In cases where there is an immunosuppressed person in the household, the killed vaccine is recommended.
I'm unsure as to whether or not you have had chicken pox. If you have had chicken pox (also called varicella), there is no risk in giving the vaccine to your son now. You should not get varicella again, so there is no risk of it affecting the baby before she is born.
If you have not had chicken pox then there is basically a theoretical risk that you could get chicken pox after your son is vaccinated. Only three people with normal immune systems (out of 15 million doses of chicken pox vaccine given so far) have developed chicken pox after being exposed to a child who was recently vaccinated. And keep in mind that if your son doesn't get the vaccine now, and then develops chicken pox in the next five months, you almost certainly will get it too, and that could cause significant problems with the pregnancy. There is no official recommendation to avoid giving chicken pox vaccine to the children of pregnant women.
As to the issue of putting your son in jeopardy, there is no harm in delaying the polio vaccine, as there have been no cases of polio in the Western Hemisphere in many years. If you were to travel to a developing country, then it would be very important that he have the vaccine beforehand.
While measles is not common, there are outbreaks in various parts of the country periodically, usually when there are large segments of the population that have not been vaccinated. If your child is in day care, or exposed to other children on a regular basis, then there is a small risk that he could get measles if his MMR vaccine is delayed. Certainly if there have been cases of measles in your community recently, it would be very risky to not immunize him. Rubella is even less common, however if your son were to get the disease, it could be disastrous for your pregnancy if you also got it.
In summary, in a household where all the members are healthy and have normal immune systems, there is no reason to postpone giving live virus vaccines (MMR and varicella) simply because there is a pregnant woman present. If anything, giving the vaccines to the child will make it less likely that he will get that disease and bring it home to the mother. The pregnant woman should not be given a live virus vaccine herself, but the household members should get their immunizations on schedule. Both the American Academy of Pediatrics and the Centers for Disease Control publish specific recommendations about all vaccinations, if you or your physicians have further questions.
