A woman with a rare double uterus has given birth twice in two days.

Medical phenomenon Kelsey Hatcher, from Alabama in the US, was born with two functional uteri that each have their own cervix.After falling pregnant in both wombs - described as a one-in-a-million occurence - Mrs Hatcher has now given birth to two daughters after spending 20 hours in labour at the University of Alabama at Birmingham (UAB) Hospital.

One girl, Roxi, was born on Tuesday evening, while sister Rebel arrived the following morning, some 10 hours later.

"Never in our wildest dreams could we have planned a pregnancy and birth like this; but bringing our two healthy baby girls into this world safely was always the goal, and UAB helped us accomplish that," Mrs Hatcher said in a statement released by the hospital.

"It seems appropriate that they had two birthdays, though. They both had their own 'houses,' and now both have their own unique birth stories."

Mrs Hatcher was diagnosed with a double uterus - also known as uterus didelphys - when she was 17. It is a rare congenital anomaly that occurs in just 0.3% of women.

'You're lying'

Mrs Hatcher was already a mum-of-three before her latest pregnancies, but had never been pregnant in both wombs before.

After finding out she was pregnant again in the spring and completing her initial ultrasound, she shared the exciting news with her husband Caleb.

She recalled telling him: "I said, 'Well, there's two of them in there'. And he said, 'You're lying'. I said, 'No, I'm not'.''

While it is not as uncommon for women with a double uterus to have a pregnancy with one baby in one uterus, having a baby in both - also known as a dicavitary pregnancy - is an estimated one-in-a-million chance, according to studies of the phenomenon.

Ms Hatcher's obstetrician, Dr Shweta Patel, told NBC affiliate WVTM that the pregnancy was "very, very rare", adding: "Some obstetrician-gynaecologists go their whole careers without seeing anything like this."

Because of its uniqueness, the pregnancy was considered high-risk.

Dr Richard Davis, a specialist in high-risk pregnancies at the University of Alabama, said: "A double cervix or double uterus is way under 1%, maybe three per 1,000 women might have that."

Are they actually twins?

With two wombs and two birthdays, it's a complicated question, according to Dr Patel and and Professor Davis.

"Technically, yes and no," they explain, as a typical twin pregnancy is defined by two babies in one uterus at the same time.

While Kelsey had one baby in each uterus, two eggs were released that were fertilised during the same ovulation cycle.

"I think it is safe to call the girls fraternal twins," Professor Davis said. "At the end of the day, it was two babies in one belly at the same time. They just had different apartments."