Figure 22.7: 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.
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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 have major effects on the developing
body systems.
Childbirth
Duringchildbirth, a baby passes from the uterus, through the vagina, and out of the
mother’s body. Childbirth usually starts with the amniotic sac breaking. Then the muscles
of the uterus start contracting. The contractions get stronger and closer together. They
may go on for hours. Eventually, the contractions squeeze the baby out of the uterus. Once
the baby enters the vagina, the mother starts pushing. She soon pushes the baby through
the vagina and out of her body.
As soon as the baby is born, the umbilical cord is cut. After the cord is cut, the baby can no
longer get rid of carbon dioxide through the cord and placenta. As a result, carbon dioxide
builds up in the baby’s blood. This triggers the baby to start breathing. The amniotic sac
and placenta pass through the vagina and out of the body shortly after the birth of the baby.