Technically, the embryo doesn't develop until the end of the first week after the egg becomes fertilized. At the moment of fertilization, the fertilized egg is considered a zygote. The zygote contains all the DNA material that it needs to produce a human child. The zygote floats through the fallopian tubes, continuing to reproduce new cells as it travels. These cells join at the ends, becoming a circle that allows cellular development on the outside and the inside of the circle. If the zygote is going to split into two, thus allowing the development of twins, the split will usually occur before the pregnancy stage of embryo begins. Only after the zygote attaches itself to the uterine lining does the pregnancy stage of embryo officially start. The end of the pregnancy stage of embryo is usually about week 8 when the embryo officially develops into the fetus.

The embryo starts a process of differentiating its cells. This differentiation allows the cells to develop into the individual organs, including the skin, the liver, the heart, as well as developing into the buds that will later become the arms, legs and head of the fetus. If any major problems develop, for whatever reason, the embryo will be spontaneously aborted, causing a miscarriage. Many miscarriages occur during the first trimester of a pregnancy and many of them occur during this pregnancy stage of embryo, because the DNA recognizes that there is a problem with its own development.

During the pregnancy stage of embryo, the cellular differentiation works on the circular construction. The outside edge of the rounded embryo becomes the beginnings of the head and spinal cord. As the embryo develops, the circular shape becomes distorted and the other cells begin to form other parts of the embryo. Most of this development occurs within a relatively short period of time, considering the amount of differentiation that is necessary.

During the earliest weeks of the pregnancy stage of embryo, a woman might not realize she is pregnant. The embryo is so tiny, less than 1/5 of an inch, and the cells haven't differentiated, so the mother's body may not yet react noticeably to the embryo. As the cells differentiate, the embryo begins producing its own hormones. These hormones, combined with the mother's hormones, can cause mood swings or food cravings that the mother won't usually experience. As the embryo further develops, the mother's body has to adjust to the presence of the embryo. These adjustments might also result in morning sickness. The mother's body generally completes its adjustment to the pregnancy during the first part of the next stage of development: the fetal stage.

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