320 CHAPTER 8
THE EMBRYONIC PERIOD Once firmly attached to the uterus, the developing organism is
called an embryo. The embryonic period will last from 2 weeks after conception to 8 weeks,
and during this time the cells will continue to specialize and become the various organs and
structures of a human infant. By the end of this period, the embryo is about 1 inch long and has
primitive eyes, nose, lips, teeth, and little arms and legs, as well as a beating heart. Although
no organ is fully developed or completely functional at this time, nearly all are “there.”
CRITICAL PERIODS As soon as the embryo begins to receive nourishment from the mother
through the placenta, it becomes vulnerable to hazards such as diseases of the mother,
drugs, and other toxins that can pass from the mother through the placenta to the develop-
ing infant. Because of this direct connection between mother and embryo and the fact that
all major organs are in the process of forming, we can clearly see the effects of critical peri-
ods, times during which some environmental influences can have an impact—often devas-
tating—on the development of the infant. The structural development of the arms and legs,
for example, is only affected during the time that these limbs are developing (3 to 8 weeks),
whereas the heart’s structure is most affected very early in this period (2 to 6 weeks). Other
physical and structural problems can occur with the central nervous system (2 to 5 weeks),
eyes (3 to 8 weeks), and the teeth and roof of the mouth (about 7 to 12 weeks).
PRENATAL HAZARDS Teratogens Any substance such as a drug, chemical, virus, or other
factor that can cause a birth defect is called a teratogen. Ta b l e 8. 2 shows some common
teratogens and their possible negative effects on the developing embryo.
One of the more common teratogens is alcohol. Consumption of alcohol during
pregnancy, particularly during the critical embryonic period, can lead to fetal alcohol
spectrum disorder (FASD), a series of physical and mental defects including stunted
growth, facial deformities, and brain damage (Dörrie et al., 2014; Esper & Furtado, 2014;
Rangmar et al., 2015). Exposure to alcohol in early pregnancy is the leading known cause
of intellectual disability (previously called mental retardation) in the Western hemisphere
(Abel & Sokol, 1987; Caley et al., 2005). So how much alcohol is safe to drink while preg-
nant? The answer is clearly “none!”
THE FETAL PERIOD: GROW, BABY, GROW The fetal period is a period of tremendous
growth lasting from about 8 weeks after conception until birth. The length of the devel-
oping organism (now referred to as a fetus) increases by about 20 times, and its weight
increases from about 1 ounce at 2 months to an average of a little over 7 pounds at birth.
The organs, while accomplishing most of their differentiation in the embryonic period,
continue to develop and become functional. At this time, teratogens will more likelyTable 8. 2 Common Teratogens
Teratogenic Agent Effect on Development
Measles, Mumps, and Rubella Blindness, deafness, heart defects, brain damage
Marijuana Irritability, nervousness, tremors; infant is easily disturbed, startled
Cocaine Decreased height, low birth weight, respiratory problems, seizures, learning difficulties; infant is difficult to soothe
Alcohol Fetal alcohol syndrome (intellectual disability, delayed growth, facial malformation), learning difficulties, smaller-than-normal heads
Nicotine Miscarriage, low birth weight, stillbirth, short stature, intellectual disability, learning disabilities
Mercury Intellectual disability, blindness
Vitamin A (high doses) Facial, ear, central nervous system, and heart defects
Caffeine Miscarriage, low birth weight
Toxoplasmosis Brain swelling, spinal abnormalities, deafness, blindness, intellectual disability
High Water Temperatures Increased chance of neural tube defectsSOURCES: March of Dimes Foundation (2009); Organization of Teratology Information Specialists (2011); Shepard, T. H. (2001).
embryo
name for the develoRing organism
from to weeMs after fertili\ation.
embryonic period
the Reriod from to weeMs after
fertili\ation during which the
maLor organs and structures of the
organism develoR.
critical periods
times during which certain environ-
mental influences can have an imRact
on the develoRment of the infant.
teratogen
any factor that can cause a Dirth
defect.
fetal alcohol spectrum
FKUorFGr ((#5&)
the Rhysical and mental defects
caused Dy consumRtion of alcohol
during Rregnancy.
fetal period
the time from aDout weeMs after con-
ceRtion until the Dirth of the DaDy.
fetus
name for the develoRing organism
from weeMs after fertili\ation to the
Dirth of the DaDy.