The normal process of getting pregnant is an ovum is released, becomes fertilized and implants itself on the uterine lining where it eventually becomes a fetus.
However, some women can get pregnant a second time while they’re already pregnant with one baby.
A pregnancy like this is known as superfetation but is very rare and can only occur under specific circumstances.
It’s very rare in human beings because once a woman is pregnant, changes in the body’s hormone prevents her from ovulating.
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However, if the woman keeps ovulating and the ovum is fertilized and gets implanted in the uterus, she can carry a double pregnancy.
The reasons why women can continue ovulating while already pregnant are not widely studied and more research is being conducted.
Moreover, double pregnancy can occur if a woman has a double uterus or if she ovulates multiple times.
In common cases, once a woman gets pregnant, the body emits hormones that stop ovulation. They also block other pregnancies from occurring.
Additionally, the cervix forms a protective barrier that prevents the sperms from penetrating.
This means, for superfetation to occur, there has to be irregular ovulation or once an ovum has already been fertilized.