Your concern is valid, as chickenpox during pregnancy, especially in the first trimester, carries risks for both you (like pneumonia) and the baby (like congenital varicella syndrome). Your first and most critical step is to immediately contact your obstetrician or midwife. Inform them of your exposure and that you are in your first trimester. They will likely advise you to get a blood test as soon as possible to check for antibodies against the virus. This test will determine if you are immune from a past, perhaps unnoticed, infection or a childhood vaccination.
If the test reveals you are not immune, there are protective measures available. Your doctor may administer Varicella-Zoster Immune Globulin (VarIZIG), a concentrated dose of antibodies against the virus. For this to be most effective, it should be given as soon as possible, ideally within 96 hours (4 days) of your exposure. This treatment can help your body fight the virus and significantly reduce the severity of the illness for you and, in turn, lower the risk to your baby. While this is a worrying situation, acting quickly by consulting your doctor gives you the best possible chance for a positive outcome.