Philosopher Toby Ord wrote an article called "The Scourge: Moral Implications of Natural Embryo Loss" (The American Journal of Bioethics, 8(7): 12-19, 2008). This is an argument that comes up somewhat regularly, but Ord concocted a thought experiment to illustrate the argument. The argument usually goes as follows: Some 50% of pregnancies end in miscarriages, which means that thousands of "unborn humans" die naturally. So to be consistent, you should advocate that doctors try and find a cure for miscarriages so that you can save all those lives that are lost. Ord begins with a thought experiment:
"The Scourge struck swiftly and brutally. This terrifying new disease, more deadly than any before it, left no part of the world untouched. From the poorest countries in Africa to the richest countries of the West, it killed with equal, horrifying, efficiency. It struck quickly, killing most of its victims within a few weeks of onset, and silently, for there were no detectable symptoms prior to death. Before the Scourge, the global death rate was 55 million per annum. That is, all causes of death -- old age, war, murder, disease, and so on -- conspired to take 55 million lives each year. The Scourge changed this dramatically. It alone killed more than 200 million people every year. From that time on, more than three quarters of the deaths each year were due to the Scourge. Where life expectancy in the West had risen steadily over the past century to 78 years, it had now dropped to just 29. Perhaps worst of all, the effects of the Scourge were not felt equally across all members of society. It killed only the very young and innocent -- those who were completely powerless to prevent it.
Compared with the Scourge, all other problems seemed insignificant. The Scourge was the major issue of the age, and there was an overwhelming obligation on society to fight it. Other projects had to be put on hold and a major international effort directed towards loosening the Scourge's grip upon humanity."
Ord uses this thought experiment as an analogy to natural miscarriage. But right away, some disanalogies become apparent.
For one thing, not every union of sperm and egg results in an actual human child. Other entities can result from the sperm-egg union, such as a hydatidiform mole, a choriocarsinoma, or a "blighted ovum" (see my linked article for a brief description of each of those, and the source I drew those from). In that case, they will either be flushed out with the woman's next menstrual cycle or they must be surgically removed.
Also, we do attempt to prevent miscarriages! Our society tells pregnant women not to smoke or drink while pregnant, and advancements in medicine allow us to help unborn children now that it wouldn't have been possible to help fifty years ago.
But most strikingly, we are not necessarily obligated to save someone who dies naturally. And even if we were, the fact that people die naturally does not justify our killing them intentionally. People die of old age, but that doesn't justify murder. As my friend Josh Brahm likes to say, people die in natural disasters but that doesn't mean we can nuke cities.
So doctors should work to cure humanity of "the Scourge." But it doesn't follow from this that we have an obligation to save miscarried children (and frankly, it may not even be possible, as while we can extend someone's life through medical treatment, it is not possible to prevent anyone from ultimately dying).
I'll respond to the rest of the article in a later post, as he does argue further from his example of the Scourge. In fact, he argues that if we consider the unborn to be full human persons with full moral status that it leads to an absurd conclusion. For now, these responses are enough to show why Ord's thought experiment fails, and it's enough to show to the average pro-choice person why pregnant women miscarrying certainly does not justify killing the unborn intentionally.