The third trimester is the final stage of pregnancy. The third trimester starts at 28 weeks (6 months) of gestation and usually lasts till 40 weeks (9 months). The third trimester is the time when the baby’s major organs and body systems develop and mature.
1. The fetal development in this trimester
The fetal development in this trimester is fairly rapid with 2 major developments taking place namely the formation of the baby’s brain and the nervous system. The baby’s lungs are also well developed by now, so most babies born at their due date have a good chance of survival without any serious problems related to prematurity. The baby starts practicing breathing motions although there is no amniotic fluid in the mother’s womb. The baby also gains weight rapidly and gains about 1/2 to 3/4 pounds (230-340g) of weight every week, adding 2-3 ounces (60-90g) of weight each day. The placenta produces large quantities of a pregnancy hormone called human chorionic gonadotropin. This helps in building up the uterus and enlarging it further to provide space for the growing fetus inside the mother’s womb.
2. The changes in the uterus during this trimester
The uterus continues to increase in size. In fact, at the beginning of the third trimester, the uterus is three times as large as it was at the beginning of pregnancy and weighs up to 1.3 kilograms (2-1/2 lbs). It can be felt from outside of the mother’s abdomen by pressing gently over her pubic bone. The uterus of a one-year-old child would weigh more than twice as much!
3. The skin changes in this trimester
The skin of the mother becomes thicker and oilier due to increased secretion of oil (sebum) from sebaceous glands in the skin. This helps in protecting the skin against stretching during pregnancy. The blood vessels in the skin become more prominent making the face look redder or darker, especially in the area around the eyes, nose, and cheeks. Pregnancy hormones also make the skin more sensitive to sunlight; hence excessive exposure should be avoided.
4. The changes in other organs during this trimester
The woman may find it difficult to breathe due to pressure on her lungs caused by increased uterus size (size of a full-term baby). She may get short of breath when she stands up after sitting or lying down for some time. Shortness of breath also occurs due to heart problems, such as aortic stenosis, which is a narrowing of the aorta (the major blood vessel coming out from the heart). It can occur due to high blood pressure and it can be dangerous.
The mother’s breathing rate and the amount of oxygen in the blood return to normal towards the end of the third trimester. The mother may also feel a backache because of increased uterus size or due to standing or walking for long hours. Muscle aches and pains, cramps (in legs), sciatica (pain starting from hip and going down through buttock and leg), or numbness may occur as a result of pressure on the sciatic nerve, which passes from the lumbar spine to the lower limbs.