The three trimesters of pregnancy are a way of dividing up the 40 weeks into smaller, separate stages when the mother and baby are encountering different types of pregnancy events.

First trimester pregnancy is the early stage of pregnancy from conception to 12 weeks gestation, or about 14 weeks from the first day of the last normal menstrual period (LNMP). During this stage of pregnancy you may experience the dreaded morning sickness (which can occur anytime during the day and sore and enlarged breasts.

During the first trimester, growth and development in mother and fetus cause many changes to occur. A woman may notice no period or a light period; blue lines under the skin over her breasts and abdomen; waistline expansion; breasts that grow larger, or protruding nipples.

Other common signs of early pregnancy in women include: nausea sometimes coupled with vomiting known as morning sickness; food aversions and cravings; heartburn and indigestion; fatigue; tender breasts; complexion problems; a need to urinate often ; constipation; headaches, dizziness, or faintness.

A healthy first trimester is crucial to the normal development of the fetus. The mother-to-be may not be showing much on the outside, but inside her body all the major body organs and systems of the fetus are forming. It is during this first trimester that the fetus is most susceptible to damage from substances such as alcohol, drugs, certain medications, and illnesses such as rubella (German measles). During the first trimester, both the mother's body and the fetus are changing rapidly. The most dramatic changes and development occur during the first trimester.

During the first eight weeks, a fetus is called an embryo. The embryo develops rapidly and by the end of the first trimester it becomes a fetus that is fully formed, weighing approximately 1/2 to one ounce and measuring, on average, three to four inches in length.

This second stage of pregnancy lasts until the end of the seventh month and is many times the easiest stage of pregnancy as most women will start to regain some of their energy. During the second trimester, continuing growth and development in mother and fetus cause many changes to occur. Women may notice the abdomen begins to swell.

By the end of the second trimester, the uterus is near the rib cage; the skin on the abdomen and breasts stretches. Stretch marks may be visible; movements made by the fetus. Known as quickening, this often occurs sometime around weeks 16 to 20; a dark line forming from the navel down to the middle of the abdomen; brown, uneven marks on the face or other changes in skin pigment; darkening of the area around the nipples.

Often, the second trimester delivers a welcome energy boost to women and an end to morning sickness. It may also bring nasal congestion and occasional nose bleeds; whitish vaginal discharge; less of a need to urinate often; constipation; increase in appetite; Braxton Hicks contractions in which the uterus becomes hard and tight and then relaxes; leg cramps; backache; fatigue. In the fetus, growth continues quickly from now until birth.

Organs such as the heart and kidneys develop further, eyebrows and fingernails form, the skin is wrinkled and covered with fine hair, periods of activity and quiet occur as the fetus moves, kicks, sleeps, and wakes. 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 umbilical cord continues to thicken as it carries nourishment to the fetus. However, harmful substances also pass through the umbilical cord to the fetus, so care should be taken to avoid alcohol, tobacco, and other known hazards. During the second trimester, both the mother's body and the fetus continue to grow.

The second trimester is the most physically enjoyable for most women. Morning sickness usually abates by this time and the extreme fatigue and breast tenderness usually subsides. These changes can be attributed to a decrease in levels of human chorionic gonadotropin (hCG) hormone and an adjustment to the levels of estrogen and progesterone hormones.

The third trimester of pregnancy generally spans weeks 28 through 40, though healthy babies may be born a bit sooner or later. Although most women undergo many of the same physical changes during this time, no two pregnancies are alike.

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 mother may feel more uncomfortable now as she continues to gain weight and begins to have false labor contractions (called Braxton-Hicks contractions).

During the third trimester, it is a good idea to start taking childbirth classes in preparation for the big day - especially in the case of first pregnancies. If you plan to breastfeed, taking a breastfeeding class may be helpful. During the third trimester, both the mother's body and fetus continue to grow and change.

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, while others are free from any discomfort as they anxiously await the arrival of their new son or daughter.