It's estimated that a majority of human pregnancies naturally fail to come to term without the use of contraceptive or abortive techniques. Of those, about half of the failed gestations have no defects or abnormalities that would prevent them from coming to term.

PROF. SANDEL: Thank you. I have two questions about the rate of natural embryo loss in human beings. The first is what percent of fertilized eggs fail to implant or are otherwise lost? And the second question is is it the case that all of these lost embryos contain genetic defects that would have prevented their normal development and birth?

DR. OPITZ: The answer to your first question is that it is enormous. ...

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It's estimated that a majority of human pregnancies naturally fail to come to term without the use of contraceptive or abortive techniques. Of those, about half of the failed gestations have no defects or abnormalities that would prevent them from coming to term.

PROF. SANDEL: Thank you. I have two questions about the rate of natural embryo loss in human beings. The first is what percent of fertilized eggs fail to implant or are otherwise lost? And the second question is is it the case that all of these lost embryos contain genetic defects that would have prevented their normal development and birth?

DR. OPITZ: The answer to your first question is that it is enormous. Estimates range all the way from 60 percent to 80 percent of the very earliest stages, cleavage stages, for example, that are lost.

Testimony before Presidential Bioethics Panel, Thursday, 16 January 2003

The number of abortions in the US was 1.21 million in 2005 (down from 1.31 million in 2005, by the way). There are approximately 4 million births in the US annually (4,138,349 in 2005) This means that nature (or god, if you will) accounted for between 2.5 million and 3.3 million spontaneous abortions, more than twice as many that occurred through human intervention.