Building from the Basics
By the 12th week of development, the circulatory, nervous, and skeletal systems have become to form through this stage of fetal development. Her eyes and ears are beginning to take shape. Going into the second trimester, the fetus takes on more of a human form. It marks a rapid period of change in the life of the unborn child.
Fourth Month of Development
Going into the fourth month of pregnancy, the changes in growth and development during second trimester initiate a period of repaid development. the skeleton has started to become ossified, taking on its rigid skeletal form. With the skeleton, the joints connecting bones also develop. In these early stages, the fetus looks unnatural with a head that is disproportionate to the rest of her body.
Despite the odd shape, other human characteristics begin development including facial features and the growth of hair. Digits on the hands and feet are fully formed. The fetus will weigh just under 4 ounces at this point with a height of about 6.5 to 7 inches.
Fifth Month of Fetal Growth
During the fifth month, the body will grow, reducing the disproportionate appearance of the head. At this point, the mother may feel her baby move. The systems of the body are developing rapidly, including the muscular system which accounts for the movement.
Perhaps most exciting, by the end of the fifth month, it may be possible to determine the gender of your unborn baby through an ultrasound. Your baby will end the fifth month at about 0.5 to 1 pound and 10 to 12 inches long. Your baby will also develop a protective layer of fine hair called lanugo over the body.
Sixth Month of Pregnancy
Subtle changes in your baby's face happen during the sixth month. The once-fused eyelids will separate, and eyelashes will form. Your baby will begin to gain weight rapidly, making you appear obviously pregnant. Her skin will still be wrinkled, but her shape will appear more normal.
Your baby's heartbeat may be heard at this point. Her skin will change from its earlier transparent appearance to a more opaque color. Her weight will be about 1 ¼ to 1 ½ pounds, with a height of 11 to 14 inches. During the sixth month, your unborn baby is preparing for life on the outside with the initial production of white blood cells to fight infections and other disease-causing agent.
The changes in growth and development during second trimester result in more readily identifiable human characteristics. Most importantly, the unborn baby is being to take on traits which will define its appearance. For the mother-to-be, the second trimester is an exciting time of growth and first communication through fetal movement in the uterus.
Photo by Sam Hatch, stock.xchng
L. Sherwood. Human Physiology: From Cells to Systems. 2008.
G. Tortora et al. Principles of Anatomy and Physiology. 2005.
Web MD: Your Pregnancy Week by Week: Weeks 21-25 webmd.com