Hi M.,
As a pre/postnatal personal trainer that specializes in the stomach muscles post birth, I will tell you that SIT UPS are the absolute WORST thing you can be doing for your body. 98% of postpartum women have split stomach muscles (called a diastasis) and sit ups actually INCREASE the split in the stomach muscles making your diastasis LARGER and worsening the appearance of your abs. We call this the "mommy tummy."
There are some GREAT things you can do for your abs - great exercises that will close the gap and correct your diastasis. I see women all the time that have been told by surgeons that the only way that can correct their stomach postpartum is by surgery. I have worked with some women in this situation that have brought their muscles totally back together in EIGHT weeks.
Even my women who are exercise fanatics come to me and can't understand why their stomachs just don't look right. They all have diastisis; those stomach muscles DO NOT come back together on their own.
A great book is "Lose your mummy tummy" - its on Amazon. Also there are DVDs that go along with it. I am the only one certified in the state to train with this technique. I teach classes in Oswego and Orland Park currently and i train individually as well. PLEASE feel free to contact me with any questions. My email is ____@____.com information I just gave you is HARD information to find! Good luck!!!