A birth defect is an abnormality present at birth. Also called a congenital defect, it can be caused by a genetic mutation, an unfavorable environment during pregnancy, or a combination of both. The effect of a birth defect can be mild, severe, or incompatible with life.
When I was a genetics fellow, one of the things I was often called to do was to go to the newborn intensive care unit to try to help evaluate a child who had been born with an obvious birth defect. And when we use the term birth defect, we're usually talking about circumstances where it is apparent at birth--a congenital heart defect, or a limb reduction, or some other malformation that's apparent often times in the delivery room. In reality, virtually any genetic condition could be called a birth defect, but we usually reserve the term for something that's immediately apparent on a physical examination, and we don't use birth defect to talk about something like, say, sickle cell anemia, Tay-Sachs disease, and cystic fibrosis, even though you could argue that those are present at birth, and they are problems with the DNA. They don't give you a visual signal of their presence, and so they're generally not given that term.