During an ultrasound a doctor uses a device in which sound waves detect bones and tissue in a woman’s uterus. Ultrasounds can be used to
confirm pregnancy, determine the gender of the baby, detect physical abnormalities or multiple fetuses, and evaluate fetal growth and health.
(Richard Nowitz, FPG International)
velops into sensory cells, skin, and the nervous sys-
tem. The middle layer, or mesoderm, becomes the
excretory system, muscles, and blood. The inner
layer, the endoderm forms the digestive system,
lungs, and thyroid gland.
By the end of the third week of development the
embryo’s heart is beating and its nervous system is
forming rapidly. After the fourth week the legs are
curled and the eyes have appeared as dark circles.
During the fifth and sixth weeks arms and legs can be
seen. After eight weeks all of the major body organs
are present. The liver is making blood cells, and the
kidneys are removing waste products. The mouth,
nose, eyes, and head are clear and distinct. The head
is roughly half the total body size at this time. Fingers
and toes are blunt, and ribs show under the fetus’
skin.
The eight-week time span of embryonic develop-
ment is a particularly vulnerable period in human
growth. Chemicals, drugs, hormones, or viruses pres-
ent in the mother’s system can very easily affect the
embryo, as is described in more detail below in the
section on ‘‘Prenatal Environmental Influences.’’
Fetal Stage
The fetal stage begins in the ninth week of preg-
nancy and continues until the birth of the baby, usual-
ly about thirty weeks later.
The thrill of the first-time mother feeling the
movements of the fetus in the fourth or fifth month
of pregnancy is unforgettable. By this time, the fetus
can open and close its mouth, swallow, and make cer-
tain head movements. It may even suck its thumb.
The fastest growth period for the fetus is the fourth
month, when it almost doubles in length, reaching six
inches (15.2 centimeters) from crown to rump. Limbs
become sensitive to touch, and a heartbeat can be
heard with a stethoscope, a thrilling and sometimes
mystifying experience for parents-to-be.
The fetus is becoming more of an individual dur-
ing this stage. Some fetuses move around a great deal
while others are relatively quiet. The fetus’ sense of
hearing has also begun to function during this period,
as evidenced by startled reactions to loud sounds.
After five months the skin of the fetus is fully de-
veloped. Hair, nails, and sweat glands are apparent,PRENATAL DEVELOPMENT 329