[samspot]

They wouldn't be able to tell which babies were premies because of the cocaine, and which were because of other factors.

A study of only premies would be interesting, but might yield the same result as this study.

[clavalle]

Highly unlikely. Prematurely born crack babies are often in very, very bad shape.

The rate of premature birth among all pregnant women is around 4.3% (that includes smokers and drug users so the rate amongst healthy non-smokers is even lower than that). Amongst crack cocaine users the number is between 17–27%. So the prematurity issue is pretty significant. You might even say dominant.