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Rebecca Roberts, age 39, was stunned to find out that she was pregnant again just a few weeks after discovering her first pregnancy.

Rebecca was diagnosed with superfetation after her 12-week ultrasound scan, when it was revealed that she was carrying another baby who was less developed than the first. Superfetation is the formation of a fetus while another fetus is already present in the uterus.

Rebecca, from Wiltshire, said: “I thought something awful had happened. The sonographer looked at me and was like, ‘do you know you’re expecting twins?’”

In actual fact, Rebecca and her partner Rhys Weaver had been struggling with fertility issues. Learning that they were expecting one baby was a surprise, let alone two!

David Walker, Rebecca’s obstetrician said: “We were concerned because the second twin was much smaller. It was only by regularly scanning and seeing that the rate of growth was consistently three weeks behind that we realised it was superfetation.”

In 2008, a report published in the European Journal of Obstetrics & Gynecology and Reproductive Biology noted that, at the time of writing, fewer than 10 cases of superfetation had been recorded worldwide.

The condition is rarely diagnosed because when a woman becomes pregnant, her hormones tell her body not to conceive again. Women who are undergoing fertility treatment are at a slightly higher risk of superfetation, but the chances of it happening are still slim.

Not only are the chances of being diagnosed with the condition very rare, but it is also extremely difficult to prove superfetation.

Walker was not convinced that it was Rebecca’s fertility drugs that had played a role in the superfetation. “She didn’t release two eggs at the same time, which is what the medication normally does”, he explained.

As with all twin pregnancies, Rebecca was monitored closely. The babies were delivered by C-section, with big brother Noah weighing in at 4lbs 10oz and little sister Rosalie weighing a dainty 2lbs 7oz. The little ones are now thriving at home.

AlphaBiolabs offer a variety of DNA testing services including sibling DNA tests and twin DNA tests, which can determine whether twins are genetically identical or non-identical.

You can order your twin test online, or call our friendly Customer Services team on 0333 600 1300. You can also email info@alphabiolabs.com.

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AlphaBiolabs is an award-winning DNA testing lab. DNA testing starts from just £99.