blastocyst transfer, especially when using frozen embryos by the vitrification method.