Pregnancy-6

Physiological changes in pregnancy

The body must change its physiological and homeostatic mechanisms in pregnancy to ensure the fetus is provided for. Increases in blood sugar, breathing and cardiac output are all required.[citation needed]

Hormonal changes

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

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

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

Musculoskeletal changes

The body's posture changes as the pregnancy progresses. The pelvis tilts and the back arches to help keep balance. Poor posture occurs naturally from the stretching of the woman's abdominal muscles as the fetus grows. These muscles are less able to contract and keep the lower back in proper alignment. The pregnant woman has a different pattern of gait. The step lengthens as the pregnancy progresses, due to weight gain and changes in posture. On average, a woman's foot can grow by a half size or more during pregnancy. In addition, the increased body weight of pregnancy, fluid retention, and weight gain lowers the arches of the foot, further adding to the foot's length and width. The influences of increased hormones such as estrogen and relaxin initiate the remodeling of soft tissues, cartilage and ligaments. Certain skeletal joints such as the symphysis pubis and sacroiliac widen or have increased laxity.[citation needed]

Physical changes

One of the most noticeable alterations in pregnancy is the gain in weight. The enlarging uterus, the growing fetus, the placenta and liquor amnii, the acquisition of fat and water retention, all contribute to this increase in weight. The weight gain varies from person to person and can be anywhere from 5 pounds (2.3 kg) to over 100 pounds (45 kg). In America, the doctor-recommended weight gain range is 25 pounds (11 kg) to 35 pounds (16 kg), less if the woman is overweight, more (up to 40 pounds (18 kg)) if the woman is underweight.

Other physical changes during pregnancy include breasts increasing two cup sizes. Also areas of the body such as the forehead and cheeks (known as the 'mask of pregnancy') become darker due to the increase of melanin being produced.

Illustration of fundal height at various points during pregnancy

Cardiovascular changes

Blood volume increases by 40% in the first two trimesters. This is due to an increase in plasma volume through increased aldosterone. Progesterone may also interact with the aldosterone receptor, thus leading to increased levels. Red blood cell numbers increase due to increased erythropoietin levels.

Cardiac function is also modified, with increased heart rate and increased stroke volume. A decrease in vagal tone and increase in sympathetic tone is the cause. Blood volume increases act to increase stroke volume of the heart via Starling's law. After pregnancy the change in stroke volume is not reversed. Cardiac output rises from 4 to 7 liters in the 2nd trimester.

Blood pressure also fluctuates. In the first trimester it falls. Initially this is due to decreased sensitivity to angiotensin and vasodilation provoked by increased blood volume. Later, however, it is caused by decreased resistance to the growing uteroplacental bed.[citation needed]

Respiratory changes

The partial pressure of oxygen in arterial blood (PaO2) increases slightly and that of carbon dioxide (PaCO2) decreases during pregnancy. The resulting effects on blood pH are compensated for by increased excretion of bicarbonate via the urine, maintaining a normal acid-base balance.[41]

Progesterone may act centrally on chemoreceptors to reset the set point to a lower partial pressure of carbon dioxide. This maintains an increased respiration rate even at a decreased level of carbon dioxide.[41]

Decreased functional residual capacity is seen, typically falling from 1.7 to 1.35 litres,[citation needed] due to the compression of the diaphragm by the uterus. Tidal volume increases, from 0.45 to 0.65 litres,[citation needed] giving an increase in pulmonary ventilation. This is necessary to meet the increased oxygen requirement of the body, which reaches 50 mL/min, 20 mL of which goes to reproductive tissues. Overall, the net change in maximum breathing capacity is zero.[41]

Dyspnea (shortness of breath) is a symptom reported by the majority of women at some point during pregnancy. It typically begins during the first or second trimester, before chest volume is significantly restricted by growth of the uterus, so decreased lung capacity is not the primary cause. Possible factors include slightly decreased PaCO2 and the effects of progesterone on respiration, as well as the woman's subjective interpretation of increased respiratory rate in pregnancy.[41]

Metabolic changes

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

Maternal insulin resistance can lead to gestational diabetes. Increased liver metabolism is also seen, with increased gluconeogenesis to increase maternal glucose levels.[citation needed]

Renal changes

Renal plasma flow increases, as does aldosterone and erthropoietin production as discussed. The tubular maximum for glucose is reduced, which may precipitate gestational diabetes.[citation needed]