The only proven way to stay lean during pregnancy is to follow a healthy diet composed of all the key nutrients your body needs.
Do your best to drink enough water every day. In general, aim to drink 6-8 glasses of water per day, or enough so that your urine is a pale yellow color.
The easiest and most fundamental form of exercise in pregnancy is walking. If it’s safe, you should be walking every day of pregnancy. Walking will help improve your total body circulation, activate and strengthen the muscles in your legs, and help you burn calories.