I lean towards the former. There are multiple reasons for this being a biological choice. One, women's childbearing period is more limited than men's, so it's rational for them to prioritize "taking care for the children" over "having a great career". Another is that because they give birth, they pretty much have to be absent from work (for a few days at least), whereas men don't, so again it's rational for a woman to take additional leave for childcare, so that only one parent gets penalized (by lower pay and less experience and future promotions) by taking time off. Of course, the latter only applies given everything else is equal; if the mother earns significantly more than the (step-)father, then it would be rational for her to take off only as many days an necessary for health reasons, with her partner providing the rest of childcare.
Impossible to tell with our current experimental setup. I lean towards the latter though.