the unlock says you need to beat every chapter without heart producing babies, which I have done and it is still locked. what am i doing wrong?

I assure, because of the wording, that you don't need to do each chapter in a row in one run, which would be just fucking insane.

but do you? and are there any other babies other than chad that are consider heart producing babies?

You do need to do all chapters in a row.

Fortunately, there are only two heart-producing babies, and you unlock the second by getting that achievement.

the wording is really weird. i have completed the whole lap without chad just to be sure i get the achievement. getting imp from mystery gift and sanguine bond + isaac's fork helped me a lot