Rebecca Roberts and her partner struggled with infertility for more than a year, so when they got a positive result with an at-home pregnancy test, they were overjoyed. On Roberts's medical chart, her obstetrician wrote: singleton. But their excitement abruptly shifted to shock five weeks later at the 12-week ultrasound appointment, when the sonographer spotted something astonishing.  It appeared as though Roberts was suddenly carrying two babies - one of which was considerably less-developed than the other. Her pregnancy was diagnosed as superfetation, a rare condition in which a woman who is already pregnant conceives another baby. Superfetation is so uncommon, that Roberts doctor David Walker struggled to diagnose it. Ordinarily, hormonal and physical changes during pregnancy prevent another conception from occurring. That was not the case for Roberts. Although Roberts's children had different due dates, she had a Caesarean section and gave birth to both babies on the same day. Roberts and Weaver have been documenting the twins' growth over the past few months on an Instagram page, and thousands of people around the world, fascinated by their story, are following along.