I haven't done this, but a pregnant friend of mine is nervous b/c she lifted something heavy and was worried that she may have caused something to go wrong. She called her doctor and she's fine though.
I guess I'm sensitive on this topic b/c my first pregnancy was a m/c. While I was still pg, my MIL told me not to lift anything heavy and I told her that I had lifted a heavy bag of dogfood but wasn't worried b/c a m/c caused by heavy lifting during early pregnancy is an old wives tale. Her response was silence - which is what she does when she disagrees with me. Then I miscarried. She told me to be very, very careful next time.
Anyway... I guess if you're lifting heavy crap over and over it might be harmful, but just once?