are low, whether looked at in terms of live births per embryo produced in the laboratory or live births per embryo transferred to the uterus (see Table 1). Note that the two highest percentages are derived from one experiment and are outliers; in this experiment, the numbers are small and half the newborns (four of eight) died soon after birth . In monkeys, reproductive cloning with adult nuclei has not been successful, but cloning with nuclei from the individual cells of several eight-cell embryos yielded 53 embryos for transfer; these resulted in four pregnancies, two of which gave normal offspring and two of which were lost [30; 31].
The results summarized in Table 1 and the cloning literature can be looked at from several points of view. It is clear that many healthy, apparently normal, clones have been born and have survived to fertile adulthood (for example, see [21; 27; 28; 33]). Dolly has given birth to lambs [34-36], and in the case of mice, six generations of clones have been produced serially, although the efficiency declined with succeeding generations . While some cloned mice may die soon after birth , one detailed follow-up of five surviving cloned mice revealed no serious problems, and the weight gain seen after several weeks might have been caused by non-cloning-related genetic effects . On the negative side, however, it is quite clear that across multiple species there are far more failures in the development of cloned fetuses than there are live normal births.
This low efficiency of cloning reflects, among other causes, a high rate of fetal loss after embryo transfer and implantation. Spontaneous abortion is also common in natural pregnancies, but there is a major difference in the timing of fetal and neonatal loss between animal reproduction based on reproductive cloning and reproduction based on in vitro fertilization (IVF). Whereas most fetal losses in conventional zygotic pregnancies occur in the first trimester, with reproductive cloning, fetuses are lost throughout pregnancy and in the early neonatal period [6; 8; 9; 13; 23; 24; 29; 32; 38; 39].
In humans, late gestational fetal loss causes increased maternal morbidity and mortality. Cloning studies in animals have shown that a high proportion of pregnancies involving cloned fetuses have abnormalities, including abnormal placentation, pregnancy toxemia, and hydroallantois—excessive fluid accumulation in the uterus often associated with fetal abnormality [14; 33; 43; 100; 101; 115]. Those pregnancy complications can cause fetal loss and risk maternal health. For example, in the cow-cloning study by Hill et al. (1999), four of the 13 pregnant mothers and their fetuses died because of complications late in pregnancy. Results of animal studies suggest that reproductive cloning of humans would similarly pose a high risk to the health of both fetus or infant and mother and lead to associated psychological risks for the mother as a consequence