Christie Fletcher, 22, said she had been taking the oral contraception 'religiously' at the same time every day.
But despite this, she fell pregnant and found out she was expecting non-identical triplets - something that usually only happens with the help of IVF. The chance of having a naturally conceived triplet pregnancy is approximately 1 in 6,000 to 8,000 births.
She is now the proud mum to sons Jorj, Isaac and Jac, who are four months old.
Christie, from Glyn Ceriog, Wrexham, was already mother to two-year-old Osian, when she and her partner Carwyn, 27, got the incredible news.
Speaking to the Mirror, Christie insisted: "I had been taking my pill carefully and never in a million years did I expect to end up with triplets.
"I was on the pill when I fell pregnant with Osian, but I hadn't really been that careful in making sure I took it at the same time.
"After I had him, I was so strict about taking my pill every day as according to instructions - I took it religiously at the same time every single day.
"There was no way on earth I ever thought I'd end up pregnant again."
But she suspected something was up when she randomly began to start feeling sick. She decided to do a pregnancy test and was 'gobsmacked' to find out she was expecting.
The couple were in for more even more of a shock when they attended their first scan and the nurse broke the news they were expecting triplets.
Christie said: "I couldn't believe it - I thought I was in a dream."
She gave birth at eight months to baby Jorj, the biggest of the three who was born weighing 4lbs 13oz and his brother Isaac, 4lb 8oz and Jac who weighed just 4lbs 3.5oz.
Christie said: "I now have my hands full with four boys and I don't plan on having any more. I'm not relying on the pill again though - this time I'm making sure I get the implant."
IS IT REALLY POSSIBLE TO BECOME PREGNANT WHILE ON THE PILL?
* With perfect use, the failure rate of the combined pill is 0.3 per cent and with typical use is around nine per cent.
* Certain medications including some antibiotics or anti-epilepsy drugs may reduce the effectiveness of the contraceptive pill and thereby increase the failure rates.