Fetal organs begin developing between weeks 4 and 8.
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From the time of conception until birth, a human fetus undergoes complex changes in its cells and tissues that ultimately produce fully functioning organs. Each developing organ is programmed to carry out its specific role in maintaining the baby's body after birth, but all form from just a few cells that make up the early embryo.
After fertilization, cells in the tiny embryo divide many times to form a ball of undifferentiated cells. Early in development, the ball of fetal cells flattens and cells organize themselves into three layers, called endoderm, mesoderm and ectoderm. A few days after fertilization, the embryo implants into the uterine lining. Once implantation occurs, the placenta begins forming and the fetal organs start developing. Generally, all major fetal organs establish themselves in primitive form between week 4 to 8 of pregnancy. This is one of the most critical developmental periods for the fetus.
During the third week, when the fetus is about the size of a poppy seed, a groove forms that extends from the fetal head region down the length of the back. Its edges move toward each other and fuse into a tube called the neural tube. The front portion of the neural tube develops into the brain, while the remainder becomes the spinal cord. The wall of the tube thickens during the next few weeks, and the front portion enlarges greatly to form the brain. By the end of the 7th week, the brain begins forming the specific regions. From this stage until birth, nerve cells in the brain and spinal cord multiply and take up their final positions in the nervous system. At birth, the fetal brain weighs about 14 ounces and resembles a miniature version of the adult brain.
The heart begins developing around the middle of the third week of pregnancy. It forms from 2 side-by-side, primitive tubes. These tubes fuse into a single tube, the heart tube. Other tissues are added to this tube as it thickens, grows and begins to bend and fold as it enlarges. By the fourth week of pregnancy, blood flows through the primitive heart, which begins contracting to send blood through early blood vessels to the entire fetus. As heart development continues, the walls of the folded tube thicken and certain parts of the folding tube fuse with each other. Several walls, each called a septum, also develop to divide the heart chambers from each other. By the end of the fifth week, the heart is made up of its final four chambers, and valves have developed to manage blood flow. By 20 weeks, nerves and other special tissues that control heart function develop fully, and the heart, now completely formed, continues to enlarge until birth.
Most of the fetal abdominal organs develop from a tube called the gut, which extends from the mouth region to the anus. The primitive digestive tract, including the stomach, small intestine and large intestine, begins forming by about week 4. The uppermost portion of the gut tube forms the esophagus, which connects to the primitive stomach. The next portion of the gut, connecting to the stomach, develops into the small intestine. Because it elongates rapidly, the intestines become folded and coiled within the confined space in the fetal abdomen. The final part of the gut develops into the large intestine, the rectum and the anal canal. By 6 weeks, all these parts of the digestive tract are recognizable and continue enlarging until birth. Other abdominal organs, including the pancreas, liver and gall bladder, begin as buds of tissue that appear along the gut between weeks 4 and 5. By birth, these organs are fully formed and functional.