Twin Pregnancy Ultrasound

When a patient becomes pregnant with twins following an IUI or IVF cycle, we have been billing CPT 76817 for the early monitoring ultrasound on the first sac and 76817 -59 for the additional sac examined in the multiple pregnancy, during the same encounter. We have never had a problem getting paid for both ultrasounds done on the same day when the diagnosis is twin pregnancy. Recently, Horizon Blue Cross and Blue Shield has denied payment for the ultrasound done on the second sac stating denial is based on “payment methodology and guidelines” and that 76817 can only be billed once per encounter. The CPT book neither states that the code can or can’t be billed twice per exam. I have read the description of the other pregnancy codes that specifically state they can be used more than once per exam and they involve greater work then we can provide at this early stage of monitoring. Do you have any thoughts and is BCBS correct in denying payment for the second ultrasound exam?