I know this is an old thread but peoples lack of understanding terrifies me (not meant in an insulting way)
Basically, a mother who has had a cold sore prior to being pregnant will have antibodies against the virus. If she then gets a cold sore whilst pregnant, the risk is minimal to the unborn baby because of the antibodies. If the mother has never had a cold sore before and gets one while pregnant, the risk is greater so go to the doctor right away (you may be able to get anti viral treatment).
When your baby is born, keep anyone who has a cold sore away from your baby, yes they may have had your antibodies passed onto them (if you've previously had coldsores) but why take the risk? If you've never had a coldsore and your baby is born, the baby will have no antibodies at all. Don't kiss a baby on the mouth or allow anyone else to. Ask people to wash their hands before touching your baby, as you should also be doing.
Cold sores can also be spread to other parts of the body, i.e, eyes, inside nose and genitals. Always wash your hands before going to the toilet if you have a cold sore (you may have touched it without realising) so this minimises the risk of passing to genitals. Use separate towels from other family members, keep toothbrushes separate whilst infected, don't share anything that goes near the mouth, i.e sampling food as you're cooking and using the same utensil you've just used to sample, sharing a drink, having a bite of chocolate bar etc....and if you can, carry hand sanitiser with you because other people may not take such precautions and you then touch something they've touched then it could be passed to you and then to your baby or other family members etc. For example, someone on the bus may touch their cold sore, then press the bell to get off, you sit where they sit and you also press the bell to get off.....virus spread! Once the virus is in you, it never leaves your body, you will forever have the herpes virus. It doesn't mean you'll ever have a cold sore breakout after your first ever one, or you may get one every 10 years, there is no rule to this, but once you have it, you have it for good.
I have and do suffer with the occasional cold sore and I'm very, very, very careful when I get one because I know how disgustingly contagious they are but sadly not everyone else is the same. I hope this helps anyone that might read this thread, however old it is, because the virus will never stop so hopefully this reaches and helps somebody today. I'm not a doctor or anything but this is common knowledge (not the bit about it affecting babies but general cold sore information) so I'm just putting out there what I know in order to help. I'm 6 weeks pregnant myself and I know how worrying these things are.