Pregnancy is when a woman has a developing fetus inside her. It is divided into three phases, called trimesters. Each trimester in pregnancy has its own significant milestones. The trimesters are divided as follows: First trimester: 0 to 12 weeks Second trimester: 12 to 24 weeks Third trimester: 24 to 40 weeks During the first trimester, both the mother's body and the fetus are changing rapidly. All the fetus' major organs and body systems are forming. The eyes, ears, and nose develop, and the limbs fully form. This is also the most vulnerable time for the fetus. Major organs and body systems can be damaged if exposed to drugs, radiation, tobacco, and chemical and toxic substances. Many changes are also occurring in the mother-to-be's body. The mammary glands enlarge causing the breasts to swell and become tender in preparation for breast-feeding. A woman's areolas (the pigmented areas around each breast's nipple) will enlarge, darken, and may become covered with small, white bumps called Montgomery's tubercles. The uterus is growing and begins to press on the woman's bladder, causing the need for her to urinate more frequently. Partly because of surges in hormones, a pregnant woman may experience mood swings similar to premenstrual syndrome. Increased levels of hormones may also cause "morning sickness," feelings of nausea and sometimes vomiting. The woman may also experience fatigue and digestive problems, such as heartburn, indigestion, and constipation. The second trimester marks a turning point for mother and fetus. The mother usually begins to feel better and will start showing the pregnancy more. The fetus has now developed all its organs and systems and will now focus on growing in size and weight. During the second trimester, the weight of the fetus will multiply more than seven times over the next few months, as the fetus becomes a baby that can survive outside the uterus. The fetus is developing reflexes such as swallowing and sucking and can respond to certain stimuli. Fingernails have grown on the tips of the fingers and toes, and the fingers and toes are fully separated. Hair is growing on the fetus' head. Eyelids are beginning to open, and the eyebrows and eyelashes are visible. The second trimester is the most physically enjoyable for most women. Morning sickness, extreme fatigue, and breast tenderness usually subside. The mother may be able to feel the movement of the fetus for the first time. The skin on the belly may itch as it stretches and there may be pain down the sides of the body as the uterus stretches. The lower abdomen may ache as ligaments stretch to support the uterus. A mother's nose may become congested, and she may experience nosebleeds. This is because of the increase in hormones that affect the mucous membranes in the nose. A woman may also experience varicose veins, hemorrhoids, backaches, and continued heartburn, indigestion, and constipation. The third trimester marks the home stretch, as the mother-to-be prepares for the delivery of her baby. The fetus is continuing to grow in weight and size, and the body systems finish maturing. The lungs are still maturing, and the fetus begins to position itself head-down. The brain continues to develop. In the third trimester, some women become increasingly uncomfortable as their due date nears. As the fetus grows in size and crowds the abdominal cavity, some mothers-to-be have difficulty taking deep breaths or getting comfortable at night for sleep. The mother may also experience increased skin temperature as the fetus radiates body heat; increased need to urinate; swelling of the ankles, hands, and face; leg cramps; stretch marks; and continued heartburn, indigestion, and backaches.