When a woman becomes pregnant, she and her partner may have questions about what is happening to her body and how the baby is developing. During the pregnancy stages, first trimester, the process starts with the fertilization of the womans egg and progresses to the point that the fetus is starting to resemble the baby it will become.

The entire pregnancy takes approximately 40 weeks. This period is divided into sections called trimesters, each of which is about 13 weeks long. In the pregnancy stages, first trimester, the womans body has to adjust to the changes created by the pregnancy. Most of these changes dont occur immediately.

In the very first days of the pregnancy stages, first trimester, the fertilized egg, called a zygote, travels through the fallopian tubes and finally attaches itself to the wall of the uterus. There, the zygote becomes an embryo and the embryo starts the process of differentiation so that the individual cells become different parts of the body. During this time, the basics of the spinal column, the head, heart, digestive system begin to develop. Little nubs that will become the arms and legs also begin to grow. Facial features are not readily evident, though a rudimentary eye can be seen by the end of the first month. At this stage, the embryo is only about an inch long.

During the second month of the pregnancy stages, first trimester, the embryo becomes a fetus. The organs of the fetus begin to work and it begins to produce its own hormones. These hormones, together with the hormones of the mother, can cause morning sickness to develop since the mothers body is not accustomed to the changes. Generally, it is after the fetal hormones begin to develop that the mothers menstrual periods stop. Even though all of these changes have taken place, the fetus is only about an inch long.

During the third month of the pregnancy stages, first trimester, the fetus completes the basic developmental processes. The mothers morning sickness will have stopped, and she will have started a noticeable weight gain. The fetus will have obvious arms and legs. Tiny little fingers and toes become evident. Teeth will begin to develop. The facial features are now distinct and the fetus would be recognizable as a human. The internal organs are much more advanced. The liver and the digestive systems will have begun working. The gender of the fetus has been established, though it may not be noticeable yet on ultrasound. At four inches long, the fetus is applying increased pressure on the bladder. It will apply pressure to the bladder and the woman will become aware of an increased need to urinate. This is natural and should not be a concern to the woman.