Have jsut found out today that my main supervisor is three months pregnant! Should I get a card or something? What is the done thing in this situation?

Save the card etc for after the event probably - it's still early days. Maybe just say 'congratulations' and ask whether the morning sickness is bothering her much . She may have bad days too, so prepare yourself! A lady likes to be complimented too, so the odd 'your hair looks nice today' would go down well as time goes on.