A second conception some time after a prior one, by which two fetuses of different age exist together in the same female: often used figuratively. [emphasis added]
The last part is the key: a figurative use is the addition of a second thing before the first thing is brought to fruition.
This came up in a discussion of Obamacare:
… The new American mandatory insurance system has shortcomings that are not necessarily related to the idea of mandatory insurance - as instead with the bureaucratic superfetations of such a system engendered by the way the US Administration has planned it.
I like that idea: layers of bureaucracy being added onto existing layers of bureaucracy (perhaps, because the earlier layers aren’t working well).
I’ve always described bureaucracy as continuing revisions of a Frankenstein monster: it didn’t quite work the first time, so we’re going to staple on some new part and hope for the best.
Now I have a word for that: superfetation.