That is, very few of the cloned animal embryos implanted in a surrogate mother survive. The others either die in utero, sometimes at very late stages of pregnancy or die sometime later. It is simply unacceptable to subject humans to those risks. Rudolf Jaenisch, a leading cloning expert at MIT's Whitehead Institute, agrees. I don't believe there's a single normal clone in existence.
All clones have some subtle defects.
If the defects are serious, they die early in the development. If they are less serious, they go to birth and die at birth. The apparent reason, Jaenisch told lawmakers, is that the process in which the eggs must reprogram the newly inserted genes is greatly speeded up in cloning. That can lead to a variety of deformities and other problems. In normal development, also, reprogramming has to occur.