After the first two months of development, the embryo is called a fetus. During the remainder of development there is an enormous increase in size, with weight increasing by a factor of 600. By the end of the first trimester (3 months), sexual differentiation has occurred, the fetus kicks and sucks its thumb, and breathing movements bring amniotic fluid in and out of the lungs. By the end of the second trimester, the fetus is 12 inches long, has hair, eyebrows, and eyelashes, and is moving a great deal. The third trimester is a time of maturation of the digestive and respiratory systems.
