"WHILE ON HORMONAL birth control I noticed so many changes that I didn't like mentally or physically," says an enthusiastic young woman in an Instagram ad for Natural Cycles, which claims to be the world's first FDA-cleared birth control app. "Ever since switching," she gushes, "my health has only been on the up & up!"

Natural Cycles uses a proprietary algorithm and daily body temperature readings to track ovulation and identify fertile periods. For around $120 a year, the company aims to help users either achieve or avoid pregnancy-without the hormones present in birth control pills and many IUDS.

That pitch seems to be working: Natural Cycles' customer base grew to more than 3 million in 2024. It's one of the premier products in the booming market for "femtech," app-based software with a focus on fertility and menstrual tracking and, increasingly, sexual satisfaction. (Consider the Lioness orgasm-tracking vibrator: "Don't just masturbate. Masturbate smarter.") The femtech industry is already valued at around $50 billion by market researchers and expected to be worth more than $100 billion by 2032.

While these companies' fertility algorithms and app interfaces might be new, the technique of tracking ovulation to prevent pregnancy isn't. The Catholic Church, which forbids most birth control, popularized "natural family planning" decades ago, and women have been using their menstrual cycles to inform their reproductive choices long before that.