The stages of pregnancy are called the first, second and third trimester, according to MedicineNet. The first trimester takes place from conception to about 12 weeks, the second begins at week 13 and lasts until week 27, and the final trimester begins on week 28 and lasts until delivery.
During the first semester, the pregnant woman may start experiencing physical changes related to pregnancy, states MedicineNet. She may have a missed period, develop tender or swollen breasts, or become extremely fatigued. During this time, the embryo begins to develop its nervous system first, followed shortly by the rest of its major organs. By the end of the first trimester, the fetus develops sex organs and measures roughly three inches long.
The second trimester may be an easier experience when compared to the first trimester, states MedicineNet. Many side effects, such as nausea and fatigue, diminish or go away entirely. Meanwhile, the baby continues to grow, first developing its neuromuscular system and skin. By the end of the second trimester, a baby's bone marrow begins to produce blood, and it has developed a regular sleep cycle.
When a woman reaches the final trimester, new types of discomfort may develop, such as having to go to the bathroom more frequently. During this time, the baby completes the rest of its development and gains about a half pound a week. By week 37, a baby is considered full term.