A 32-year-old woman from Alabama who was born with two uteruses and became pregnant in both gave birth to twin girls on different days, she announced on Friday.
"Our miracle babies were born!" Kelsey Hatcher, who is documenting her story on her Instagram account "doubleuhatchlings", wrote in a post.
She added that the girls "decided they were rare enough statistically that they should just go ahead and have their own birthdays too".
The first one, named Roxi Layla, was born on Tuesday at 7:49 pm (0149 GMT). She was joined by Rebel Laken on Wednesday at 6:09 am.
Doctors had estimated a Christmas due date, but the sisters arrived just in time to be at home for the holidays with their siblings.
The mother and daughters have been discharged from the hospital, with Hatcher, promising to share details about the delivery in future.
Hatcher knew from the age of 17 that she has "uterus didelphys," a rare congenital condition thought to affect about 0.3 percent of people born female.
It was during a routine eight-week ultrasound visit in May that the massage therapist and then-mother-of-three learned not only that she was having twins this time, but that a fetus was present in each of her uteruses.
Pregnancies in both uteruses are exceedingly rare, according to Shweta Patel, the obstetrician-gynecologist who cared for Hatcher at the University of Alabama at Birmingham's Women & Infants Center.
Hatcher said she was told the odds were 1 in 50 million -- with the last widely known case in Bangladesh in 2019 when Arifa Sultana, then 20, gave birth to healthy twins 26 days apart.