Growth and Development of the Fetus
Table(22.1) also shows changes that take place after the embryo becomes a fetus. Compare
the 18th-week fetus with the 8th-week embryo. Some of the differences between them are
obvious. For example, the fetus has ears and eyelids. Its fingers and toes are also fully
formed. The fetus even has fingernails and toenails. In addition, the reproductive organs
have developed along male or female lines. The brain and lungs are also developing quickly.
The fetus has started to move around inside the uterus. This is usually when the mother
first feels the fetus moving.
By the 28thweek, the fetus is starting to look muchmore likea baby. Eyelashesand eyebrows
are present. Hair has started to grow on the head. The body of the fetus is also starting to
fill out, as muscles and bones develop. Babies born after the 28thweek are usually able to
survive. However, they need help breathing because their lungs are not yet fully mature.
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. Compare the pictures inTable(22.1) of the
fetus at 28 weeks and 38 weeks. Do you see how much chubbier the older fetus looks? By
the end of the 38thweek, all the organs are functioning, 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. They are shown inFigure22.7.
The amniotic 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 the umbilical 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 food during pregnancy. She must
takeinenoughnutrientsforthefetusaswellasforherself. SheneedsextraCalories, proteins,
andlipids. Shealsoneedsmorevitaminsandminerals. Inadditiontoeatingwell, themother
must avoid substances that could harm the embryo or fetus. These include alcohol, illegal