Haig notes that:
Pregnancy is absolutely central to reproduction, and yet pregnancy doesn't seem to work very well. If you think about the heart or the kidney, they're wonderful bits of engineering that work day in and day out for years and years. But pregnancy is associated with all sorts of medical problems. What's the difference?
The difference, he has concluded, is that the heart and the kidney belong to a single individual, while pregnancy is a two-person operation. And this operation does not run in perfect harmony because mother and unborn child engage in an unconscious struggle over the nutrients she will provide it.
The New York Times has more here.