Hamal / Pregnancy is not an illness itself but a condition that can cause discomfort and trigger illness.
if carrying a baby and finds it difficult to qiyaam from sujuwd (or sit, stand or bend in anyway) then I am sure it is better she prays however she feels able rather than cause herself harm or worse miss the salaah itself. Islam is all about sense.
Maybe she can sit and pray and have the turbah/sajdagah elevated to her needs. Or use a piece of furniture to help her qiyaam/stand and juluws/sit.