use your best judgement. I helped my sister carry a couch when I was pregnant, 'cause i too was moving house. Most pregnant women usually do something crazy like move house, so i'm sure hundreds do the lifting, and many have 2-3 yo's that they would lift while pregnant weighing anywhere from 10-17kgs. Don't overdo it, play the girly card and make the boys do the really heavy stuff. my sil was climbing around in maccas play equipment looking for a 2yo the other day and she is in her second timester.
A pregnant women is no where near as delicate as most people think. Just trust what your body is telling you.