A woman was left shocked after doctors discovered an unborn twin growing inside her ovaries since her birth some 45 years ago.

The mother of two, Jenny Kavanagh, 45, from south London discovered the odd growth after going for a routine procedure of having a contraceptive coil inserted inside her.

The fact that it had long black hair – just like mine – a face, with one eye and one baby tooth makes it even more believable... I felt shocked, very scared, horrified and it felt like an alien was inside me. - Jenny Kavanagh

After doctors conducted an ultrasound on Kavanagh, they found a 10cm dark mass growth inside her left ovary.

It was later revealed that the mass was in fact an undeveloped twin that had formed inside Kavanagh since her birth. Over the years, the twin had formed black hair like hers, along with an eye, tooth and a face.

Kavanagh was warned that if the mass is not removed, it can possibly get twisted or rupture resulting in her death.

"I try to remember that it had no heart and no brain. And that it would have almost certainly killed me if they hadn't found it and removed it," said Kavanagh, reported The Telegraph.

"The fact that it had long black hair – just like mine – a face, with one eye and one baby tooth makes it even more believable... I felt shocked, very scared, horrified and it felt like an alien was inside me."

The unborn twin was eventually removed after Kavanagh was operated on during a three-hour surgery at The Mediterranean Hospital in Cyprus.

Over the years, Kavanagh's left ovary had been protecting the stem cell, which was the weaker embryo engulfed by Kavanagh –the stronger embryo.