But this wasn’t a typical set of twins, Roberts learned. Her pregnancy was diagnosed as superfetation, a rare condition in which a woman who is already pregnant conceives another baby.Doctors found a second baby during the 12-week ultrasound appointment, and after several scans, they diagnosed the pregnancy as superfetation. In this image, the larger baby is 13 weeks, while the smaller one is 10. (Courtesy of Rebecca Roberts) Roberts’s pregnancy is one of very few superfetation cases recorded in medical literature, her obstetrician, David Walker, said.