The fact that you can get pregnant even whilst you are already pregnant is not something that many expect to hear. It can happen, and has happened, but it’s rare and thought to only happen once in every few million pregnancies. The medical term for this unlikely event is superfoetation.
Superfoetation is when an additional baby is conceived days or even weeks after its “elder sibling”. Whilst one is older, gestationally, they will often be born at the same time. They will never be identical.
There are only around 10 recorded cases of superfoetation happening in medical literature. It may be more common and mistaken for normal twin pregnancies or may possibly be even rarer. The difference in size can sometimes be attributed to medical conditions such as congenital infection, chromosomal abnormalities and twin-to-twin transfusion syndrome, a condition in which twins share a placenta (most twins don’t) and one consumes a larger share of the nourishment it provides. This discrepancy between the size of twins is known as growth discordance and is not uncommon.
Whilst it is accepted that it does happen within the animal kingdom and is well documented in mammals (badgers, buffalos, mink and panthers), there is much discussion within the medical community as to the likelihood of superfoetation even being possible in humans. – This paragraph is confusing because it seems to contradict the title. I think more explanation/clarification is needed.