rebecca roberts british mom twins
'Medical marvel': Mom got pregnant while already pregnant
03:08 - Source: CNN
CNN  — 

At first, baby Noah was all alone, growing beatifically in his mother’s womb.

Noah’s first close-ups, taken by ultrasound at seven and 10 weeks into the pregnancy, showed 39-year-old mom Rebecca Roberts and 43-year-old dad Rhys Weaver the baby they had tried for more than a year to conceive.

Then, at three months into the pregnancy, Noah suddenly had company.

An ultrasound taken at week 12 showed that Noah had an unexpected little sister – fraternal twin Rosalie.

“I got pregnant whilst I was already pregnant, which was absolutely crazy … because that’s not supposed to happen,” Rebecca said.

Baby Noah was joined in utero by fraternal twin Rosalie some three weeks after Noah's conception.

Called a superfetation, getting pregnant while already carrying a baby is so rare that one 2008 study found fewer than 10 recorded cases in the world.

Doctors told the couple the babies were actually conceived about three weeks apart, Rebecca said.

“They realized that the baby was growing at a consistent rate of three weeks behind the first one, and it was then that they said to me, they think this is a superfetation pregnancy,” she said.

“I couldn’t believe it had happened to me,” Rebecca added with a laugh. “But it did – it’s lovely. It’s like winning the lottery.”

Dad felt the same: “I was elated to be having one child, but even more so for twins. The job is done in one go! And then Rebecca did some research, and we realized how unique and how lucky we were.”

An incredibly rare event

Superfetations are rare for a variety of reasons, said Atlanta gynecologist Dr. Lillian Schapiro.

First, women typically ovulate only once per cycle, releasing one or more eggs simultaneously. If fertilization by the man’s sperm is successful, the egg or eggs then implant in the uterus, the pregnancy begins and no further ovulation occurs.

By being concieved while Rebecca was already pregnant, baby Rosalie was a rare "medical marvel."

“If a woman has twins,” Schapiro said, “two eggs are released at the same time. And in the unusual case of triplets, those eggs are all released with one ovulation.” Identical twins happen if a freshly fertilized egg splits.

In Rebecca’s case, the egg was fertilized and implanted during the first ovulation, and “somehow she ovulated again during that same cycle,” Schapiro explained. “Another egg was also fertilized – became another embryo – and at different times both embryos implanted in the uterus.”

Another reason superfetations are exceptional, Schapiro said, is that once the pregnancy begins, the uterus is no longer a hospitable place for implantation. That means the second embryo “must have managed to implant and grow at a stage when we would not have thought it would be able to grow.”

“We have almost never seen where two embryos start developing at different times,” Schapiro said. “That is just nothing short of amazing.”

Rebecca had just taken one dose of fertility drugs designed to stimulate ovulation before she conceived Noah. While that may be one reason for this rare occurrence, Rebecca said, it could also just be a “medical marvel.”

‘Tiny, tiny baby’

At first Rebecca and Rhys fretted over baby Rosalie’s development in the womb, worried that being so far behind her bigger brother might affect her health at birth.

“It could go either way,” Rebecca said. “Because the baby is so much smaller, there could be something wrong with it and it might not survive. That’s usually the case.

“But they said this baby’s actually growing consistently,” she said. “It was a relief. It was a great relief.”

Born by cesarean in September 2020, both babies did have to spend time in separate neonatal intensive care units (NICU). Rosalie, who was born at a tiny 2 pounds, 7 ounces, was sent to a specialty NICU about 15 minutes away from the NICU that cared for Noah, who was born at 4 pounds, 10 ounces.

“She was a tiny, tiny little baby just fitting in our hands, and even though he was tiny, you could see that he was a much bigger baby,” Rebecca said.

“It was very hard. I had a major operation and then our babies were in two separate hospitals as well,” she said. “So it was really hard work having to travel between them both.”

At birth, Noah was nearly twice as big as his little sister.

Noah was able to come home in three weeks, but little Rosalie remained in intensive care for 95 days, coming home just before Christmas.

Today, at close to 7 months old, Noah is still ahead of Rosalie, but Rhys said, “She’s a little trooper. She’s got her own little personality, but I have no real worries at all.”

Noah’s rolled over and is “actually showing signs of trying to crawl, so he might be off soon, which will be fun,” Rebecca added.

“They’re both talking, which is really really sweet, because they talk to each other. It’s lovely. They interact with each other really, really well. It’s beautiful to watch.”

Noah is still ahead of Rosalie, but Rhys said, "She's a little trooper, she's got her own little personality, but I have no real worries at all."

CNN’s Elizabeth Cohen and Justin Lape contributed to this story.