When a woman becomes pregnant, her body changes in many ways as she awaits the birth of her baby. Within the uterus, the fetus goes through several stages of development. Over a period of approximately 40 weeks, your baby will develop from a two-celled zygote to a full-term baby, weighing an average of 7 to 8 lb. Tracking your baby's development throughout your pregnancy can be exciting, particularly for a first-time parent.
The First Trimester
The first trimester consists of the first 13 weeks of pregnancy. When the woman's egg is fertilized by her partner's sperm, the egg travels down the fallopian tube until it reaches the uterus. It implants in the uterine lining and begins to develop. By the time you find out that you are pregnant, usually around week four or five, the baby has already begun developing its brain, spinal cord and heart. The heart begins to beat, and arm and legs begin to form. During the second month of pregnancy, the eyes, ears, lungs, hair follicles and all organs will begin to develop. By the end of the first trimester, your fetus's liver will begin to function, and the baby will begin producing his own red blood cells. He will be about 4 inches long and will weigh about 1 oz. During the first trimester, you may feel nauseous and tired. This is the body's reaction to the hormones produced during pregnancy.
The Second Trimester
The second trimester begins at week 13 and ends at about week 28. During the second trimester, you may begin to feel some of your energy come back and may notice that you feel less nauseated. Also during this time, you will need maternity clothes and will begin to feel your baby kicking. Your fetus will begin developing fingernails and toenails during the fourth month. Sexual organs will develop, and the baby will be covered in lanugo, which is fine hair. By the end of the second trimester, the baby's brain develops rapidly. She will have a startle reflex and will sometimes suck her thumb. Her lungs will be getting ready to breathe air.
The Third Trimester
The third trimester begins at week 28 and ends when the baby is born. During this time, you may begin to feel tired again. You may find it hard to take deep breaths and may have to use the bathroom quite often. This is due to the growing baby pressing up toward your diaphragm and down onto your bladder. During the third trimester, your baby is gaining weight, growing eyelashes and getting ready for delivery. His fingernails will grow longer than his fingertips, and most of his lanugo will disappear. He will be covered in a coating of vernix, which will protect his skin until he is born. The lungs will continue to develop until birth. Babies born at full term come between 37 and 42 weeks gestation, most commonly in the 40th week.
