It's really up to you to decide if you want to wait till after you get pregnant and have your baby. Realize though that post-partum hair loss does not occur immediately after having the baby. For example, my daughter is 7 months old and I am now shedding my hair.
I think your doctor said it wouldn't affect the transplants for two reasons: the hair that we lose postpartum is hair that we didn't lose during pregnancy (hence the gorgeous mane when we're pregnant). So, basically it's just hair that we would have lost regardless. Secondly, hair transplants are done with the follicles that are fully capable of growing hair so even if this did affect the new hair growth, chances are it would grow back just as your other hair does.
In the meantime, here are some tips on how to avoid additional hair loss:
Avoid tight hairstyles such as: ponytails, buns, and pigtails.
Be gentle with fragile hair
Avoid heated treatments on your hair
Keep your diet high in proteins, fruits, vegetables and whole grains