By Sam Frizell
November 6, 2014

A small fluorescent object in your pocket tells you when to have sex in order to create children. It may sound like a dystopian future a la Aldous Huxley, but it’s actually a helpful way of figuring out when to have sex in order to conceive.

There are a number of fertility tracking smartphone apps that tell you when you’re most likely to conceive. They’re becoming increasingly sophisticated and accurate, and are likely to become more popular in the future.

Here are three apps that tell women when or when not to have sex to maximize their chances at getting pregnant:

NaturalCycles asks users to input their temperature each day, and by inputting the readings into an algorithm, the app says it can judge with 99.9% accuracy if the user is fertile and may get pregnant on a particular day. The app uses green color codes on a calendar to indicate infertility, and red to indicate that a user is fertile on marked days. It’s marketed chiefly as a form of digital birth control.

Glow, an app funded by PayPal entrepreneur Max Levchin, pinpoints users’ ovulation days and sends you message updates like “Whoohoo! You’re ovulating!” and includes message boards for women.

Aesop Fertility Clock does what NaturalCycles and Glow do, but it also shows users how to influence whether the child is a boy or girl.

Contact us at editors@time.com.

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