My son was possibly exposed to chicken pox just before we concieved, and came out with a rash etc.

We went to the GP, because if they get the booster as soon as they get the rash it is less severe etc.

When i found out i got pregnant the day after we were there (i meant, i concieved then, found out 10 days later), i asked for my CP immunity to be checked. I knew i had it from 1st pregnancy, but i got CP when i was 5 and very mild, so just to reassure me, she checked it.

My BT came back that my CP immunity was fine, but that i had be exposed to slap cheek, another viral infection the children had. I had to get retested as they weren't sure if i had it during early pregnancy, or before that (turns out it was before). Although slap cheek is differemt (higher risk of miscarriage, and other problem), those few weeks were very very stressful worrying about the baby being ok.

Chicken pox is different in that there is a minor chance if you have it early the baby can have birth defects...

Here is some info from the net:

What can happen to my baby if I get chicken pox while I'm pregnant?
Chances are good that no harm will come to your baby, but timing is a factor.

If you get chicken pox during the first half of pregnancy, there's a slight risk that your baby will get something called congenital varicella syndrome. This condition is characterized by birth defects, including skin scarring, malformed limbs, an abnormally small head, vision or hearing problems, and motor or mental developmental disabilities. A baby with congenital varicella syndrome may also suffer poor growth in utero.

If you do contract chicken pox, you'll have a detailed ultrasound at 18 to 20 weeks to look for signs of defects or other problems and at least one follow-up sonogram later to see how your baby is doing. You may also choose to meet with a genetic counselor to discuss the risks in your particular case and decide how you want to proceed.

If you get chicken pox in the second half of pregnancy but more than five days before giving birth, your baby will probably be fine. Here's why: About five days after coming down with chicken pox, your body develops antibodies to the virus and passes them to your baby through the placenta, offering protection that his own immature immune system can't provide.

If you develop chicken pox five to 21 days before your baby is born, he might develop chicken pox days after birth, but because of the antibodies he received from you, it's much less likely to be serious. (Some babies exposed to chicken pox in utero, particularly those exposed five to 21 days before birth, develop a case of shingles during infancy or early childhood without having had chicken pox after birth, but it's usually not serious.)

The most risky time to come down with chicken pox is between five days before giving birth and two days after delivery, because then your baby is exposed to the virus but doesn't have had time to receive antibodies from you before birth. In this case, he has a 30 to 40 percent chance of developing what's called neonatal varicella, or newborn chicken pox, which can be serious and even life threatening, especially if left untreated.

Fortunately, your baby's risk of a severe case can be greatly reduced if he gets a shot of varicella zoster immune globulin (VZIG), a blood product that contains chicken pox antibodies. He'll be given the shot soon after birth if your chicken pox showed up within five days of delivery or as soon as you discover your rash if it's within two days after delivery.

If your baby shows any sign of developing the infection — such as coming down with a fever or showing a rash of even a few spots — he'll be treated intravenously with the antiviral drug acyclovir.
Personally, i think if you were worried you were exposed to it (and lots of kids have it without knowing as they come out with a rash later etc), it will be a very stressful time for you..

I would have the immunisation, and at least you can relax about it, but thats just my personal opinion.. I remember the worry about the slap cheek incident, i wouldn't want to worry about something like that again, and quite frankly, there is enough 'normal' stuff to worry about in pregnancy without adding something like this ontop of it.

Good luck with making a decision.