What are the various physiological changes during pregnancy?

The body changes its physiological and homeostatic mechanisms in pregnancy in order to ensure that the fetus is provided for. Increases in blood sugar, breathing as well as cardiac outputs are all required.

Hormonal changes

Levels of progesterone and estrogen rise continually throughout pregnancy, suppressing the hypothalamic axis and subsequently menstrual cycle. The woman and the placenta also produce several hormones.

Prolactin levels rise due to maternal Pituitary gland enlargement by nearly 50%. This mediates a change in structure of the Mammary gland from ductal to lobular-alveolar. Parathyroid hormone is also increased which leads to increase of calcium uptake in the gut and also reabsorption by the kidney. Adrenal hormones like cortisol and aldosterone also increase.

Placental lactogen is produced by placenta and stimulates lipolysis and fatty acid metabolism by the mother, conserving blood glucose for use by the fetus. It can also reduce maternal tissue sensitivity to insulin, resulting in gestational diabetes.

Musculoskeletal changes

The body's posture is changed as the pregnancy progresses. The pelvis tilts while the back arches to help keep proper balance. As the fetus grows, poor posture occurs naturally due to the stretching of the woman's abdominal muscles. The muscles have less ability to contract and keep the lower back in proper alignment. The pregnant woman has a different pattern of walk. The step lengthens as pregnancy progresses, due to weight gain as well as because of the changes in posture. On an average, a woman's foot can grow by half size or more in pregnancy. Also, the increased body weight of pregnancy, fluid retention, and weight gain lowers arches of the foot, adding to the foot's length and width. The influences of increased hormones like estrogen and relaxin initiate the remodeling of soft tissues, cartilage and the ligaments. Certain skeletal joints like the symphysis pubis and sacroiliac broaden or have increased laxity.

Physical changes

The enlarging uterus, growing fetus, the placenta and liquor amnii, acquisition of fat and water retention, all add to increase in weight. The weight gain differs from person to person and could range anywhere from 5 pounds (2.3 kg) to over 100 pounds (45 kg).

Other physical changes during pregnancy comprise breasts increasing two cup sizes. Also areas of the body like the forehead and cheeks become darker due to increase of melanin being produced.

Cardiovascular changes

Blood volume increases by about 40% in the first 2 trimesters due to an increase in plasma volume through increased aldosterone. Progesterone can also interact with the aldosterone receptor leading to increased levels. Red blood cell numbers increase because of the increased erythropoietin levels.

The blood pressure also fluctuates. In the first trimester it tends to fall. Initially this is because of decreased sensitivity to angiotensin and vasodilation provoked by the increased blood volume. But later, it is caused by decreased resistance to the growing uteroplacental bed.

Respiratory changes

The partial pressure of oxygen in arterial blood increases slightly and that of carbon dioxide decreases in pregnancy. The consequential effects on the blood pH are compensated for by increased excretion of bicarbonate through the urine, maintaining a normal acid-base balance.

Progesterone acts centrally on chemoreceptors to reset the set point to a lower partial pressure of carbon dioxide. This maintains increased respiration rate even at decreased level of carbon dioxide.

Decreased functional residual capacity is observed, typically falling from 1.7 to 1.35 litres, due to compression of the diaphragm by the uterus. Tidal volume increases, from 0.45 to 0.65 litres, giving an increase in pulmonary ventilation. Overall, net change in maximum breathing capacity is zero.

Metabolic changes

An increased need for nutrients is given by fetal growth as well as fat deposition. Changes are caused by the steroid hormones, lactogen, and cortisol.

Maternal insulin resistance may lead to gestational diabetes. Increased liver metabolism is also observed, with increased gluconeogenesis to increase maternal glucose levels.