The fact these innocent looking children can make you do anything with the power of their mind. Like by sticking your hand into boiling water, or cutting yourself open. You're aware of it the entire time.
The 'remains' of the stillborn aka David's "twin" in the remake. An alienesque bloated thing floating in a tube of liquid. Terrifying.
Dialogue from the remake suggests that it's happened before. It could always happen again.
The remake doesn't just suggest it...it outright says that this has happened before. And in at least two of the instances there weren't just ten children; there were twenty or even thirty children born at once. In the latter instance none of them survived very long after birth, but still.