Answer:
Not all birth defects are detected at birth, such as, for instance, Spina Bifida Occulta, a mild form of Spina Bifida which is often not detected until the person has an X-ray or other test that reveals the presence of the defect. But even that can be detected at birth, if the child were to undergo such an X-ray or other test at that time. I know a person who was unaware that he had Spina Bifida Occulta until he had a spinal X-ray at the age of 65. Another common birth defect that sometimes is not detected at birth is a heart defect that causes a heart murmur, which may be detected later by a careful doctor while listening to the person's heart.