If the disease that your wife developed at 15 yrs of age is CHICKEN POX, then she cannot contract that now and she will be immune for life. But the problem is people label all exanthematous fever as chicken pox. Your baby is absolutely safe inside the womb. It will not get affected by chicken pox by any means. But if your wife develop that when she is pregnant, there is a 'slight' chance of some birth defects in the child only if it happens during the first three months. Wait till April 28 to May 5, of your wife doesn't develop it means she is immune. If she develops, give MEDICAL TREATMENT for that