I would suggest that you ask your provider to give you the "quad" test. The quad test examines your blood for four different substances, not just AFP (alpha-fetoprotein), and is much more sensitive than the AFP test alone. If your provider can't do the quad test it isn't available everywhere at least ask for a repeat blood test using triple screening (testing for three substances). The result of that test will give you a better idea if your baby is at risk for Down's syndrome or a neural tube defect like spina bifida. The question of whether to have an amniocentesis is an important one: Amniocentesis does carry a small risk of miscarriage, and although the risk is small that has to be a part of your decision.
Here are some things to consider when making your decision about amniocentesis: Amniocentesis can tell you more than the MSAFP, triple screen, or quad test. If you are going to be worried for the rest of your pregnancy, it might be worth the risk to get some answers about your baby's health. The odds are that your baby will be fine. If, however, a problem is found with your baby, you would at least have the rest of the pregnancy to prepare yourself for how to best help your child. Best of luck.