A 28-year-old woman presents to the antenatal clinic at 12 weeks of gestation with a 3-day history of low-grade fever, maculopapular rash starting on the face and spreading downward, and mild arthralgias. She has no history of MMR vaccination. On examination, she has posterior cervical lymphadenopathy and a blanching rash on the trunk. Her rubella serology shows IgM antibodies positive with negative IgG. What is the most significant concern regarding her current pregnancy?