Annabelle, may I ask, is there any particular reason why you are considering these tests? Do you have a particular risk factor that has been identified by other tests (eg nuchal translucency), or a preexisting genetic condition common to your family or your partner's?

Both CVS (chorionic villi sampling - ie: a small sample of the placenta is taken) and amniocentesis (sample of the amniotic fluid) carry a high risk of miscarriage - 1:100 for CVS and 1:200 for amniocentesis. CVS can be performed earlier in the pregnancy (towards the end of the first trimester), whereas amniocentesis cannot be performed until later in the pregnancy (from 16-20 weeks). Both are generally considered accurate although amnio can also identify spina bifida.

Due to the risk factor, neither test is considered standard, even for women above 35. If you are considering it, like SJH said, consider what implication a less than positive result would have and whether you would choose not to keep the pregnancy as a result.

Good luck!