Pregnancy contains three phases known as trimesters, according to the U.S. Department of Health and Human Services. Each trimester runs for about twelve weeks or three months. Although the phases of pregnancy are often thought of as three three-month periods, these phases have distinct month-by-month events. These stages may vary from person to person. Therefore, expecting mothers should always seek the supervision of a physician.
First Month
Starting out as a ball of cells, the embryo begins development of the brain, spinal cord, and heart. At about the end of the first month, the embryo may begin development of the the gastrointestinal tract. The length of the embryo is about 1/8 of an inch.
Second Month
The embryo begins to form a head, hands, and feet, according to the National Institutes of Health. Muscle growth occurs throughout the second month. By week eight, the heart of the embryo begins to beat. Ultrasound may detect this heartbeat. The length of the embryo is about an inch.
Third Month
Sex organs begin development in the third month. Fingers and toes become identifiable. The embryo becomes a fetus. By the end of the third month, the fetus develops vital organs. Chance of miscarriage significantly decreases. The length of the fetus is about three 1/2 inches.
Fourth Month
The fourth month of pregnancy marks a growth stage for the fetus. The fetus presents fingernails and hair. At this point, the fetus may appear to suck on thumb or curl fingers. The length of the fetus is about five 1/2 inches.
Fifth Month
The fifth month of pregnancy continues the growth phase from month four. However, the fetus develops rudimentary sensation. Until sensation develops, the fetus cannot perceive sensory stimuli. The length of the fetus is about 7.5 inches.
Sixth Month
Although the fetus continues to grow in the sixth month, the rate of growth significantly slows down. Nevertheless, the fetus acquires more control over body movement; the mother may feel this motion. In addition, the immune and digestive systems begin maturity. The length of the fetus is about 10 inches.
Seventh Month
By the time the fetus completes the seventh month, major organs become functional, including the brain, which continues to develop. Completing the seventh month of pregnancy marks a good chance of fetus survival. The length of the fetus is about 11 inches.
Eighth Month
During the eighth month, the fetus is nearing full development. The size of the fetus may cause it to decrease movement, such as kicking and twisting. During this month, the fetus is able to sleep, reaching consistent REM cycles. The length of the fetus is about 12 inches, from head to rump.
Ninth Month
The fetus may be called full term at about halfway through the ninth month. It is common for birth to occur after the ninth month, usually up to two weeks after the due date. When the fetus reaches the birthing process, he or she is considered a newborn baby. The length of the fetus is about 14 1/2 inches from head to rump or about 20 inches from head to toe.
