“Ordinarily, the release of eggs ceases once a woman is pregnant, and the hormonal and physical changes of pregnancy work together to prevent another conception," the column answers.
But in some poorly understood and mystical way, sometimes things happen that go against what a woman's reproductive cycle is inherently programmed to prevent, and an entirely different sperm and egg manage to join together when the person is already pregnant.
According to the Scientific American, "In order for superfetation to occur in humans, therefore, it would appear that three seemingly impossible things need to happen: ovulation must take place during an ongoing pregnancy, semen must somehow find its way through the blocked cervix to the oviduct, via the occupied uterus and finally, the conceptus has to successfully implant itself in an unsuspecting already-occupied uterus."
In other words, there are some almost impossible hoops for the sperm to jump through for this to happen. It's incredibly rare for superfetation to occur. There are maybe 10 known cases of it in humans up to now, but it is fairly common in mammals like cats, badgers and panthers.
"Most doctors believe alleged reports (of superfetation) are primarily due to markedly unequal growth and development of twin fetuses of the same gestational age," Jason James, M.D., medical director at Miami's FemCare Ob-Gyn tells SELF.
But if a second pregnancy does occur, there's a risk that the second baby will be born premature. In some recent cases, though, both pairs of babies were successfully delivered.
In 2010, Julia Grovenburg, a 31-year-old Arkansas woman, became pregnant twice, two and a half weeks apart. The baby girl and boy were both born by C-section. And in 2015, an Australian mom named Kate Hill conceived two girls 10 days apart. Hill said because she had polycystic ovary syndrome, she was on hormones to aid ovulation and claimed she and her husband only had sex once during that 10-day period. The girls, Charlotte and Olivia, were also delivered on the same day through C-section despite different due dates and have different blood types.
It only took three visits to the hospital, but Belinda Waite finally found out she was pregnant—a few hours before she gave birth. The Devon, England resident felt unwell for months before the birth. According to The Telegraph, the first time she went to the hospital, she was told it was Irritable Bowel Syndrome. The second time, when she arrived with swollen hands and feet, the doctors thought it was allergies or gout. "I did feel like something was moving inside me as the months went on. But I never considered I was pregnant and it doesn't seem to have crossed the doctors' minds," Waite told The Telegraph. Finally, at her third visit to the hospital, doctors realized that she was, in fact, pregnant—three months so, they told her. As it turned out, she was actually nine months pregnant, and four hours later gave birth at home to a healthy, 8-pound 14-ounce girl named Louise.