1.73. Pregnancy and Childbirth http://www.ck12.org
During the last several weeks of the fetal period, all of the organs become mature. The most obvious change,
however, is an increase in body size. The fetus rapidly puts on body fat and gains weight during the last couple of
months. By the end of the 38thweek, all of the organs are working, and the fetus is ready to be born. This is when
birth normally occurs.
The Amniotic Sac and Placenta
During pregnancy, other structures also develop inside the mother’s uterus. They are the amniotic sac, placenta, and
umbilical cord (Figure1.136).
FIGURE 1.136
This fetus is 38 weeks old and ready to
be born. Surrounding the fetus is the
fluid-filled amniotic sac. The placenta
and umbilical cord are also shown here.
They provide a connection between the
mother’s and fetus’s blood for the transfer
of nutrients and gases.- Theamniotic sacis a membrane that surrounds the fetus. It is filled with water and dissolved substances.
Imagine placing a small plastic toy inside a balloon and then filling the balloon with water. The toy would be
cushioned and protected by the water. It would also be able to move freely inside the balloon. The amniotic
sac and its fluid are like a water-filled balloon. They cushion and protect the fetus. They also let the fetus
move freely inside the uterus. - Theplacentais a spongy mass of blood vessels. Some of the vessels come from the mother. Some come from
the fetus. The placenta is attached to the inside of the mother’s uterus. The fetus is connected to the placenta
by a tube called theumbilical cord. The cord contains two arteries and a vein. Substances pass back and
forth between the mother’s and fetus’s blood through the placenta and cord. Oxygen and nutrients pass from
the mother to the fetus. Carbon dioxide passes from the fetus to the mother.
It is important for the mother to eat plenty of nutritious foods during pregnancy. She must take in enough nutrients
for the fetus as well as for herself. She needs extra calories, proteins, and lipids. She also needs more vitamins and
minerals.
In addition to eating well, the mother must avoid substances that could harm the embryo or fetus. These include
alcohol, illegal drugs, and some medicines. It is especially important for her to avoid these substances during the
first eight weeks after fertilization. This is when all the major organs are forming. Exposure to harmful substances
during this time could damage the developing body systems.