Fetal Development During Pregnancy

First Trimester

Gender is determined immediately upon conception, report doctors at Pregnancy.org. The chromosome that carries the male or female chromosome is in the sperm that fertilizes the egg. During the first 14 weeks of pregnancy, the fetus grows rapidly. Limbs and organs are formed. By the fourth week, Mayo Clinic doctors report that the baby's neural tube in the spine closes and blood begins pumping from the heart. Facial features begin to appear, as well as the passageways that will form the inner ear and the arches that will form the jaw. Stubs show up that will become arms and legs. The nostrils and eyes begin developing in Week 7, and by Week 8 the upper lip and nose form. By the end of the first trimester, a fetus should be approximately 3 1/2 inches long and weigh close to 1 1/2 oz.

Second Trimester

During the second trimester, the kidneys and heart continue to develop and the baby grows fingernails. A fine layer of hair forms over the skin. The baby's limbs will be fully developed during this period, and the gender of the child can be determined at approximately 14 weeks. Movement begins during this period, and the fetus begins to develop hearing and swallowing abilities. The baby may begin to move her eyes during the second trimester; facial muscles allow her to make expressions such as frowning and squinting. By the end of the second trimester, a fetus is expected to weigh between 2 and 2 1/2 lbs. and measure from 11 to 14 inches long.

Third Trimester

A baby has developed a bone structure made up of about 300 bones by the beginning of the third trimester. Mayo Clinic doctors report that about 90 percent of babies born prematurely at Week 28 or later will survive and be healthy. Muscle and fat begin to accumulate during this period, and the baby's hair begins to grow. By the 29th week, the baby begins to store calcium and iron. The lungs continue to mature until the time of delivery. Independent breathing begins around Week 32, and by Week 33, the baby can detect light. The baby gains weight rapidly after Week 36 and crowds the belly. In another week, the fetus is fully developed and the head may move into position for delivery. By the time the baby is born, she weighs in the neighborhood of seven lbs. and measures close to 20 inches.