<P> Pregnancy is typically broken into three periods, or trimesters, each of about three months . Each trimester is defined as 14 weeks, for a total duration of 42 weeks, although the average duration of pregnancy is 40 weeks . While there are no hard and fast rules, these distinctions are useful in describing the changes that take place over time . </P> <P> Minute ventilation increases by 40% in the first trimester . The womb will grow to the size of a lemon by eight weeks . Many symptoms and discomforts of pregnancy like nausea and tender breasts appear in the first trimester . </P> <P> Weeks 13 to 28 of the pregnancy are called the second trimester . Most women feel more energized in this period, and begin to put on weight as the symptoms of morning sickness subside and eventually fade away . The uterus, the muscular organ that holds the developing fetus, can expand up to 20 times its normal size during pregnancy . </P> <P> Although the fetus begins to move during the first trimester, it is not until the second trimester that movement, often referred to as "quickening", can be felt . This typically happens in the fourth month, more specifically in the 20th to 21st week, or by the 19th week if the woman has been pregnant before . It is common for some women not to feel the fetus move until much later . During the second trimester, most women begin to wear maternity clothes . </P>

When do females start showing signs of pregnancy