Rebecca Roberts' first shock when she went for an ultrasound at around 12 weeks during her pregnancy was finding out she was having twins. The second shock was more of a mystery: Doctors were baffled as to why one of the UK woman's fetuses was much bigger than the other—a size difference you'd normally see in fetuses with a three-week age difference. Soon enough, they figured it all out: Roberts' pregnancy was a rare superfetation, in which the woman becomes pregnant for a second time even though she's already pregnant. "[I] was slightly relieved that it was not my mistake but a quite extraordinary pregnancy," Roberts' OB-GYN told Good Morning America, per ABC News. Live Science reports that the superfetation case of the 39-year-old, who already has a teen daughter and had been taking fertility drugs to try to get pregnant, is "extremely rare," first of all because pregnancy hormones usually nix further ovulation.
But there are two other "improbable events" that also have to happen: fertilization, usually prevented during pregnancy by a mucus plug that blocks sperm; and implantation, which calls for more space in the uterus for a second embryo. Somehow, a perfect storm in this case allowed it all to take place. There were some scary moments, as doctors feared one of the twins, a girl, wouldn't make it. After it was found she wasn't growing properly, doctors induced labor in September at 33 weeks: Little Rosalie was born at 2 pounds, 7 ounces, while brother Noah was 4 pounds, 10 ounces. Noah spent three weeks in the NICU; Rosalie came home around Christmas. Roberts' "super twins," as she calls them," boast one of the biggest age differences between superfetation babies. "Twins have an amazing bond ... but the story between these two, when they're old enough to find out, they'll feel even more special," she told GMA, adding that Rosalie is catching up in size to her brother (pics here).