The first trimester, or first three months, of pregnancy is a critical time in a baby's development. For the first 11 weeks, the baby is called an embryo. The heart, lungs, and brain are beginning to develop, and the tiny heart starts to beat. During this time, the mother may not notice weight gain, but her breasts may become larger and feel tender. She may also have some nausea or 'morning sickness.' During the second month, the embryo becomes a fetus. Arms with tiny hands and fingers, and legs with the beginnings of knees, ankles, and toes start to form. Organs such as the stomach and liver also begin to develop. The head now seems very large compared to the rest of the body because the brain is growing so quickly. Tiny ears and the beginnings of elbows and knees appear. The mother still may not have a weight gain, but may tire more easily and need to urinate more frequently. In the third month, the baby will grow to be 3 inches long, weigh close to 1 ounce, and begin to show signs of its sex. Finger- and toe-nails will develop. The baby is now starting to move its hands, legs, and head, though the mother will not yet feel this movement. She will, however, gain about 3 to 4 pounds and begin to feel warmer than usual. The first trimester is an important time in a baby's development. Mothers should take special care to eat well, get adequate rest, abstain from tobacco, alcohol, and drugs, and take only those medications prescribed by a doctor. For more information on fetal development during the first trimester, contact a health care specialist.