As a general rule, it's a good idea to avoid heavy lifting during pregnancy. According to the book "The Complete Idiot's Guide to Pregnancy and Childbirth," you should be able to lift objects of up to 25 lbs. with no problems. For occasional lifting--not constant carrying for long periods of time---some women can handle up to 50 lbs.
However, that depends on your physical condition and how much you were able to lift before pregnancy. When in doubt, don't lift; talk to your doctor to determine what's safe for you.
Pain and Injuries
During pregnancy, your muscles, ligaments and joints become weaker and looser. This makes it easier to get hurt, even when doing the same activities you did before pregnancy. Your lower back is especially vulnerable when lifting heavy objects.
Falls
As your pregnancy progresses, you’re more likely to fall. This is because your center of gravity shifts as your belly gets bigger. If you bend forward to pick up something heavy, you might lose balance and fall. Falling is very dangerous during pregnancy, especially in the third trimester. A hard fall could send you into premature labor or cause injury to the baby.
Safety
Sometimes you can’t avoid lifting or carrying objects during pregnancy. For example, you might need to pick up your toddler if she fell. "The Complete Idiot's Guide to Pregnancy and Childbirth" recommends that you cease heavy lifting--more than 50 lbs.--after the 20th week of pregnancy, so ask for help if you’re in your last trimester. Otherwise, make sure you bend your knees to pick up something, rather than bending forward and putting pressure on your back.
Expert Opinion
It’s not entirely true that pregnant women shouldn’t lift heavy objects. It all depends on a number of factors. According to women's health expert Dr. Judith Reichman in an article for NBC News online, pregnant women should reduce their heavy lifting, but not necessarily avoid it completely. Dr. Reichman recommends reducing your capacity by 25 percent.
This means that if you could safely lift 50 lbs.
before pregnancy, you shouldn’t lift anything over 37.5 lbs. during pregnancy.