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11.72: Pregnancy

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    Does a growing embryo always look like a tiny human?

    The above picture shows the very early stages of human development. Notice that, at first, the embryo doesn't look very human! It takes time for the human features to take shape.

    Pregnancy and Childbirth

    While a woman is pregnant, the developing baby may be called an embryo or a fetus. Do these mean the same thing? No, in the very early stages the developing baby is called an embryo, while in the later stages it is called a fetus. When the ball of cells first implants into the uterus, it is called an embryo. The embryo stage lasts until the end of the 8th week after fertilization. After that point until birth, the developing baby is called a fetus.

    Growth and Development of the Embryo

    During the embryo stage, the baby grows in size.

    • 3rd week after fertilization: Cells of different types start to develop. Cells that will form muscles and skin, for example, start to develop at this time.
    • 4th week after fertilization: Body organs begin to form.
    • 8th week after fertilization: All the major organs have started to develop.

    Pictured below are some of the changes that take place during the 4th and 8th weeks (Figure below). Look closely at the two embryos in the figure. Do you think that the older embryo looks more human? Notice that it has arms and legs and lacks a tail. The face has also started to form, and it is much bigger.

    Most organs develop in the embryo during weeks four through eight
    Figure \(\PageIndex{1}\): Embryonic Development (Weeks 4–8). Most organs develop in the embryo during weeks four through eight. (Note: the drawings of the embryos are not to scale.)

    Growth and Development of the Fetus

    There are also many changes that take place after the embryo becomes a fetus. 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 to make the baby a male or female. 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 28th week, the fetus is starting to look much more like a baby. Eyelashes and 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 28th week are usually able to survive. However, they need help breathing because their lungs are not yet fully mature. A baby should not be delivered prior to this time, unless absolutely necessary. A baby born prior to week 28 will need considerable medical intervention to survive.

    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 38th week, all of the organs are working, and the fetus is ready to be born. This is when birth normally occurs. A baby born before this time is considered premature.

    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 (Figure below).

    The fetus is surrounded by the fluid-filled amniotic sac and is linked to the mother by the umbilical cord
    Figure \(\PageIndex{1}\): 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.
    • The amniotic sac is a membrane that surrounds the fetus. It is filled with water and dissolved substances, known as amniotic fluid. 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.
    • The placenta is 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 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.

    Childbirth

    During childbirth, a baby passes from the uterus, through the vagina, and out of the mother’s body. Childbirth usually starts when the amniotic sac breaks.

    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.

    Summary

    • A zygote develops into an embryo and then a fetus.
    • During pregnancy, the amniotic sac, placenta, and umbilical cord develop inside the mother’s uterus.

    Explore More

    Use the resource below (printable version) to answer the questions that follow.

    1. What hormone does the mother secrete at the start of labor? What is the affect of this hormone
    2. What signals the start of "active labor"?
    3. What happens at the "pushing stage" of childbirth?
    4. What is the leading cause of maternal death in developing countries?

    Review

    1. What's the difference between an embryo and a fetus?
    2. What is the amniotic sac and amniotic fluid? Explain their roles.
    3. What is the placenta and umbilical cord? Explain their roles.
    4. What triggers a baby to start breathing after birth?

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