From the first moment that the cells begin to divide and create an embryo, fetal development and the stages of pregnancy progress in an established sequence. When the egg is fertilized, a zygote develops. In this first stage, which takes place during the first two weeks of the pregnancy, the cells multiply and develop.
The next phase of fetal development and the stages of pregnancy occurs when the zygote becomes an embryo. During week three, the embryo starts to develop the buds that will become the arms and legs of the fetus. The digestive system also has rudimentary beginnings at this time.
By the fifth week of fetal development and the stages of pregnancy, the facial features have started to form. The embryo has started to produce hormones and the presence of these hormones stop the mother’s menstrual cycle. When the embryo becomes about an inch long, about the sixth week of the fetal development and the stages of pregnancy, the brain has begun to develop its component parts, facial features, toes and fingers are noticeable and a heartbeat can be detected in an ultrasound test.
During the next several weeks, the organs continue to develop and the features become more distinguishable. The genitalia form, though they can not be distinguished at this stage in an ultrasound. Bones are beginning to form and the muscles can contract. By the end of the first trimester, the fetus is approximately three inches in length and weighs only about an ounce.
During the second trimester of fetal development and the stages of pregnancy, the fetus continues to develop the characteristics that make it recognizable as a human baby. Development has progressed to the point that the fetus will start to move slightly, it is during this time that a mother will start to feel movement. As this period progresses, the fetus will develop waking and sleeping cycles that the mother will soon be able to detect and recognize. At about the 24th week, a normally developed fetus is about twelve inches long and weighs about 2 pounds. It would be able to survive, with the help of current medical technology, if it were delivered.
During the last weeks of fetal development and the stages of pregnancy, or the third trimester, the bones are developed, though they are still pliable. The fetus can inhale, exhale, and even cry. The eyelids open and the lungs continue to develop until the moment of birth. The fetus gains in size and moves less because there is less room to maneuver. Most of the movement occurs as the fetus shifts its position in preparation for birth.
At the age of 30 weeks, the fetus would be small and considered premature, but it could be capable of living on its own if it were delivered. The fetus is considered fully developed at the age of 38 weeks and would be fully capable of living on its own if delivered, though the normal delivery date is considered to be at the end of the 40th week.