TABLE TABLE 2.3 Vulnerability During Prenatal Development
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, most often because of chromosomal abnormalities. This is usually called an early miscarriage.
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 developing nations.
Birth
Because of all these factors, only about 31 percent of all zygotes grow and survive to become living newborn babies. Age is crucial. One estimate is that less than 3 percent of all conceptions after age 40 result in live births.
Sources: Bentley & Mascie-Taylor, 2000; Corda et al., 2012; Laurino et al., 2005.