At 12 weeks, the embryo is a fetus. According to the Endowment for Human Development, "a second huge burst of growth" has occurred between 11 and 12 weeks and increased its weight "by roughly 58 percent." The fetus is about 2-1/2 inches in length and weighs about 1/2 oz. It is fully formed and, with the head curling into the chest, shaped like a shrimp. The most critical portion of the baby's development has passed, and chances of miscarriage are remote.
Head and Neck
The fetal head is about half of the crown-to-rump length, resting on a well-defined neck. There's a little chin and tiny nose. Jaws are in place and tissues forming in the mouth, where its roof separates the oral and nasal cavities. Sucking muscles fill out the cheeks. A week earlier, the eyes were far apart on the face; now, they are closer together. Once low on the neck, each ear has found its permanent place on one side of the head. The basic structure of the brain is evident, its mass growing constantly.
Limbs
The arms have reached the point where they are almost normal in length, size and proportion to the body. The hands have wrist bones and interact continuously with the head, face, and feet. The fingers flex and occasionally form a fist. Their touch pads have formed fingerprints, and fingernails have arrived. The legs, ankles, toes and toenails still have some growing to do.
Heart and Lungs
External instruments track the heartbeat of the fetus. The tiny muscle moves about 145 to 165 beats per minute, depending upon the time of day or night, and the mother's activity. In a few days, it will be pumping several quarts of blood each day. The lungs develop steadily as the fetus inhales and exhales amniotic fluid vital to functioning of the air sacs within.
The Organs
The kidneys have formed and soon the fetus will be passing urine into the amniotic fluid of the uterus. Intestinal loops, which had expanded into the umbilical cord, have retreated back into the abdomen. Boys have formed testicles, and the girls have ovaries. Bone marrow grows from cartilage, muscles develop, skin begins to acquire the qualities of an adult. Sweat glands appear and body hair begins to grow.
Expert Insight
The fetus derives tastes and odors from the numerous chemicals crossing the placenta from the amniotic fluid. "The amniotic fluid surrounding the fetus bathes the oral, nasal and pharyngeal cavities, and babies breathe it and swallow it," according to the Association for Prenatal and Perinatal Psychology and Health. The association also notes that while the mother's abdomen does muffle sounds from outside, the fetus still lives in "a stimulating matrix of sound, vibration and motion." Studies have shown that sounds can have "a surprising impact" on the heartbeat, and music can stir changes in metabolism.
References
- Fetal Development: Week 12: Fingernails and Toenails Appear
- Child Development Institute: http://www.baby2see.com/development/week12.html
- Visible Embryo: Fetus begins to move around
- Endowment for Human Development: Prenatal Form and Function -- The Making of an Earth Suit
- MayoClinic.com: Fetal development: The first trimester
