It is laughingly said that pregnancy is the best antidote to pregnancy; that you cannot get pregnant while you are pregnant. This is true in all but the rarest of rare cases.
There are many reasons that a woman cannot get pregnant when she is already pregnant.
For one a woman will stop ovulating once a pregnancy has occurred; also a mucous plug forms at mouth of the cervix, sealing off the uterus for all practical purposes.
However there are freak occurrences in nature which make it possible for a woman to fall pregnant even while she is pregnant.
Some women have two uteruses. In most cases a second uterus would not be a healthy or a fully functioning one, and would therefore have to be removed. It is not common even for those women who have two uteruses to be able to get pregnant a second time when already pregnant.
Very recently a bizarre case of what is known as Superfetation happened, wherein, a pregnant woman conceived another child while already a few weeks pregnant.
A couple by name of Todd and Julia Grovenburg from Arkansas had this strange experience when the woman got pregnant with a female fetus and two and a half weeks later, a male fetus was conceived.
According to the attending doctor, the explanation is, “egg and sperm, implant. Of course, that’s your first pregnancy. But if you ovulate more than one time a month — and women do — and a sperm happens to meet that egg and they, too, implant, guess what, you get a second fetus.” These particular two babies will be born at the end of 2009 and the beginning of 2010 respectively.
In another bizarre case, a woman conceived twins from two different men. Twins Justin and Jordan from Dallas are twins who have different fathers and are born from a phenomenon known as heteropaternal superfecundation.
The explanation for this is that a “woman can release more than one egg during ovulation, and if she has intercourse with more than one partner at around that time, then sperm from different partners can fertilize each egg.”
Sperm cells take a while to travel and eggs take a while to travel, there can be an overlap, and this extremely rare occurrence can then take place! This is extremely rare among humans, but heteropaternal superfecundation is more common in other animals such as cats and dogs.