The evolutionary biology of spontaneous abortion in humans

Human mothers possess 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 explanations have focused on a rising production of defective zygotes; in contrast, an evolutionary approach suggests a relaxed maternal screen. Relaxed screening potentially explains the rising incidence of chromosomal abnormalities in live-births, the falling incidence of normal embryos in spontaneous abortions, and the increased incidence of spontaneous abortions with maternal age. If true, then restoration of the screen may be possible, reducing the incidence of birth defects.