when taken by a girl, the drug just causes them to become even more "girly" (physically speaking).
the first thing that happens to mahiro when the drug begins to wear off, is the genitalia reverting back to male ones. i don't want to even start imagining what will happen if the uterus happens to be occupied at that time.
best option (realisticly) is to wait for the drug to start wearing off, then for momiji to get pregnant.
though there's nothing to prevent them doing it just after the drug begins to wear off and before mahiro completely turns back, leading to some femdom girl X femboy action.