CHAPTER 4
PRENATAL DEVELOPMENT
Section 1- The Developing Baby
THE FEMALE REPRODUCTIVE SYSTEM
THE MALE REPRODUCTIVE SYSTEM
WHEN CAN CONCEPTION OCCUR?
CONCEPTION
• About once every 28 days, an ovum- an egg cell- is released
by one of a woman’s two ovaries. This occurs as part of a
woman’s menstrual cycle and is called ovulation. At the same
time, a woman’s body of releases specific hormones. These
prepare the uterus in the event that the ovum is fertilized. The
uterus is the organ in a woman’s body in which a baby
develops during pregnancy. It is a pear-shaped muscle able
to expand during pregnancy.
• During ovulation, the inner lining of the uterus grows and
thickens. If the over me is not fertilized, aligning breaks down
and passes out of the body. This is the bleeding that women
experience as a menstrual period.
CONCEPTION (CONTINUED…)
• When an ovum is released from ovary, it moves through the
Fallopian tube, which connects the ovary to the uterus. The
journey from the ovary to the uterus takes about two or three
days.
• When the ovum reaches the uterus, it usually disintegrates and
leaves the body with the menstrual flow. When a sperm, or
male cell, reaches the Fallopian tube, it may penetrate and
fertilize the ovum. This process is called conception. Pregnancy
begins at that time.
• An ovum usually lives 12 to 24 hours, while a sperm is capable of
fertilizing along for approximately 48 to 72 hours. During a
woman’s menstrual cycle, there are approximately three or four
days during which intercourse could lead to conception.
CONCEPTION (CONTINUED…)
THE GERMINAL STAGE
• The baby’s development during
pregnancy is called prenatal
development. It is often grouped
into three stages called the
germinal stage, embryonic
stage, and fetal stage.
• The germinal stage is the first
stage in a baby’s development.
It includes the formation of the as
the zygote, or fertilized egg. This
stage lasts only about two weeks,
but includes the key steps in
establishing a pregnancy.
THE GERMINAL STAGE (CONTINUED…)
• Cell division. While the zygote is still in the Fallopian tube, it
begins to grow by cell division. This single cell splits into two
cells, then the two cells rapidly multiply to four, then to
eight, and so on. After about four days of growth and slow
movement, the zygote reaches the opening to the uterus.
• Implantation. By this time, the lining of the uterus has
thickened enough to provide a place for the zygote to
attach itself and continue to grow. The zygote usually
implants in the lining of the uterus and is covered by that
lining. Despite the rapid growth of the zygote during the
two weeks after fertilization, is only the size of the head of a
pin.
THE EMBRYONIC STAGE
• The second stage of pregnancy is the embryonic stage. The
embryo is what the developing baby is called from about the third
week of pregnancy through the eighth week. During this time, the
embryo grows rapidly. It is also during this stage that several
important and amazing changes occur.
• Organs and body systems. First, the cells begin to separate and
develop into major systems of the human body- heart and lungs,
bones and muscles. These internal organs and their systems are
not ready to function yet, however. They continued to develop
throughout the pregnancy. Approximately 27 days after
conception, the neural tube, a tube in the back of the developing
baby that will become the brain and spinal cord, has closed. At
this point, the brain systems begin to take control of the various
body systems.
THE EMBRYONIC STAGE (CONTINUED…)
• Organs and body systems (continued…). By about the sixth the week
after conception, the connections between the brain and the spine
allow the first movements of the embryo. The developing brain is
sensitive to damage from any drugs or alcohol the mother might
take, especially at this crucial stage. It is vital that a pregnant
woman avoid these substances throughout her pregnancy.
• Amniotic sac. Second, a sac filled with fluid forms around the
embryo. This amniotic fluid protects the developing baby. (At the
peak of pregnancy there is approximately 1 quart of amniotic fluid in
the amniotic this point in development, the embryo is still verysac).
The amniotic sac is formed from special layers of cells in the uterus. It
cushions the embryo from any bumps or falls that the mother might
have. At small (about 1 inch or 2.5 cm long) and can float freely in
the in the amniotic fluid.
THE EMBRYONIC STAGE (CONTINUED…)
• The placenta and umbilical
cord. Third, a tissue called the
placenta develops. The
placenta is also formed from
special layers of cells in the
uterus. It is rich in blood vessels
and attached to the wall of the
uterus.
• The mother’s bloodstream
carries food and oxygen to the
placenta. The placenta’s job is
to absorb oxygen and nourishment from the mother’s blood to be transmitted
to the baby through the umbilical cord, which connects the baby to the
placenta. In addition, the umbilical cord takes carbon dioxide and other
waste products away from the baby and to the placenta, which releases
those wastes into the mother’s bloodstream.
THE EMBRYONIC STAGE (CONTINUED…)
• The umbilical cord is usually stiff and firm, like a garden hose filled with
water. It is generally not flexible enough to loop around the fetus,
although this may occur in rare cases. The placenta and umbilical cord
provide everything a baby needs until birth.
THE FETAL STAGE
• The third and final stage of development, the fetal stage, is also the
longest time. It begins around the eighth or ninth week of pregnancy
and lasts until birth. During this stage, the developing baby is called
a fetus. At this time, the buds for all 20 “baby” teeth appear. The
vocal cords develop, and the digestive system and kidneys begin to
function. By the end of the third month, spontaneous movements are
possible.
• Making movements. Sometime during the fourth or fifth month, the
kicks and other movements of the fetus touch the wall of the uterus.
These movements are faint and infrequent at first. These are usually
the first fetal movements that the mother can feel. She may feel her
baby’s movement as a kind of fluttering, like a butterfly. Gradually,
these sensations become stronger and more frequent, telling the
mother that she is indeed carrying the a live child within her.
THE FETAL STAGE (CONTINUED…)
• A pregnant woman’s doctor usually asks her when she first felt these
movements. This information helps the doctor estimate the baby’s
age and makes sure the baby is developing normally. This
information can also be used to help project an accurate due date.
• Completing development. During the last few months of pregnancy,
development continues, preparing the fetus to live independently. By
the seventh month, the baby is capable of living outside of the
uterus, but not without a great deal of medical help. The body’s
major organs become ready to function without any help from the
mother’s body. The fetus also gains weight rapidly. Fat deposits,
which will help the baby maintain body heat after delivery, are
formed under the skin. The fetus, which had been thin and wrinkled,
takes on a smoother, rounder appearance of a baby. During this
time, the fetus also stores nutrients and builds immunity to diseases
and infections.
THE FETAL STAGE (CONTINUED…)
• Staying active. The fetus can do a surprising
number of things- suck its thumb, cough,
sneeze, yawn, kick and hiccup. A fetus can
even cry. Even though the uterus is crowded,
the fetus is still very active and change
positions.
• Growing bigger. As the fetus grown, so does
the amount of surrounding amniotic fluid.
The uterus also expands, causing the
woman’s abdomen to grow. When the fetus
grows large during the last few months of
pregnancy, it no longer has room to stretch
out. It curls up inside the uterus in what is
called the fetal position.
PREPARING FOR BIRTH
• The common length of
pregnancy is about 40 weeks
or 280 days, from the first day
of the last menstrual cycle. By
this time, the fetus is fully
developed and can usually
survive outside the mother’s
body without a great deal of
medical assistance. Some
babies are born either a few
weeks early or a few weeks
late. This is generally not a
problem.
PHYSICAL CHANGES FOR THE MOTHER
• Toward the end of the 40 weeks, the baby’s weight seems to
shift downward, and most mothers feel more comfortable in
their upper abdomen. This shift is called lightening. Sometimes
there is a visible change in the shape of the mother’s abdomen,
giving signs that the baby has dropped into the birth canal- the
channel through which the baby passes during birth.
• In the majority of births, the fetus is usually upside down at this
point, with the head nestled in the mother’s pelvis. This is the
easiest and safest position for birth because the baby’s head is
the largest part of the body. If the head can be expelled from
the mother’s body easily, the rest of the body usually delivers
easily as well. The baby is generally less active because there is
very little space to move.
PHYSICAL CHANGES FOR THE MOTHER
(CONTINUED…)
• In some cases, the
fetus does not turn to
the head-down
position in the last few
weeks. Instead, the
fetus is in a seat-down
or a feet down position
in the mother’s pelvis.
This is called a “breech
presentation”.
• In many of
these
instances, a
doctor uses a
surgical
procedure
called a
cesarean
section to
deliver the
baby.
PHYSICAL CHANGES FOR THE MOTHER
(CONTINUED…)
PHYSICAL CHANGES FOR THE MOTHER
(CONTINUED…)
• The skin of the mother’s abdomen
appears stretched to capacity. The
muscles of the uterus and abdomen can
be stretched to many times their original
capacity during pregnancy. At the end
of the pregnancy, the muscles of the
uterus contract to expel the baby during
delivery. The mother’s abdominal and
uterine muscles generally return to nearnormal sizes approximately six weeks
after delivery.
SECTION 4-1 REVIEW
1) What cells join together to cause conception?
2) What happens during the germinal stage? How long does this
stage last?
3) What changes might a pregnant woman feel in the germinal and
embryonic stages?
4) What major changes take place in the developing baby during the
embryonic stage?
5) What are some characteristics of a fetus at five months’
development?
6) Why do babies born at full term have a better chance of survival
than babies born a few months early?
7) When a mother feels lightening, what does this tell her about the
fetus?