Pregnancy is divided into three developmental stages: the first trimester, or first three-month period; the second trimester, from three to six months; and the third trimester, from six to nine months, according to the University of Iowa. Each stage is necessary for fetal development and growth, and each has unique pleasures and discomforts for a pregnant woman.
First Trimester
Pregnancy is measured as 40 weeks, but the first four weeks are counted from the last menstrual period to the first missed period. Ovulation occurs around Week 2 and implantation by Week 3. By Week 4, when you miss your first period, implantation has occurred. You may have light spotting as the embryo burrows into the uterine lining. By Week 6, the embryo looks like a bent-over pole with no arms or legs yet, but the underdeveloped heart will start to beat and can be seen on ultrasound as a little flicker, according to the American Pregnancy Association. By Week 12, all of the major organs, including the brain and spinal cord, are present, along with facial features, fingers and toes, according to the Mayo Clinic.
Morning sickness is most common in the first trimester and affects about 50 percent of women, according to the American Pregnancy Association. Breasts enlarge, nipples darken, and sleepiness is almost overwhelming at times, according to the Mayo Clinic.
Second Trimester
The middle period of pregnancy is usually the most enjoyable for the mother. The baby weighs only an ounce or so at the beginning of the fourth month and about 1 lb. to 1 1/2 lbs. by the start of the seventh month, so he's easy to carry around. In this stage of pregnancy, morning sickness has usually disappeared, along with the sleepiness that marks the first three months. The baby begins to kick and move around, and may even suck his thumb. The baby's sex can be determined by 14 weeks or shortly thereafter, according to the Mayo Clinic. By Week 15, the baby's bones are beginning to develop, and by 18 weeks, he is able to detect sound.
Third Trimester
The last three months of pregnancy are a growth period for the baby. Every physical element is present, but finishing touches are added; fingernails, toenails and head hair grow, and body hair falls off. Eyes are partially open by 28 weeks, and eyelashes start to form. The lungs develop enough tissue to support life with ventilator assistance around Week 24. Body fat develops and skin becomes less wrinkled, according to the Mayo Clinic.
The eighth and ninth months are uncomfortable for the mother. The baby presses on her back and bladder, and changes in the pelvic joints make it hard to walk sometimes. Breasts may begin to leak milk. Heartburn is common, and uterine contractions called Braxton-Hicks contractions can be uncomfortable. Feet swell, and weight gain rises to a total of 25 lbs. to 35 lbs., according to the Mayo Clinic.
