By the end of the 36th week of pregnancy, the enlarged uterus almost fills the abdominal cavity, despite the fact that the cavity is greatly expanded by the stretching of its front and side walls. The well-developed, plump baby, in the membranous sac within the uterus, lies wholly within the abdominal cavity, with the abdominal muscles supporting much of its weight.
During this week, the fundus is at the tip of the xiphoid cartilage of the breastbone, which is shown pushed forward. The liver, transverse colon, stomach, and spleen (which is behind the upper portion of the stomach) are crowded into the vault of the abdominal cavity. The small intestines are crowded above, behind, and to the sides of the uterus. The diaphragm is pressed upward, reducing the vertical diameter of the chest cavity sometimes as much as 4 centimeters; to compensate, the space on the side, front, and back of this cavity increases. The capacity of the chest cavity is not diminished. This displacement of the diaphragm changes the position of the heart, and the increased blood volume may cause the heart to dilate slightly.
Although there is an increase in the amount of blood pumped from the heart per minute and in the amount of air respired per minute, the change in the position of the heart and the upward pressure of the diaphragm probably account for the difficult breathing and the smothered feeling you may experience during this week. The crowding of the stomach and intestines contributes to the discomfort after eating.
The cervix is long, thick, and filled with the mucous plug. By the 36th week, the vagina and urethra are elongated and all the tissues in the perineal region are enlarged, so the swollen perineum projects beyond the pelvic outlet in the last weeks of pregnancy, and is readily expandable during labor.