The Germinal Period

An estimated 60 percent of all zygotes do not grow or implant properly and thus do not survive the germinal period. Many of these organisms are abnormal; few women realize they were pregnant.

The Embryonic Period

About 20 percent of all embryos are aborted spontaneously. This is usually called an early miscarriage, a term that implies something wrong with the woman, when in fact the most common reason for a spontaneous abortion is a chromosomal abnormality.

The Fetal Period

About 5 percent of all fetuses are aborted spontaneously before viability at 22 weeks or are stillborn, defined as born dead after 22 weeks. This is much more common in poor nations.

Birth

Because of all these factors, only about 31 percent of all zygotes grow and survive to become living newborn babies. Age of the mother is crucial. One estimate is that less than 3 percent of all conceptions after age 40 result in live births.