Birth control app Natural Cycles is under fire in Sweden after 37 women reported getting pregnant while using the app as contraception.
The app uses an algorithm to tell women when they have the highest and lowest chances of getting pregnant, but ultimately, it relies on men and women changing their behavior.
It takes into account many factors involved in fertility, including woman's temperature, the regularity of her periods, and sperm survival, then suggests which days to have or refrain from sex to avoid pregnancy.
"We give red and green days and clear recommendations on which days to abstain and which days we consider the risk of pregnancy to be negligible," Natural Cycles co-founder Scherwizl told Business Insider last spring.
The app was initially portrayed by multiple news outlets — including Business Insider — as "as effective as the pill using only math," but the problem is that it relies on couples to change their behavior and abstain from sex when the app says to do so.
Fertility awareness-based methods of birth control, like Natural Cycles, have been used for thousands of years.