Sweetie your employer is being horrid to put it honestly. Someone who has been pregnant themselves (assuming your employer is the Mum?) should realise that you have to be careful with lifting etc, your body is telling you your limitations and you need to listen to it. Honestly. Please don't let them walk all over you, the law is on your side and you do not have to risk your health, or your babies, for someone who needs to find their humanity.