The evolutionary biology of spontaneous abortion in humans.

Human mothers an efficient screening system to eliminate genetically abnormal embryos. However, the incidence of certain birth defects - most notably Down's syndrome - rises with maternal age. Conventional eplanations have focused on a rising production of defective zygotes; in contrast, an evolutionary approach suggest a relaxed maternal screen. Relaxed screening potentially explains the rising incidenee of chromosomal abnormalities in live-births, the incidence of normal embryos in spontaneous abortions, and the incidence of spontaneous abortions with maternal age. If true, then restoration of the screen may be possible, reducing the incidence of birth defects.