They are a popular source of information for women trying to conceive, and also those who are keen to avoid pregnancy, but new research suggests fertility apps are rarely reliable.
The authors of the review, published in the journal BMJ Sexual & Reproductive Health and which examined 18 studies from 13 countries, said the apps that were designed to track women's periods and ovulation cycles were mostly inaccurate.
They noted that, with a few exceptions, the apps rarely involved health professionals in the "design, development or deployment of menstruation and fertility apps."
"There is a lack of critical debate and engagement in the development, evaluation, usage and regulation of fertility and menstruation apps" the authors of the review wrote in the journal.
The review acknowledged that there are huge variations in the types of apps available, ranging from "very simple diaries through to apps that use complex, sometimes proprietary, algorithms to determine ovulation and fertility windows."
"I would rely on an app as a contraceptive if it was certified for that use and I was very familiar with fertility awareness based methods," research author Sarah Earle told CNN.
She continued: "I would probably seek out support and advice from a fertility specialist first."
According to the review, the main reason women used a fertility app was to keep track of their menstrual cycle. Other reasons included conceiving a baby or as a form of contraception.
Experts are worried more women will rely on the apps as a form of contraception while in isolation due to COVID-19.
They are urging women to call or visit a doctor, or try different methods of contraception or fertility tracking if they're trying to plan or prevent pregnancy during this time.
Wendy Vitek, a fertility specialist from the University of Rochester Medical Centre in New York told MedPage Today many reviews of menstruation tracker apps show that predictions can sometimes be off by two to three days, making them inaccurate.
"They are not as effective for contraception as more proven methods," Vitek said. "It's basically digitising the rhythm method, which we all know has a high rate of failure."