Validation of Effectiveness on NaturalCycles Users

Brief Summary

The goal of this observational study is to measure the effectiveness of the Natural Cycles digital birth control application at preventing pregnancy in sexually active women aged 18 to 35 with no known history of infertility. The main questions it aims to answer are: * What is the pregnancy rate of the application over one year, for all users? * What is the pregnancy rate of the application over one year, for users who follow the application's instructions perfectly? Participants will: * Use the Natural Cycles application for birth control for up to one year. * Log information about daily sexual activity in the app. * Complete a questionnaire at the beginning and end of their participation. * Perform urine pregnancy tests (provided by mail) upon entry to the study, at the start of every cycle during the study, and upon exit from the study.