How A Fetus Develops in the First Trimester
In the first weeks after conception, the single cell rapidly divides into many cells. A hollow ball of cells is formed and becomes attached to the wall of the uterus (womb). Some of the cells become the placenta; the rest become the embryo. The latter group of cells develops into a four-layered disk. Each layer is converted into different areas of the body. The outer layer, or ectoderm, for example, develops into skin, hair, nails, and the nervous system. The inner layer, or endoderm, develops into the intestines and lungs. The middle layers develop into the heart, bones, and muscles.
By three weeks after fertilization, or about one week after the first period is missed, the embryo is 1/10 inch long and has an oval shape. In the next few weeks, it becomes more curved, and a head and tail become discernible. The beginnings of the spinal cord and brain take shape. A heart begins to form. Tiny eyes can be seen. Arms and legs begin to bud.
By the fourth week after fertilization, traces of all the organs of the body are present. Bulges that will become the ears and nose appear. The gut is formed from blind pouches within the embryo; these pouches push forward, creating an opening in the head that will become the mouth. A crude face begins to take shape. At this point, the embryo is only 1/4 inch long.
The embryo is called a fetus at the seventh to eighth week. It has grown to a length of one inch. The head is disproportionately large because of the size of the developing brain, while the abdomen seems large because of the growing liver. Fingers and toes appear. The rudiments of all the major hormone-producing glands -- the pituitary, thyroid, and adrenal glands -- are present. Amazingly, the tiny heart begins to beat.
By the end of the third month, the fetus is two to three inches long and weighs less than one ounce. Nails form on the fingers and toes. The bones begin to calcify. The sex organs begin to develop. The tooth buds form in the mouth. The fetus begins to make breathing movements and starts to swallow amniotic fluid. The muscles of the intestines contract and relax, as if digesting food. Skeletal muscles begin to work as well, so the fetus can move in response to local pressure.
After three months some women might not even look noticeably pregnant. However, during the second trimester the baby really starts to grow and develop. Learn about what changes to expect in the fourth through sixth months in the next section.
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