We all know that fertility varies from woman to woman: Some struggle to conceive, while others are surprised by a pregnancy when they're trying to avoid it. But getting pregnant while you're already pregnant? That seems to go beyond the extremes of human fertility.

And yet – at least for a handful of women – it has happened.

In an odd phenomenon known as superfetation, a pregnant woman releases an egg a few weeks into her pregnancy. The second egg is fertilized, and the woman is then pregnant with two babies simultaneously. (Don't confuse this with superfecundation – when two eggs released during the same cycle are fertilized at separate times. Read more about these twins with different fathers.)

Biologically speaking, things aren't supposed to happen this way. "Pregnancy hormones usually shut down a woman's system, making it impossible for her to ovulate during her pregnancy," says Connie Hedmark, an obstetrician at Marquette General Hospital in Michigan. "This is why superfetation is so remarkable."

No one knows why it happens. But according to Karen Boyle, a nationally recognized expert in reproductive medicine, superfetation may occur when implantation of the first embryo is delayed, preventing that initial spike in hormones.

Delayed implantation is said to occur in rabbits and some other animals (although this hasn't been well studied). In humans, the phenomenon seems to be so rare that when Boyle was preparing for a segment about superfetation on ABC's Good Morning America, she was able to find only about ten cases on record.

After the show aired, however, Boyle was inundated with comments and emails from people who suspect that they experienced superfetation themselves. Many others wrote to say that they knew of twins, one of whom appeared to have been conceived much later than the other. "I was shocked at the response," says Boyle.

Couldn't some of these cases be the result of growth retardation in one of the twins? Definitely. But, as Boyle points out, we can't dismiss the possibility that superfetation is simply not recognized when it does happen.

Are the babies born at the same time? Yes. In the earliest known cases, the babies' age difference was discovered after both children were delivered naturally. These days, the condition shows up during an ultrasound, giving Mom and the medical team time to plan the delivery – usually a planned c-section, since only one of the babies is full-term.