In most cases my main concern is for the first trimester, but in some cases does extend al the way through infancy. I believe that if a baby's immune system is sheltered early on, that as it develops it will be stronger and more able to withstand later assaults. It makes sense to me to isolate a newborn from contact with anyone outside of the family, and to limit exposure to strangers the entire first year. While it is true that the immune system requires stimulation for proper development, any home no matter how clean is virtually swimming with microorganisms. These can provide the necessary challenge to an infant's system without overwhelming it, since the infant receives antibodies to these normal household germs via its mother's milk.
Many vaccines contain live viruses.
So while I was pregnant I considered it prudent to avoid contact with recently vaccinated babies and pets. Even though I have no data to back this up, it is my gut feeling that many of the chemical additives used in foods and the supposedly benign over the counter medications that are commonly taken by pregnant women, might serve to subtly undermine the baby's immune system, and so I am careful to avoid these too. The fact is we just simply don't have nearly enough data on the variety of environmental factors that influence health. We can't even begin to predict the combined affects of these factors or the synergies that might develop. None months is a very short time to be so cautious, but the consequences of not being cautious could last a lifetime.