Growing numbers of women are using contraceptive apps, but experts have warned they could lead to unwanted pregnancies.

The Swedish app Natural Cycles, the only certified app for contraception, has seen a surge in the number of members from the UK in the past year with almost 200,000 signed up, an increase from 5,000 in 2016.

Sarah Hardman, the director of the Faculty of Sexual and Reproductive Healthcare’s clinical effectiveness unit in Edinburgh, expressed concern that while some women were very pleased with contraception apps, others had had unwanted pregnancies while using them.

“We don’t have a good indication yet of how successful the average woman in the street in the UK is going to be when using them,” she said.

She added: “I would express concern about people knowing what they need to know before they use it and not just thinking: ‘this is modern because it’s an app and it’s something new, so it’s better and more effective’. It’s not – it’s just very different.”

Hardman said she understood there would be women who wanted to avoid hormones. “For them, they may say: ‘I understand that this is not as effective as lots of contraceptives, you need to work hard to get it right and it’s not a disaster if I did get pregnant.’ It’s about choice for women but it’s about making choices in an informed way and knowing something’s limitations.”

In the past few years, there has been a growth in the number of contraceptive apps. These include Kindara – which allows you to track everything from your body temperature to the appearance of your cervical fluid – to Ovia, which lets you monitor your moods, periods and other metrics. The apps work by pinpointing when you ovulate and when you are most fertile.

Natural Cycles requires women to input their temperature every morning. It then calculates their menstrual cycle and informs them when they can have sex without protection.

The app met with controversy earlier this year when it was reported to Swedish authorities after a hospital found 37 cases of unwanted pregnancies among women relying on it for contraception.

Natural Cycles said the efficacy of the app was backed by a wealth of clinical data: a study of 22,785 women demonstrated an effectiveness rate of 93%.

Hardman said: “It’s a good study as it has lots of women involved but they are women who went out to specifically look for something different for contraception ... maybe they had problems on the pill ... they chose to use this app, so may be more motivated than the average woman on the street.”

Raoul Scherwitzl, the chief executive of Natural Cycles, said: “It is important to note that our typical users are age 30 on average, in a stable relationship with a regular daily routine – and are willing to take their temperature on a daily basis and use protection on fertile days. Our users do tend to be highly motivated.”

The concept of tracking fertility has roots in the work of gynaecologists Hermann Knaus and Kyusaku Ogino, who in the 1930s revealed that ovulation occurs in the mid-point of the cycle. Their discovery led to the development ofcontraceptive practices such as methods involving daily temperature readings and checking the consistency of cervical mucus.

Hardman said: “We will not know how it works for the general population until it’s been out there and used and, at that point, we would love to be pleasantly surprised. But looking at the general population, we know people can use pills and condoms and not always use them perfectly, and the overall failure rate can be quite high. That is why we are concerned.”