Noah's first ultrasound close-ups, taken at seven and ten weeks into the pregnancy, showed 39-year-old mom Rebecca Roberts and 43-year-old dad Rhys Weaver the baby they had struggled to conceive more than a year.

Then, three months into the pregnancy, Noah found himself with business. Noah had an unexpected little sister, fraternal twin Rosalie, according to an ultrasound taken at week 12.

"I got pregnant while already pregnant, which was insane... because that's not supposed to happen," Rebecca explained. 

Known as a superfetation, being pregnant while already bearing a child is so uncommon that a 2008 study found less than ten documented cases worldwide.

Doctors told the couple that the babies were born about three weeks apart, according to Rebecca.

"They noticed the baby was developing at a steady rate of three weeks behind the first one, and that's when they told me they think this is a superfetation pregnancy," she said.

"I couldn't believe it had happened to me," Rebecca laughed. "But it did, and it's beautiful. It's similar to winning the lottery."