1. family and parenting
2. motherhood
3. pregnancy

Student text pages
1034-1039
SECTION
343 Fetal Development
KEY CONCEPT
Development progresses in stages from zygote to fetus.
A zygote takes about nine months to develop from a single cell into a
fully formed human being. The rate of growth in the first few weeks is
amazing. If you grew at the same rate after birth, you would be 4 meters
(13 ft) tall at one month of age!
The fertilized egg implants into the uterus and is
nourished by the placenta.
After fertilization, the zygote continues its journey through the fallopian
tube. As it travels, it divides through the process of mitosis.
Implantation in the Uterus
VISUAL VOCAB
The zygote continues to divide until it has become a ball of cells,
called a blastocyst. The blastocyst attaches, or implants, itself into the
uterine lining. Then it continues to grow. It develops three cell layers:
the ectoderm, the mesoderm, and the endoderm.
The ectoderm develops into the skin and nervous system. The
mesoderm forms many of the internal tissues and organs. The
endoderm develops into several digestive organs and their lining.
Once these layers appear, the ball of cells is called an embryo.
The blastocyst is a hollow ball of
cells that implants in the uterus.
Embryonic Membranes
As the embryo develops, membranes form to nourish it and keep it
safe. One membrane, the amnion, fills with fluid and is called the
amniotic sac.' This sac cushions the embryo and protects it from sudden
changes in temperature. Another membrane, the chorion, also forms to
nourish the embryo as it develops. •
Together, the chorion and the lining of the uterus form an important
organ called the placenta The placenta allows mother and embryo to
exchange oxygen, nutrients, and wastes. The placenta is connected to the
embryo through the umbilical cord. The umbilical cord carries nutrients
and oxygen from the mother's blood to the embryo. It also carries the
embryo's wastes back to the mother, who can then excrete them in her urine.
The placenta ensures that the blood of the mother and her baby flow
past each other but do not mix. If proteins from the embryo entered the
mother's circulatory system, her immune system might attack them as
foreign invaders. This attack could end the pregnancy.
Highlight the structures that protect and nourish an embryo as it
e develops.
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lining of
uterus
A zygote develops into a
fully formed fetus in
about 38 weeks.
Human pregnancies can be divided
into trimesters; or three periods that
are each three months long. During
those months, hormones help to keep
the pregnancy going and stimulate the
embryo to develop.
The amniotic sac, placenta, and umbilical cord
connect the fetus and the mother.
placenta —____
umbilical cord —
uterus --amniotic sac
First Trimester
In the first trimester, cells are rapidly
_
dividing and forming the specialized
tissues and organs that make up the body. The embryo can be easily
damaged by genetic mutations and by the mother's poor nutrition. It
can also be damaged by any toxic chemicals, such as drugs and alcohol,
that the mother may consume.
Even this early in the pregnancy, the embryo begins to look like a
small human being. At five weeks, its heart begins to beat. At nine weeks,
the embryo has developed enough to be called a fetus.
Second Trimester
In the second trimester, the fetus continues to develop and becomes
more active. The mother can feel the fetus moving its arms and legs. Its
heartbeat can be heard with a stethoscope, and its fingers and toes are
fully formed. The mother's uterus expands enough that she begins to
look pregnant.
Third Trimester
In the third trimester, the fetus grows to its largest size as the time for
birth approaches. In the last month, its lungs fully develop so that it can
breathe on its own once it leaves the mother's body. A baby born a week
or two early can easily survive. However, a baby born toward the end of
the second trimester has a much harder time surviving because its lungs
are not yet fully formed.
'etr‘
als ir
Why do you think a fetus is most at risk during the first trimester?
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569
The mother affects the fetus, and pregnancy
affects the mother.
Throughout the pregnancy, the mother and fetus continually affect each
other's health.
Health of the fetus The fetus gets all of its nutrients from the mother, so
it is important that the mother eat well during the pregnancy. For example, nutrients such as folic acid help prevent defects in the spinal cord. The
mother should also avoid drugs and alcohol, because these substances can
cause birth defects.
Health of the mother To supply enough energy for herself and her
developing baby, a mother needs to add about 300 more Calories a day
to her diet. Most mothers gain about 12 kilogram (26 lbs). Gaining too
much or too little weight can interfere with the fetus's development.
The mother's health is also affected by hormones during pregnancy.
Sometimes these hormones make it hard for the mother's body to
maintain homeostasis. Women may develop morning sickness or even
pregnancy-related diabetes. To protect her health during pregnancy, the
mother should have regular medical checkups, eat well, and exercise.
/t-N
iss tr 41 Highlight one way that pregnancy can affect a woman's health.
eta:,
blasto cyst
umbilical cord
embryo
trimester
amniotic sac
fetus
Go back and highlight
each sentence that
has a vocabulary
word in bold.
placenta
Choose the correct term from the listfor each description.
1. the fertilized egg after three cell layers develop
2. the fertilized egg after nine weeks
3. three months of pregnancy
4. the structure that connects the embryo to the placenta
5. When is a fetus most at risk for birth defects?
6. Name two things a mother can do to protect her fetus.
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McDougal Littell Biology
Pregnant women
should talk to their
doctors before taking
any medication.
344 Birth and Development
SECTION
Student text pages
1040-1043
KEY CONCEPT Physical development continues through adolescence and
declines with age.
Birth occurs in three stages.
When the fetus is fully developed, the placenta can no longer provide
enough nourishment. It is time for the baby to be born. There are three
stages in the process of birth: dilation of the cervix, emergence of the
baby, and expulsion of the placenta. The physical changes the mother's
body goes through are called labor.
Dilation of the Cervix
Labor begins with regular contractions of the uterus. The
mother and fetus release a hormone, orytocin, that stimulates the muscles in the walls of the uterus. However, not all
contractions mean that the baby is about to be born. True
labor does not begin until the contractions are frequent,
intense, and painful. In the early stages of labor, the amniotic
sac may break, releasing amniotic fluid through the vagina
(also called the birth canal).
Labor contractions push the walls of the cervix apart. The
baby cannot be born until the cervix dilates, or widens, to at
least 10 centimeters (4 inches). This space allows most babies
to pass through. If the cervix does not dilate enough, a doctor
can deliver the baby by Cesarean section, or C-section. During
a C-section, the doctor makes an incision in the abdominal
wall and removes the baby.
Emergence of the Baby
The contractions of the uterus cause the baby to turn so that
its head is toward the cervix. (Sometimes babies are born feet
first, which is more difficult.) The muscles of the uterus push
the baby into the birth canal. Once the baby's head emerges,*
the rest of the body slips out easily.
Soon after birth, the baby begins to breathe on its own.
The hormone oxytocin stimulates the mother's breasts to
produce milk and makes her want to bond* with the baby.
STAGE 1
During the first stage of labor, the
uterus contracts, the cervix dilates,
and the baby turns head down.
STAGE 2
During the second stage, the baby
emerges from the mother's body.
* ACADEMIC VOCABULARY
emerges comes out
bond become emotionally attached to someone or something
Interactive Reader
571
Expulsion of the Placenta
When the baby emerges, birth is still not complete. The
placenta must detach from the uterus and be expelled
through the vagina. The uterus continues to contract, constricting blood vessels. This reduces bleeding from the uterus.
A nurse or doctor cuts the baby's umbilical cord, leaving a
few inches of it attached to the baby's abdomen. Later, the
umbilical cord dries up and falls off. It leaves behind a scar
called the navel, or belly button.
717-‘
in$ er•What role does oxytocin play in the birth process?
STAGE 3
In the third stage, contractions of the
uterus expel the placenta and help to
control bleeding.
Human growth and aging also occur in stage!
' Hormones do not stop affecting a child's growth after it is born. Instead,
they continue to regulate how a child grows and develops.
Infancy and Childhood
Infancy lasts from birth to age 2. At birth, a baby's ability to maintain
homeostasis is not fully developed. During the first year, the baby gets
better at controlling its heart rate, temperature, and breathing. Babies
grow dramatically during infancy. They learn to sit, stand, walk, and
speak a few words.
Childhood begins at age 2 and lasts until about age 12. Growth slows
down during this period. The nervous system matures, and children
develop more fine motor skills, such as writing. They also develop more
language and reasoning skills.
Puberty and Adolescence
At puberty, children begin to mature sexually. For girls, puberty begins
between the ages of 10 and 14. For boys, puberty begins between the
ages of 10 and 16. During this time, young people grow very fast, averaging 5 to 7 centimeters (2 to 3 inches) per year. The brain also develops
rapidly during this period.
Adolescence begins with sexual maturity. For girls, sexual maturity
occurs with ovulation and the beginning of the menstrual cycle. For
boys, it begins when sperm start to appear in the semen. Adolescents
continue to grow and become more coordinated. Their brains also
continue to develop, making more neural connections.
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McDougal Littell Biology
Adulthood and Aging
You might think that adulthood is a period of time in which humans are
fully developed and at their peak. This is partly true. Adults become
independent, and may raise families of their own. However, the body
continues to change as people age. The immune system begins to
decline, and the body produces lower levels of key hormones, such as
testosterone and estrogen. Women go through menopause, and men's
sperm count drops. The body's metabolism and digestion slow down.
Skin becomes thinner, bones lose calcium, and muscle mass decreases as
fat deposits increase.
The picture is not all gloomy, however. Scientists are finding that
genetics, lifestyle, and environment make a big difference in how people
age. Eating a healthy diet, getting lots of exercise, and continuing to learn
can slow down the effects of aging. Exercise helps the body maintain
bone and muscle mass. Learning new things allows the brain to continue
making new neural connections, just as it did during a person's youth.
tr
7-7—
Ists mn
Name one way that people can slow down the aging process.
Ulf
infancy
adolescence
childhood
adulthood
Go back and highlight
each sentence that
, has a vocabulary
word in bold.
Fill in the blanks with the correct term from the list above.
1.
begins with sexual maturity.
2. During
, people become independent and may raise
families.
3.
lasts from birth to age 2.
4.
lasts from age 2 until about age 12.
5. List the three stages of birth.
6. People grow the fastest during which stages of life?
Interactive Reader 573
Chapter 34 Review
.
1. Label the parts of the male reproductive
system in the drawing.
a.
a
2. Where in the female reproductive system
does fertilization of an egg usually take
place?
3. A fertilized egg matures in
a.
b.
c.
d.
a follicle.
the epididymis.
a fallopian tube.
the uterus.
4. The menstrual cycle's three phases are
5. What structure helps sperm to penetrate an egg's membrane?
a. vas deferens
b. acrosome
c. testes
d. tail
6. When the zygote becomes a ball of cells and implants itself into the
lining of the uterus, it is called a
7. What does the placenta do?
8. When is a fetus most at risk for developing birth defects?
9. How are the second and third stages of birth similar?
10. What is the difference between puberty and adolescence?
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