The successes in animal cloning suggest to some that the technology has matured sufficiently to justify its application to human cloning. An in vitro fertilization specialist and a reproductive physiologist recently announced their intent to clone babies within a year's time (1). There are many social and ethical reasons why we would never be in favor of copying a person. However, our immediate concern is that this proposal fails to take into account problems encountered in animal cloning.

Since the birth of Dolly the sheep (2), successful cloning has been reported in mice (3), cattle (4), goats (5), and pigs (6, 7), and enough experience has accumulated to realize the risks. Animal cloning is inefficient and is likely to remain so for the foreseeable future. Cloning results in gestational or neonatal developmental failures. At best, a few percent of the nuclear transfer embryos survive to birth and, of those, many die within the perinatal period. There is no reason to believe that the outcomes of attempted human cloning will be any different. The few cloned ruminants that have survived to term and appear normal are often oversized, a condition referred to as “large offspring syndrome”(8). Far more common are more drastic defects that occur during development. Placental malfunction is thought to be a cause of the frequently observed embryonic death during gestation. Newborn clones often display respiratory distress and circulatory problems, the most common causes of neonatal death. Even apparently healthy survivors may suffer from immune dysfunction, or kidney or brain malformation, which can contribute to death later. So, if human cloning is attempted, those embryos that do not die early may live to become abnormal children and adults; both are troubling outcomes.