I lifted a heavy box while 13 weeks pregnant. can this cause miscarriage?

Pregnant & lifting: As the pregnancy grows, the uterus expands and changes the curvature of the spine making it more difficult to lift heavy objects without stress to the spine and uterine ligaments. Ask your OB MD what constitutes too heavy to lift. Generally, if you have to hold your breath to lift it, it's too heavy. If vaginal bleeding starts call your OB immediately for advice.