This web site says that zovirax has been partly studied in pregnant women and animals and does not appear to cause birth defects but that these studies were neither large, nor conclusive.
So this certainly doesn't say conclusively that zovirax is 100% safe, but what is? I think weighing up the possible risks vs benefits is a personal thing. And for me, the 3rd bout of cold sores in 10 weeks is the end of the line. I suffered through the first two and at this point I just can't do it again.
Unknown side-effects not withstanding, I think it's definitely a benefit for the baby to not have a feverish, stressed-out mother every two weeks. Also I would think that any possible side effects would be lessened with a topical application as opposed to an oral since the medicine is delivered straight to where it was needed. I'm also asthmatic and the obstetrician told me that this is why asthma medicine is condidered safe in pregnancy - because it's delivered straight to the site.
Everyone has to make their own choice about what you deem safe or not in pregnancy. And certainly nothing is without risk. But for me, the advice of the obstetrician weighed with my absolute desperation to get these things out of my system is enough.
Yoyo - Zovirax is definitely better at preventing cold sores from getting nasty if you get them at the beginning. But it also speeds up the healing and dries them out if use it after. Again, I think this is because it acts against the virus that travels along the nerves to the cold sore. Any way, for me it's the only thing that ever works.