A U.K. mom got the shock of a lifetime when she found out she was pregnant—when she was already pregnant. No, that's not a typo.

Rebecca Roberts told CBS News that she'd been trying to get pregnant for over a year when she went to a fertility clinic for help. She began taking a drug to stimulate ovulation, and it worked. She got pregnant, and everything looked great. Her scans at 7 and 10 weeks showed a healthy baby growing. But a few weeks later, she got the shock of a lifetime. "I found out there was a second baby at my 12 weeks scan," she explained. "I was so shocked, I didn't feel real at all. It was a good job I was laying on the couch or I would have fainted on the floor."

It's called superfetation—when a second fertilized egg implants in the womb days or even weeks after the first one—and it's an extremely rare occurrence. Roberts says there have only been 14 documented cases in the world, and when she realized it was happening to her she said it "felt magical."

"Nothing could've prepared me for this news. It was only a few weeks before that I saw 1 little baby wiggling around. What on earth had happened, how did they miss the other baby?! The Sonographer was gobsmacked, I was gobsmacked!" she wrote on Instagram shortly after finding out.

Before recognizing that superfetation was in play, Rebecca and her husband Rhys were initially very concerned about why one twin was developing weeks behind the other. "After worrying that something was wrong with Twin 2 as it was a lot smaller, more scans have concluded that these babies were conceived 3 weeks apart. Not only is T2 smaller, it's also younger than T1," she wrote. "This is a super rare 'phenomenon' and it's happened to me!"

That fear then gave way to relief and excitement. "All that worry, the stress, the panic and all that it was, is that they are different ages."

Roberts gave birth to the babies in September at 33 weeks. Baby boy Noah, the twin conceived first, weighed 4 pounds and 10 ounces at birth. Rosalie weighed just 2 pounds 7 ounces, and her smaller size led to a much longer NICU stay than her brother. While Noah went home in about three weeks, Rosalie stayed in the hospital for about three months.

Proud mom Rebecca says both twins are now doing great at home, and even though Rosalie is still noticeably smaller than her twin she's making great progress. Rebecca says the estimated three weeks between when they were conceived is the biggest gap recorded—making these super special twins even more special.