The most likely abortifacient pills used in the 1920s would have been
ergotamine, a derivative of the fungus ergot, which commonly grows on rye and 
caused frequent outbreaks of ergot poisoning in the Middle Ages.

Ergot pills were commonly available over the counter in some areas of the
US well into the 1940s and 1950s.  In sufficient doses, the drug causes
muscular contractions.  Nowadays its chief medicinal use is to alleviate
migraine headache, as in smaller doses it helps the vascular system contract.  
Even so, I often experience muscle contractions especially in the lower
leg for the 24 hours or so after I take the drug.

John Parsons