In the United States, there were 167,689 cycles to freeze eggs and embryos in 2021, nearly double the number in 2017, according to the Centers for Disease Control and Prevention. In most clinics, reproductive material is preserved in liquid nitrogen in double-walled tanks at nearly minus-200 degrees Celsius. The CDC collects data on success rates from more than 450 clinics in the United Kingdom, where a shortage of trained lab workers has grown worse.