Hi!
Relationship wise, it sounds to me like you're in a good spot. Having a guy who loves & supports you through your difficulties and whom you know will be a good father * provider for your child is a blessing!
With regard to the bi-polar disorder, have to exhausted all efforts in terms of medication? Your mother may be right about you being more susceptible to postpartum depression. Seeing a doctor and trying to find a drug that works (if at least for after the delivery) may be a good option to have or keep in mind.
with regard to the legg calve perthes disease, do you have a doctor for this? Ask your doctor how it will affect your pregnancy (if at all) and your OB GYN too. I'm sure they can find a good solution/compromise for you.
Good luck!