I am 24 weeks pregnant & my son may have hand, foot, mouth disease. Is this harmful to my unborn baby?

See note. . Considering hfamd is a virus that is transmissible during the early stages of first recognizing the signs and symptoms. The virus is not commonly transmitted to adults so your chances of contracting this virus and affecting your pregnancy is unlikely.
Potentially. The entero/coxsackie viruses do not readily cross the placenta, although they have been rarely reported to cause placental abruption with dire results. They can lead to maternal dehydration and preterm birth. My recommendation would be to maintain as strict hygienic precautions as possible, minimize contact with your infected son and stay hydrated and rested for a few days/weeks. Notify physician!
Doubt a problem. This pattern is so common in childhood you likely had it and are immune.It is sometimes hard to know when you have had it, since you can get it with no spots or spots in several locations. It is passed during the fever in the saliva but can remain in stool for weeks after resolution. Normal hand washing and hygienic practices can reduce your risk.