At how many weeks is it safe to have a prenatal massage?

12 weeks. Most massage therapists won't touch a pregnant woman until after her first trimester because this is the riskiest time for miscarriage. Once you are passed week 12 you should no longer lie on your stomach unless your therapist has a pregnancy pillow or table. If you lie on your back you should have a small pillow or towel under your hip/back. No deep tissue massages.
Anytime. There is never any harm to your baby from a body massage during pregnancy. Common sense would dictate that slight modifications in positioning during the massage you, using pillows as props would be helpful. Finally massages wonderful in helping to relieve low backache and fatigue in the muscles of the legs and feet.