Most people are under the assumption that once you're confirmed pregnant, you no longer have to deal with periods and fertilization, because your egg has clearly already been fertilized. But those people would be wrong, in once case at least.
Although pregnant women don't get menstrual periods, they can experience irregular bleeding similar to a period.
But come to find out, it is very much possible to become pregnant with an additional fetus when you already are pregnant, although it's rare in humans.
The term for this phenomenon is called superfetation, and it consists of two fetuses in separate amniotic sacs who are both in two different stages of development, depending on when conception happened for each fetus.
But don't call them twins, because although fraternal twins are conceived when two eggs and two sperm cells meet, twins are considered to be the same age, while fetuses conceived by means of superfetation are not. Superfetation can also occur with different fathers.
"Pregnancy hormones usually shut down a woman's system, making it impossible for her to ovulate during her pregnancy. This is why superfetation is so remarkable." Connie Hedmark, an OB/GYN for a Michigan hospital told Baby Center.
This seems to be something that stuns both professionals and mothers who find themselves pregnant. There have only been around 10 documented cases of superfetation, but although it's rare, it proves that even pregnancy can't protect you from pregnancy.