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.
Hormonal
Pregnant women experience adjustments in their endocrine system.
Levels of progesterone and estrogens rise continually throughout pregnancy, suppressing the hypothalamic axis and subsequently the menstrual cycle. Estrogen is mainly produced by the placenta and is associated with fetal well–being. Women also experience increased human chorionic gonadotropin (β-hCG); which is produced by the placenta. This maintains progesterone production by the corpus luteum. The increased progesterone production, first by corpus luteum and later by the placenta, mainly functions to relax smooth muscle.
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.
Human placental lactogen (hPL) 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.
Musculoskeletal
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.
Physical
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| Illustration of fundal height at various points during pregnancy |
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.
Cardiovascular
The woman is the sole provider of nourishment for the embryo and later, the fetus, and so her plasma and blood volume slowly increase by 40-50% over the course of the pregnancy to accommodate the changes. The increase is mainly due to an increase in plasma volume through increased aldosterone. It results in an increase in heart rate (15 beats/min more than usual), stroke volume, and cardiac output. Cardiac output increases by about 50%, mostly during the first trimester. The systemic vascular resistance also drops due to the smooth muscle relaxation and overall vasodilation caused by elevated progesterone, leading to a fall in blood pressure. Diastolic blood pressure consequently decreases between 12–26 weeks, and increases again to pre-pregnancy levels by 36 weeks. If the blood pressure becomes abnormally high, the woman should be investigated for pre-eclampsia and other causes of hypertension. 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.
Respiratory
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. This, in turn, results in that the partial pressure of oxygen in arterial blood (PaO2) increases slightly and that of carbon dioxide (PaCO2) decreases during pregnancy. The resulting rise in blood pH (respiratory alkalosis) are compensated for by increased excretion of bicarbonate via the urine, maintaining a normal acid-base balance.
Decreased functional residual capacity is seen, typically falling from 1.7 to 1.35 litres, due to the compression of the diaphragm by the uterus. The compression also causes a decreased total lung capacity (TLC) by 5% and decreased expiratory reserve volume. Tidal volume increases with 30-40%, from 0.45 to 0.65 litres, and minute ventilation by 30-40% 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.
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.
Hematology
During pregnancy the plasma volume increases by 50% and the red blood cell volume increases only by 20-30%.Consequently, the hematocrit decreases on lab value; this is not a true decrease in hematocrit, however, but rather due to the dilution. The white blood cell count increases and may peak at over 20 mg/mL in stressful conditions. Conversely, there is a decrease in platelet concentration to a minimal normal values of 100-150 mil/mL.
A pregnant woman will also become hypercoagulable, leading to increased risk for developing blood clots and embolisms, due to increased liver production of coagulation factors, mainly fibrinogen and factor VIII (this hypercoagulable state along with the decreased ambulation causes an increased risk of both DVT and PE). Women are at highest risk for developing clots, or thrombi, during the weeks following labor. Clots usually develop in the left leg or the left iliac venous system. The left side is most afflicted because the left iliac vein is crossed by the right iliac artery. The increased flow in the right iliac artery after birth compresses the left iliac vein leading to an increased risk for thrombosis (clotting) which is exacerbated by the aforementioned lack of ambulation following delivery. Both underlying thrombophilia and cesarean section can further increase these risks.
Edema, or swelling, of the feet is common during pregnancy, partly because the enlarging uterus compresses veins and lymphatic drainage from the legs.
Metabolic
During pregnancy, both protein metabolism and carbohydrate metabolism are affected. One kilogram of extra protein is deposited, with half going to the fetus and placenta, and another half going to uterine contractile proteins, breast glandular tissue, plasma protein, and haemoglobin.
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.
Nutrition
All about: Nutrition and pregnancy
Nutritionally, pregnant women require a caloric increase of 300 kcal/day and an increase in protein to 70 or 75 g/day. There is also an increased folate requirement from 0.4 to 0.8 mg/day (important in preventing neural tube defects). On average, a weight gain of 20 to 30 lb (9.1 to 14 kg) is experienced.
All patients are advised to take prenatal vitamins to compensate for the increased nutritional requirements. The use of Omega 3 fatty acids supports mental and visual development of infants. Choline supplementation of research mammals supports mental development that lasts throughout life.
Renal
A pregnant woman may experience an increase in kidney and ureter size. The glomerular filtration rate (GFR) commonly increases by 50%, returning to normal around 20 weeks postpartum. Plasma sodium does not change because this is offset by the increase in GFR. There is decreased blood urea nitrogen (BUN) and creatinine and glucosuria (due to saturated tubular reabsorption) may be seen. Persistent glucosuria may suggest gestational diabetes. The renin-angiotensin system is upregulated, causing increased aldosterone levels.
Gastrointestinal
During pregnancy, woman can experience nausea and vomiting (morning sickness); which may be due to elevated B-hCG and should resolve by 14 to 16 weeks. Additionally, there is prolonged gastric empty time, decreased gastroesophageal sphincter tone, which can lead to acid reflux, and decreased colonic motility, which leads to increased water absorption and constipation.
All about pregnancy:
- Pregnancy
- Complications of pregnancy
- Gestational hypertension
- Abortion
- Abortion law
- Health risks of unsafe abortion
- Drugs in pregnancy
- Immune tolerance in pregnancy
- Nutrition and pregnancy
- Prenatal care
- Prenatal development
- Human embryogenesis
- Implantation (human embryo)
- Human fertilization
- Obstetrics
- Postnatal
- Sex during pregnancy
- Sex after pregnancy
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