Toximia baby too small when due in four more weeks

My sister was told by the doctor that she has toximia. This is her first pregnancy, I know that it is most likely during the first pregnancy. However, the doctor just told her today that the baby is too small in her womb and she is due in about four more weeks. She is to go back to the doctor tomorrow to have a sonogram done to see how the baby is being affected. I just wanted to be prepared and see what kind of things to expect...I won't be going to the doctor tomorrow with her, so I wanted to see if I could find out for myself what might be the cause of the baby's weight being too small. Could the toximia be the reason, or would that be due to something else!? She doesn't smoke or anything like that, she has been eating healthy. I know my mom had smoked and had low birth weight babies, but that shouldn't affect how her children give birth.......atleast I don't think! My sister does have P.S.V.T.......I know that giving birth can trigger a reaction from that.....but I don't think that would cause the baby to be too small......I keep thinking about everything trying to evaluate it all......I'm not asking for definite answers, I know only the doctor can give me that, but if you have any information about how a baby.....and the mother can be affected by it, and I don't mean the symptoms, I know what those are......then please let me know!! I am aware also that it can be a lead to death...I don't think they will die during the birth, but I know it is always a possibility, and I just want to have as much knowledge about it as I can, good or bad!! I want to know! Please, and thank you!! If you are weary about posting info that might freak other people out, then please contact me personally through my e-mail: _[removed]_