I've been currently babysitting my 2 1/2 year old sister which weighs about 50-60 Lbs, she is very hyper and is very hard to deal with, I put a lot of pressure on my belly and have to lift her many times to change her because she doesn't like to get her diaper changed and we are currently potty training her, but I'm scared because I've heard that a lot of lifting heavy objects while pregnant can result into preterm birth.

Without knowing you and how much you are already used to lifting in a non pregnant state, your height, weight, physical fitness... well you get the point we can't really say yes or no you should or shouldn't. That would really be for your doctor to really determine what you are capable of safely but here is some reading that might help you... Safe Lifting in Pregnancy

Are you sure she weighs 50-60 lbs? According to " what to expect when you are expecting" (considering the facts that we don't know your medical history, and physical make up, etc, so this is just an average explanation from a pregnancy book, and should be taken as just advice, and you should seek medical advice on the subject as well before taking my word on it)so long as your doctor has not specifically ordered you not to lift heavy loads/children, then you should be fine, so long as you use the proper lifting technics and don't over exert yourself. However, As Tawnynkids said, this is something you really need to discuss with your doctor, ESP if she really is 50-60 pounds. Try not to lift her more then you have to, if for no other reason then that it adds more strain to your back then it already has. Carrying around baby and pregnancy weight is already hard enough as it is, without adding another kids weight into the mix. Just use caution, and make sure you talk to your doctor about it. If it makes you uncomfortable, regardless of what the doctor or anyone else says, don't do it. You are the mommy, and you know your instincts. Follow them. Good luck to you hun.

Thanks, yeah she is 50 pounds she's pretty heavy.