This message is for Sandra. I am seventeen years old and have classical PKU. I have done an enormous amount of research on PKU on women in pregnancy. I also educate the young and the pregnant. I have found (through studies and doctors) that it is NOT good at all to be off diet when a woman is pregnant. This can effect the baby mentally (if it has PKU) even when in the womb of the mother. When the baby is born and mother has not been on diet, this can often times, result in mental retardation (if the baby has PKU) when they are born. Now, depending on the severity of how much the mother has been off diet, will be the severity of how much the babies brain will be damaged (if baby has PKU). Though its completely up to your sister, I would strongly recommend she stay on diet, as she may not know if her baby has PKU or not until it is born. Though I am only seventeen years old, I have seen a fair share of what most decisions can do to a child who was born with PKU.