During the nine months that a woman is pregnant, many changes occur. In addition to growth of the baby, there are certainly major physical changes for the woman. The changes that occur follow a time-line of development that is well documented. There are three developmental stages of pregnancy known as trimesters.
First Trimester
The first trimester begins with the onset of the menses or the woman's period. Conception occurs sometime during the two weeks after the beginning of the menstrual period (Reference 1). Once implantation occurs the embryo goes through a rapid period of development. During this trimester the increased hormonal secretion coming from the growing placenta can cause significant nausea and vomiting for the mother. She may experience breast tenderness also from the hormones. In the meantime, the embryo is developing facial features, a spinal cord and a tiny heart that begins beating and pumping blood around week six, according to the MayoClinic. By week 11 the baby is considered a fetus. Its eyes are widely spaced with eyelids that are fused shut and external genitalia are forming. By week 12 the baby is 2.5 inches long and weighs ½ ounce (Reference 1).
Second Trimester
As the second trimester begins, many women notice that the morning sickness they were experiencing subsides. This trimester begins with the 13th week of pregnancy and lasts until week 27. Many women begin to show a small protruding abdomen at this stage and begin to wear maternity clothes (Reference 2). During weeks 13 to 14 the fetus begins making urine and this becomes amniotic fluid (Reference 1). During the second trimester the genitalia are developed well enough to be able to determine the baby's sex by ultrasound, the boney skeleton is forming and the baby can now make facial expressions. If the fetus is female, at week 19 the uterus and vagina will begin to form. By week 20, the mother may actually feel the light sensations of the baby's movement called "quickening" (Reference 1). By the end of the second trimester the baby's lungs, liver and immune system are developed and begin maturing.
Third Trimester
By the third trimester, the mother may begin to have frequency of urination and backaches as the baby begins to enlarge and press on her bladder and back. During the third trimester the main goal of development for the fetus is to mature the organ systems. The baby's eyes will open and he or she will be able to see light. The baby will practice breathing and the movement of the arms and legs will become quite forceful. At week 39 the male testes descend from the abdomen into the scrotum (Reference 1). When the baby is born, he or she should have enough fat under the skin for adequate temperature control.
