Unfortunately, the word 'cloning' has multiple meanings. When a single embryo is split, forming identical twins or triplets, this can be called cloning. But so can copying a single gene.

Some people object to making human embryonic stem cells by nuclear transfer for two reasons. One is that it currently requires destroying an embryo. The other is that it creates an embryo that presumably could be implanted into a woman and grow into a baby. This technique, called reproductive cloning, has been successful for a dozen or so mammalian species, but attempts still fail more often than they succeed, perhaps because placentas fail to form properly. One team recently reported vastly improved cloning efficiency in dogs: the team was able to create 167 embryos through nuclear transfer. These were transplanted into surrogate mothers, and three live pups were born. In earlier work, it took 1,095 cloned embryos to produce two live-born pups, one of which died just after birth.

Credit: Wilmut, I et al. Nature 385, 810 (27 Feb 1997)

Reproductive cloning for humans has been banned in several countries (though not formally in the United States), and mainstream scientists consider it unethical, not least because it would almost certainly result in many miscarriages and severely debilitated offspring.