In 2016, when Australian couple Kate and Peter Hill gave birth to two healthy girls born just ten days apart, the medical community took note. The duo had been trying to get pregnant for some time, but due to a case of Polycystic Ovary Syndrome (PCOS) were told that may never happen. When it did, and Kate conceived twice in just over a week, they became a textbook example of a rare occurrence known as superfetation.

Fortunately, superfetation is as uncommon as the title sounds, and surprisingly quite harmless. Dr. Sherry A. Ross, an obstetrician, gynecologist, and author of She-ology, explains the process bluntly: "A woman ovulates, she gets pregnant, she creates an embryo, and then a few days or even a week later she ovulates again, and then gets pregnant again, creating a second embryo."

It sounds similar to twins, but Dr. Sherry says it's entirely separate biologically, as the eggs don't become fertilized at the same time. The difference in developmental stages for each fetus is also usually a week to 10 days apart—but can be up to several weeks—because the two embryos come from two separate waves of ovulation.

The reality is that women's bodies typically stop this from happening, making superfetation a scientific anomaly and something that medical practitioners are still trying to get their heads around. As Dr. Sherry puts it, "Doctors especially like hard facts; they like to show a trail as to how this actually makes sense, and I think that's sort of what makes this a mystical entity for the medical community."

According to Dr. Sherry, when a woman becomes pregnant there are certain biological processes that are triggered, resulting in a few core reasons that this phenomenon (in the vast, vast majority of pregnancies) doesn't happen. Firstly, "If you were to get pregnant during your ovulation, then you're ovaries shut down and they should not ovulate anymore … You get pregnant, your eggs [are] fertilized, and then your ovaries stop producing future follicles." Another reason is that when a woman becomes pregnant she creates a "mucus plug" which forms in the cervix after conception, and "prevents sperm from entering the uterus for that second egg to get fertilized." Despite all of this, there have been a handful of recorded cases over the years and across the globe. Kate and Peter Hill were receiving fertility treatment, which Dr. Sherry says could have been a contributing factor to their case of superfetation. "[If] you're getting fertility treatment, you're defying what typically happens in your body," she says. According to Dr. Sherry, this possibly resulted in the hyperstimulation of Kate's ovaries.The most surprising part of the Hills' story is that the couple is adamant they only had sex once.

As for becoming pregnant by two different men at the same time, is that feasible? "Yep, it totally [is]," Dr. Sherry says. This also extremely rare phenomenon is called heteropaternal superfecundation, and occurs when two ova meet two sperm, from two different parents-to-be, creating two embryos (homeopaternal superfecundation is what happens when one parent fertilizes two separate ova).