1. Section 3: Pregnancy, Development, and Birth
What are the changes that occur to the zygote, embryo, and fetus during development?
How is the developing embryo protected and nourished?
What happens during childbirth?
What changes occur as a person develops from infancy to adulthood?

2. Development Before Birth
The zygote develops into an embryo and then into a fetus. Differentiation leads to specialized cells, tissues, and organs.
Zygote

3. Development Before Birth
The zygote develops into an embryo and then into a fetus. Differentiation leads to specialized cells, tissues, and organs.
Zygote
Eight-Cell Stage
Hollow Ball

4. Development Before Birth
The zygote develops into an embryo and then into a fetus. Differentiation leads to specialized cells, tissues, and organs.
Zygote
Eight-Cell Stage
Hollow Ball

5. Development Before Birth
The zygote develops into an embryo and then into a fetus. Differentiation leads to specialized cells, tissues, and organs.
Zygote
Eight-Cell Stage
Hollow Ball
Stage When Folding Begins

6. Development Before Birth
The zygote develops into an embryo and then into a fetus. Differentiation leads to specialized cells, tissues, and organs.
Zygote
Eight-Cell Stage
Hollow Ball
Stage When Folding Begins
Embryo With Three Layers

7. Protection and Nourishment
The membranes and other structures that form during development protect and nourish the developing embryo, and later the fetus.

8. Birth
There are two types of twins: identical twins and fraternal twins.