Why do my hands get numb in pregnancy?

Swelling! When you double your blood volume in pregnancy, there is a lot of swelling. Some you see and some you don't. Swelling around the nerves in your wrist and elbow can squeeze the nerves. It is a temporary problem but really frustrating and it can be painful. Splints at night can sometimes help. It will resolved after pregnancy but may take a few weeks.
Carpal tunnel. Most common cause is nerve compression at the wrist due to fluid changes in pregnancy narrowing the carpal tunnel. This is annoying and perhaps causing poor grip, but usually fully resolves shortly after labor and delivery.