Understanding Psychology (10th Ed)

zygote
The new cell formed by the 
union of an egg and sperm.
embryo
A developed zygote that has a 
heart, a brain, and other organs.
fetus
A developing individual from 
eight weeks after conception until 
birth.
age of viability
The point at which a 
fetus can survive if born prematurely.
The complex sequence of genes can be 
harnessed in gene therapy in which 
genes are used to correct health 
problems.
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 Module 
36 
Prenatal Development: Conception to Birth 
389
point at which it can survive if born prematurely, at about prenatal age 22 weeks. At 
prenatal age 28 weeks, the fetus weighs less than 3 pounds and is about 16 inches long. 
It may be capable of learning: One study found that the infants of mothers who had 
repeatedly read aloud The Cat in the Hat by Dr. Seuss before the infants’ birth preferred 
the sound of that particular story to other stories after they were born (Spence & 
DeCasper, 1982; Schenone et al., 2010).
Before birth, a fetus passes through several sensitive periods . A sensitive period is 
the time when organisms are particularly susceptible to certain kinds of stimuli. For 
example, fetuses are especially affected by their mothers’ use of drugs during certain 
sensitive periods before birth. If they are exposed to a particular drug before or after 
the sensitive period, it may have relatively little impact; if exposure comes during a 
critical period, the impact will be signifi cant (Konig, 2005; Werker & Tees, 2005; 
Uylings, 2006). 
Sensitive periods can also occur after birth. Some language specialists suggest, 
for instance, that there is a period in which children are particularly receptive to 
developing language. If children are not exposed to appropriate linguistic stimuli, 
their language development may be impaired (Innocenti, 2007; Sohr-Preston & Scar-
amella, 2006). In the fi nal weeks of pregnancy, the fetus continues to gain weight 
and grow. At the end of the normal 38 weeks of pregnancy, the fetus typically 
weighs 7 pounds and is about 20 inches in length. However, the story is different 
for preterm infants, who are born before week 38. Because they have not been able 
to develop fully, they are at higher risk for illness, future problems, and even death. 
For infants who have been in the womb for more than 30 weeks, the prospects are 
relatively good. However, for those born before week 30, the story is often less 
positive. Such newborns, who may weigh as little as 2 pounds at birth, are in grave 
danger because they have immature organs; they have less than a 50-50 chance of 
survival. If they do survive—and it takes extraordinarily heroic (and expensive) 
medical intervention to assure this—they may later experience signifi cant develop-
mental delays.