There is no risk to your baby from your cold sore, and it doesn’t mean that your baby will be more likely to suffer from cold sores. The placenta is a barrier to infection. The antibodies do cross to the fetus and give some protection against future infections with the herpes virus.
The time when herpes infection can be a problem to a baby is when the mother has a first genital herpes infection in the last few months of pregnancy. The risk to the baby occurs as the baby (who hasn’t had time to acquire effective immunity) comes into contact with the virus as he or she passes through the birth canal. When a woman has genital herpes around the time of delivery, a caesarean section is the preferred mode of delivery. If a woman has recurrent infections with genital herpes, this is unlikely to be a problem for the baby as he or she will have acquired immunity to the herpes virus via the placenta. In this situation, unless the woman has obvious herpes infection at the time of delivery, vaginal delivery is allowed.