Before you say yes or no to anything, ask your doctor everything you want to know to reassure yourself. Doctors have to give you all the necessary information about amniocentresis, the procedure (how it's carried out and why, how long it takes) and the potential consequences (risks and precautions to take).
If you decide to have screening and you're clued up about it, you'll be relaxed and confident, so make sure you get your facts beforehand.
Before the amniocentesis, you need to sign a consent form to state that you'rve been informed about what it entails.