According to Dr. Phill, pregnant women need to avoid lifting more than 25 pounds. This is because a woman's body is already burdened with additional weight, which throws off her balance. Adding even more weight increases her likelihood of falling.
On Women's Health, Raul Artal, M.D., says that another reason why pregnant women should not lift more than 25 pounds is the temporary diversion of blood flow away from their internal organs to their muscles when lifting heavy objects. This might prevent oxygen and nutrients from getting to the baby. Even if it is just for a short period, this is dangerous for the unborn baby.