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.
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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.
Francis S. Collins, M.D., Ph.D.
Director, National Institutes of Health; Former Director, National Human Genome Research Institute
Dr. Francis S. Collins, director of the National Institutes of Health, is noted for his landmark discoveries of disease genes and his visionary leadership of the Human Genome Project, a complex multidisciplinary scientific enterprise directed at mapping and sequencing human DNA. Dr. Collins was the director of the National Human Genome Research Institute from 1993 to 2008. His research has led to the identification of genetic variants associated with type 2 diabetes and the genes responsible for cystic fibrosis, neurofibromatosis, Huntington's disease and Hutchinson-Gilford progeria syndrome. In 2007, Dr. Collins received the Presidential Medal of Freedom, the nation's highest civil award, for his revolutionary contributions to genetic research.