The second trimester begins with the sixteenth week of pregnancy. For the mother, many of the minor discomforts of the first trimester will disappear, and she will start to look pregnant and be able to feel the baby move. Good nutrition is important to ensure that both mom and baby gain weight at the right rate. In the fourth month, the baby weighs about six ounces and grows to be about eight to ten inches long. The umbilical cord continues to grow and thicken in order to carry plenty of blood and nourishment. Soon, the mother may start to feel a slight sensation of movement, like bubbles, in her lower abdomen. This movement, called 'quickening,' can actually help a doctor determine when the baby is due. By the fifth month, the baby will have grown to weigh around eight ounces, and is half as long as it will be at delivery. The doctor will now be able to hear the baby's heartbeat. This month, the mother may gain three to four pounds and begin to breathe deeper. The area around her nipples may look darker and wider as her breasts prepare to make milk. In the sixth month, the baby is a fully formed miniature with red, wrinkled skin and virtually no fat. The baby still needs to grow, being now only about twelve inches long and weighing about one and a half pounds. The baby sucks on its thumb, and the mother can regularly feel the baby's movement. Mom may gain three or four more pounds and experience fatigue or fluid retention, but gentle, regular exercise can relieve these symptoms to some degree. For more information on fetal development in the second trimester, contact a health care professional.