Twins Aylin and Alfredo Trujillo were born in the U.S. state of California over New Year's with a little twist – they were delivered in different years.

They will certainly have a tale to tell about their birthdays, which fall on different days, months and years.

"It was a surprise," mother Fatima Madrigal, 28, told Reuters on Tuesday in an interview from Greenfield, California.

At 11:45 p.m. (7:45 a.m. GMT) on New Year's Eve 2021, Fatima Madrigal gave birth to her son Alfredo Antonio Trujillo in Salinas, California. Fifteen minutes later, as the clock struck midnight and hospital staff rang in the new year, Aylin arrived.

The twins were over two weeks early, as Madrigal's due date was Jan. 16. Aylin weighed in at 5 pounds, 14 ounces (2.66 kg), while big brother Alfredo weighed 6 pounds, 1 ounce.

Madrigal said her partner Robert Trujillo and their other three children, aged 11, 3 and 1, were over the moon with the new arrivals.

"I was kind of shocked because twins don't run in my family, nor in my partner's family," she said. "So we were really surprised that we got blessed with two babies, and it's a boy and a girl, so we're complete."

For now, the twins will celebrate their birthdays on the same day, Madrigal said.

"I'll explain it to them the best I can. When they're older, if they want to celebrate their birthdays, like, different years, it's up to them, but right now that they're small and they're with me, they're going to celebrate their birthday together," she said.

According to Natividad Medical Center, where the twins were born, there are about 120,000 twin births in the United States every year, but twins with different birthdays are rarer. Some estimate the chance of twins being born in different years as one in 2 million, the hospital said.