Superfetation is the fertilization and subsequent development of an ovum when a fetus is already present in the uterus, a result of fertilization of ova during different ovulatory cycles and yielding fetuses of different ages. (Superfecundation, remember, is the fertilization of different ova within the same cycle.)
In other words, you can be pregnant and conceive a second child. Or, as in this recent case, you can be pregnant with twins and conceive a third child.
Superfetation is extremely rare in humans. There are only a handful of cases reported in the medical literature, most with a difference of three or four weeks between fetuses. The babies are typically delivered together.
The case of the Italian woman who claimed to have conceived triplets three months into her singleton pregnancy impressed even medical professionals, although it turned out to be nothing more than a hoax.
So if you're disappointed after your first ultrasound that you're not carrying twins, take heart - the possibility is not completely ruled out.