I am very fit. I got my body back.
The less you put on the less you have to lose afterwards. I have 3 DC. I put on less weight with each because I know how bloody hard it is to lose when sleep and time deprived. Sorry.
Rest as much as possible for the first six weeks. If you had a broken bone or a torn hamstring or your appendix out, you would need six weeks for your body to heal before you'd go out running. Same with the physical effects of pregnancy and child birth. The body must heal first.
Don't try to diet when sleep deprived. But don't stuff your face on chocolate biscuits either. Do online grocery shopping. Buy lots of healthy easy stuff. Don't have the crisps in the house in the first place. Make yourself a packed lunch so when you find three seconds to wolf down something, probably one handed, you've got something to grab that isn't biscuits.
Basically, make it easy on yourself to make good choices when you feel like shit.
Worry more about exercise than dieting.
I like that I can do some exercise for half an hour or an hour and it is done. It cannot be undone. I can hold the smugness all day, even if I later spend most of the day on my arse.
I can diet all day then eat loads of crap in the evening and I have undone all the day's dieting. Wasted. I hate that. It is more likely to happen when I feel like shit too.
Somewhere between baby's first smile and first giggle, start exercising. Just a walk outside with the buggy helps you both. Maybe do a DVD at nap time a few times a week. IMO classes seem like a good idea but are always a logistical nightmare.
Flattening my belly helped me feel more like myself. Try the Tupler Technique. It is bloody wonderful.
Nine months up. Nine months down.