The links on the left summarize the risks and benefits. As always, you should check with your healthcare provider before follwing medical advice, and this author assumes no responsibility for your actions.

Recommended book:

Guide to Effective Care in Pregnancy and Childbirth by Murray Enkin, et al. One of the best books I've found that describes the pros and cons (in a fairly unbiased manner) of each of the many prenatal tests.