When you have to lift something heavy, do it correctly. Stand with your feet about a foot apart. Bend at the knees (not at the waist), and lift with your arms and legs, rather than with your back.
It is not true that you can harm your fetus by lifting something heavy. You cannot dislodge it or tear the placenta from the wall of the uterus. But it's still not a good idea to heave two-ton grocery bags into your car. Your back won't like it. During pregnancy, the usually stable joints of the pelvis begin to loosen up to prepare for childbirth. This, combined with your growing abdomen, will throw your weight off balance and cause you to counterbalance by walking with a curved lower back. This puts strain on the lower-back muscles that will be aggravated by heavy lifting.
Give your back a break. If you have a young child at home, now is a good time to get some use out of the stroller and let her practice walking more often. Put your groceries in several lightweight bags. Ask someone else to carry heavy suitcases or packages. Your back muscles will be strained even more in the pregnancy later on; try to ease their burden now.