Anatomical and Physiological Changes during Pregnancy
Pregnancy brings about marked anatomical and physiological changes in the mother. These adaptations meet the increasing metabolic needs of the fetus and prepare the pregnant woman for parturition. The changes that occur in respiration, circulation, and digestion are the most relevant ones and every pregnant woman should be aware of these, so that she can tell the difference between what is normal and what is not. This will be very useful in detecting any problems during pregnancy well in advance, so that they can be managed appropriately.
A number of changes occur in the cardiovascular system during pregnancy. The blood volume increases by up to 35% during the third trimester with a rise in the plasma volume and in the number of red blood cells. A greater increase in the plasma volume leads to a decreased red blood cell count, thereby lowering the hemoglobin and haematocrit values. The change in the plasma volume is triggered by increased production of a hormone aldosterone, which happens under the influence of placental lactogen, progesterone, and oestrogens. The alteration in the number of red blood cells is caused by increased activity of erythropoietin, a glycoprotein secreted by the kidneys that stimulates the production of red blood cells. During pregnancy, the cardiac output increases by both, an increase in the heart rate as well as the stroke volume. Despite this higher cardiac output, the blood pressure decreases because of a reduction in the peripheral vascular resistance.
The respiratory system doesn't stay unaffected either. During pregnancy, oxygen consumption and carbon dioxide production increase by 60%. This is related to the metabolic needs of the fetus, uterus, and placenta as well as the increased cardiac and respiratory workload. The growing uterus displaces the diaphragm towards the head. Respiratory alkalosis is partially compensated for by increased excretion of bicarbonate by the kidneys, which is reflected by decreased serum bicarbonate levels. Increased levels of carbon dioxide in the mother's blood can lead to fetal acidosis. Vessel engorgement and increased vascularity of the upper respiratory tract cause it to swell up. This can make intubation difficult in pregnant women by reducing the diameter of the trachea, and increasing the chances of severe bleeding following trauma to the upper respiratory tract.
Alterations in gastrointestinal function during pregnancy occur because the enlarged uterus displaces the stomach upwards and towards the left side of the diaphragm. In addition, the stomach is rotated 45 degrees to the right, and the angle between the axis of the stomach and the oesophagus changes. As a result of these changes, and also hormonal fluctuations, the gastric pressure increases and the lower oesophageal sphincter tone decreases. This causes regurgitation of gastric contents, giving rise to heartburn that is frequently reported in pregnant women. According to recent studies, the gastric pH and volume do not change during pregnancy.
Early in pregnancy, due to an increase in the mucosal vasculature throughout the respiratory tract and excessive secretions in the nose, oropharynx, larynx, and trachea, women experience allergy-like symptoms, chronic colds, nasal congestion, voice changes, and mild breathlessness. In pregnancy, respiration is more abdominal than thoracic. The inspiratory capacity increases by 5 to 10% and the respiratory rate increases by two to three breaths per minute. The total lung capacity decreases by approximately 5%.
During pregnancy, the kidneys enlarge by approximately 1 to 1.5 cm with a concomitant increase in weight. At full term, both kidneys are larger than in the non-pregnant state. Both the renal pelvis and the ureters dilate during pregnancy due to the relaxing effect of progesterone and the enlargement and rotation of the uterus to the right side. Since progesterone also decreases the bladder tone, there is an increase in the residual urine after emptying the bladder. There is also a loss of urinary sphincter control as pregnancy advances because the bladder is displaced upwards and flattened by the enlarging uterus. Reduction in the bladder capacity results in increased urinary frequency.
Pregnancy leads to an increase in the vascularity of the reproductory organs. An increase in vaginal secretions produces a typical discharge. The uterus enlarges in size and becomes even more muscular. Its weight increases from approximately 70 gm in the non-pregnant state to more than 1000 gm at full term. Similarly, the volume of the uterine cavity grows from about 10 ml to approximately 5000 ml. The enlarged uterus causes an increase in the intra-abdominal pressure; and as the pregnancy progresses, the spine develops a forward curvature to compensate for the weight in the abdomen. Under the influence of progesterone and relaxin, the ligaments become more lax. There is also increased activity of PTH (parathyroid hormone), which leads to increased absorption of calcium from the intestines and decreased excretion of calcium by the kidneys.
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