Of course you know twins are a possibility, but did you know that it is possible to get pregnant while already pregnant? If that boggles your mind, let Kate Hill's story answer your questions. Last year, the Brisbane, Australia-based mother stumped her doctor when she became pregnant twice in just 10 days. Hill's obstetrician, Dr. Brad Armstrong, told Australian news program Today Tonight that he had never seen anything like it. “I could not find any literature in the medical review websites at all,” he said. “I had to go and Google it.” As it turns out, getting pregnant while already pregnant is very, very unlikely but not unheard of. It's a phenomenon known as superfetation, in which a second foetus develops in the womb while one is already present. The foetuses aren't twins — rather, they develop in two different sacs within the womb. Hill previously had trouble conceiving due to polycystic ovary syndrome, and her doctor put her on hormones to aid ovulation. Though she told Today Tonight that she and her husband, Peter, only had intercourse once during that 10-day period, that's apparently all it took. Sperm can survive for several days after sex, remaining in the cervix, uterus, and fallopian tubes. However, for a second pregnancy to occur, you would have to continue to ovulate after becoming pregnant, which would allow another fertilised egg to attach to the womb. Robert Atlas, chairman of the obstetrics and gynaecology department at Baltimore's Mercy Hospital, spoke to Time about superfetation in 2009 following another news-making case, saying that hormone changes will usually prevent ovulation from occurring when you're already pregnant. It's unclear why that doesn't always happen — in Hill's case, it may have had something to do with the hormone treatments she had undergone to help her conceive. The bottom line is that a lot of special circumstances have to line up for superfetation to occur. In a 1999 study of such a case, a woman underwent a superfetation pregnancy and ultimately delivered two healthy infants in what researchers deemed "an otherwise uneventful pregnancy." Likewise, Hill and her husband had two healthy daughters. Dr. Atlas told Time that such pregnancies shouldn't be a cause for concern — in all likelihood, the second baby will be born slightly premature but still healthy. As for the Hills, their daughters, Charlotte and Olivia, were safely delivered on the same day via Caesarean section despite initially having different due dates. "They are definitely little miracles," Kate Hill told Today Tonight about her daughters, who are now 10 months old. Though it's unclear exactly how many cases of superfetation have been recorded throughout history, the fact remains that it's super rare. Only about 10 cases have been recorded. Miracle babies indeed.