| 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 poor 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. |