I had a chat with my doctor about this the other day, as I thought that my daughter had it (for those who haven't heard of it, it's also known as slapped cheek).
She said that I wouldn't have much doubt if it really were slapped cheek as the cheek would be a livid red colour, and that most adults have already had it so developed an immunity. She also said that even if I didn't already have immunity and did get it, it's not such a worry after 20 weeks. Although it does produce an increased risk of miscarriage before 20 weeks, it's not a major risk or a certainty. She said that there's nothing that they can prescribe or do, even if you do get it, it's best to just try to relax about it as being pregnant is stressful enough!
How is your cheek now?
Welly