have you looked into finding a contractor for the work that needs done now. Also, your kidding yourself if you think someone with a new baby is going to pop right back into things with out missing a beat in a month.
^this
Managing a maternity leave is much different from the 'getting hit by a bus' scenario. if you have a key man risk and lose someone due to accident or resignation, you hire and deal with the pain of getting them up to speed. With maternity leaves, you're legally required to maintain that position for the person to come back to. Hiring a permanent staff isn't an option usually. Contractors are best bet, however for 1 month maternity leaves; it doesn't leave much time for the contractor to become proficient/worthy.
I wouldn't try pushing this person back into things right away. A start up is a big deal, and maybe even something you'll regret doing wrong. Missing time with your new born baby, that's something you'll really regret for a life time.