Hem-Onc Drs. specialize in treatment of blood disorders and cancer. They see many patients not cancer related, like iron deficiency, anemia, pregnant women with ITP, and other blood disorders, like sickle cell, etc. Often a referral by your OB is just for a second opinion, you may not even need treatment. Sometimes ITP is treated with steroids and with pregnancy it is most likely temporary and pregnancy may be the only time it shows up. In severe cases, that don't respond to conservative treatment, might require removal of spleen.
Don't hesitate to have that evaluation, one thing you don't want is something treatable to progress into a long term problem. Be glad that your OB is on top of things and astute enough to call in a specialist for something beyond his/her expertise.
RN