I was sitting in the bathroom in my pyjamas when I saw the two blue lines appear on the pregnancy test. For a moment, everything seemed to blur. I picked up the instructions, reading and rereading them, willing them to tell me something different. It didn’t make sense: I couldn’t be pregnant — I’d been so careful with my birth control. A wave of shock rose from my stomach, tightening in my chest.

What surprised me most was the feeling that followed: a deep, instinctive longing for the baby I hadn’t planned. It was completely unexpected, and I knew in that instant I was facing one of the hardest decisions I’d ever have to make — to keep the baby and embrace the idea of becoming a mother by accident, or to walk away from it. I texted my boyfriend, heart pounding. I was 26, and the fact I’d fallen pregnant without meaning to made me feel naïve and foolish. Before that moment, I’d considered myself perfectly responsible when it came to my own fertility. As a qualified sex and relationships coach with a large social media following, I had always educated myself and knew more than most about the issues involved.

And yet now I suddenly realised with a sickening jolt that the contraception I’d been using for the past two years - the fertility tracking app Natural Cycles - had left me vulnerable to unplanned pregnancy. The truth is, many women my age — if not most — are keen to come off the Pill, and if there’s an alternative that’s just as reliable, they’ll take it. That’s what I thought Natural Cycles was.

The truth is, many women my age — if not most — are keen to come off the Pill, says Millie. Now I had a thriving TikTok account and began to share my experience of the app online. I took the Pill when I got my first boyfriend at the age of 16. After two years of it, however, I began to feel the side effects more intensely: mood swings, excessive bleeding between periods, and constant bloating. I switched to the mini pill, which worked better for a couple of years. But after nearly seven years on hormonal birth control, I decided to take a break.

The change was staggering. It felt as though a fog was lifted and suddenly my brain was switched back on. I broke up with my boyfriend of six years and felt a new sense of freedom. I even left my job in marketing and started posting on TikTok and Instagram, building a substantial following as I trained as a sex and relationship coach. Like many of my friends, I first came across so-called fertility apps via adverts on social media platforms. Femtech is big business online, with apps like Flo, Clue and Ovia helping women to track their periods or conceive. But none of these claim to enable women to control their fertility like Natural Cycles. Indeed, it’s the only one officially certified as a ‘contraceptive device’ in both the US and Germany.

Claiming to be 98 per cent effective with ‘perfect use’ (ie. where you don’t have sex on days you’re not supposed to) and 93 per cent effective with ‘ordinary use’ (allowing for a few slip-ups), Natural Cycles appears to be the holy grail of contraception: a natural method with roughly the same success rate as the Pill or a condom. Now I had a thriving TikTok account, I was offered a free trial (it normally costs £49.99 for a one-year subscription) and began to share my experience of the app online.

I was never paid by the Natural Cycles team, but a request was made that I talk about the experience. At first I was excited. Created in Sweden in 2013 by Dr Elina Berglund, a Nobel Prize-winning particle physicist, the app certainly comes with stellar scientific credentials. It works by monitoring your daily temperature, which you input each day and which fluctuates throughout your menstrual cycle. Using your data, the app then trains its clever algorithm and starts to predict when you’re ovulating and, therefore, when it’s safe to have sex. Or not. ‘Green days’ are fine; ‘red days’ are not.

I was cautious at first. I knew it took three months to calibrate the data, and in the beginning I always used condoms too, just to be safe. But over time I grew more confident in its abilities. It certainly helped me better understand my body and this was the aspect I wanted to talk about online. And then, a year into using Natural Cycles, I started a new relationship and, call me paranoid, but every few months I’d take a pregnancy test for peace of mind. It wasn’t required by the app, but it helped me feel more secure, especially if stress caused a delay in my period.

I’d been using the app ‘perfectly’ when it happened. Eight days late, I was ravenously hungry and yet I was absolutely certain I wasn’t pregnant. How could I be?. When I told my boyfriend, the silence on his end of the phone stretched for what felt like hours. Now sitting on the bathroom floor in floods of tears, I definitely needed him to say something, anything... but he was just as shocked and confused as me.