A pregnancy is dated from the first day of your last menstrual period and lasts for approximately forty weeks. Pregnancy is divided into thee periods of growth and development known as the first, second and third trimesters. The first trimester lasts from week one to the end of week 12. The second trimester continues from week 13 through week 28. Your third trimester begins at week 29 and continues until delivery at or around week 40.
First Trimester
The first trimester is a time of rapid growth and development. In the first 12 weeks of your pregnancy the embryo grows from a cluster of dividing cells into a fetus with fully developed organs. Sometime between weeks five and six the baby's heart begins to beat. The baby's gender has already been determined and external sex organs are forming but are unable to view on ultrasound just yet. During the first trimester it is important to see your physician for a first prenatal visit. Certain medications and food may be harmful to your developing baby and need to be discussed with your physician.
Second Trimester
During the second trimester the baby continue to grow rapidly. During week 13 the baby begins to produce urine, which is excreted into your amniotic fluid. The skeleton develops bones during this time, and the baby is finally able to hear sounds. Facial expressions are possible during this time, and swallowing begins. Sometime around week 20, you may be able to determine your baby's sex on an ultrasound. Around this time you will begin to feel movement and kicks from your baby. At the end of the second trimester fingernails develop and real hair begins to grow.
Third Trimester
You have reached your third trimester at week 29. During this trimester the baby's eyes open and he begins to practice breathing. Rapid weight gain begins to occur at or about week 36 of your pregnancy. The baby will grow about 1/2 pound each week for the next four weeks, according to the Mayo Clinic. By 37 weeks your baby is considered full-term.
