US Rep. Alexandria Ocasio-Cortez is freezing her eggs to “feel more in control of her life,” she announced on social media Saturday. The 36-year-old has not said whether she plans to have children but said she had been saving up for the process.

Egg freezing, formally called oocyte preservation, is one of the clearest ways to extend a woman’s childbearing years. It involves extracting eggs from the ovaries and preserving them until the woman hopes to become pregnant.

Elective egg freezing has become much more popular over the past decade as the technique has gotten better and more women have chosen to delay pregnancy. Ocasio-Cortez, a New York Democrat, joins media personality Kourtney Kardashian, actor Rebel Wilson and actor Florence Pugh, among others, in sharing plans to freeze eggs. Here’s what to know about the process.

Egg freezing is most effective in a woman’s 20s or early 30s, when she typically has more viable eggs.

“The answer is typically, the younger the better,” said Dr. Pardis Hosseinzadeh, assistant professor of gynecology and obstetrics at Johns Hopkins Medicine. “Women who do this procedure before the age of 35 typically have better outcomes in terms of the quality and quantity of the eggs that are retrieved. Both the number of eggs and the quality of those eggs decline with age.”