Twins of different ages

[pne]

Today's the first time I heard about twins of different ages (here's another story): where a woman ovulated twice within a fairly short period and both eggs were fertilised—so another egg was released (and subsequently fertilised) while the woman was already pregnant with the first fertilised egg.

So when the twins were born, they share the same birthday, but have different ages! ("Gestational age", the article calls it.)