Particularly in the early stages, symptoms of pregnancy can vary among women. And while the absence of menstrual periods is the classic and hallmark pregnancy symptoms, there are many other changes in the body, even during early pregnancy that result in characteristic signs and symptoms. Missing a menstrual period is typical, although many women report having a small amount of spotting or bleeding at the time of the expected period, a phenomenon that has been referred to as "implantation bleeding." If this bleeding occurs and is mistaken for a light menstrual period, it's possible for a woman not to realize that she is pregnant. Most of the early symptoms of pregnancy develop over the first two to eight weeks. Common symptoms can include breast swelling and tenderness. Food cravings, while typical of later pregnancy stages, can also begin in the early weeks of pregnancy. Morning sickness (nausea and/or vomiting, which can occur at any time of day and not just in the morning) usually begins in the second to eighth week of pregnancy. Tiredness, mood swings, headache, and frequent need to urinate are all symptoms that can begin early in pregnancy. Changes in skin color can include a darkening of the nipple area and facial skin color changes. The so-called "mask of pregnancy" is sometimes seen, particularly in darker-skinned women, in the first trimester. Medically known as melasma or chloasma, this is a darkening of the skin on the forehead, bridge of the nose, upper lip, or cheeks that usually occurs on both sides of the face. As the pregnancy progresses, symptoms can occur that are related to the size of the growing uterus. Some women report pressure or pain in the lower back and swelling of the feet and ankles. Frequent urination, heartburn, and indigestion are common in later stages of pregnancy. Sleep disturbances and fatigue can make this time period more difficult. As the delivery date nears, most women experience what are known as Braxton-Hicks contractions of the uterus. Believed to occur as a preparation for labor, these contractions manifest as a tightening feeling in the uterus that occurs sporadically. Unlike regular labor, Braxton-Hicks contractions do not occur at regular intervals and are shorter in duration and less intense than labor pains.