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Physiological Changes
That Occur During Pregnancy
During pregnancy, the woman
undergoes many physiological changes, which are entirely normal, including
cardiovascular, renal, hematologic, metabolic or respiratory changes that
become very important in the event of complications.
Metabolism - 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 hemoglobin.
Nutrition -Increased caloric requirement by 300 kcal/day
Gain of 20 to 30 lb (10 to 15 kg)
Increased protein requirement to 70 or 75 g/day
Increased folate requirement from 0.4 to 0.8 mg/day (important in
preventing neural tube defects)
All patients are advised to take prenatal vitamins to compensate for the
increased nutritional requirements.
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. This results in overall vasodilation, 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 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 remains abnormal beyond 36 weeks, the woman should be
investigated for pre-eclampsia, a condition that precedes eclampsia.
Pulmonary - Increased tidal volume (30-40%). Decreased total lung
capacity (TLC) by 5% due to elevation of diaphragm from uteral
compression. Decreased expiratory reserve volume. Increased minute
ventilation (30-40%) which causes a decrease in PaCO2 and a compensated
respiratory alkalosis. All of these changes can contribute to the dyspnea
(shortness of breath) that a pregnant woman may experience.
Hematology - The plasma volume increases by 50% and the red blood
cell volume increases only by 20-30%. Consequently, the hematocrit
decreases, white blood cell count increases and may peak at over 20 mil/mL
in stressful conditions. Decrease in platelet concentration to a minimal
normal values of 100-150 mil/mL. The pregnant woman also becomes
hypercoagulable due to increased liver production of coagulation factors,
mainly fibrinogen and factor VIII.
Gastrointestinal - nausea and vomiting ("morning sickness")
due to elevated B-hCG, which should resolve by 14 to 16 weeks prolonged
gastric empty time decreased gastroesophageal sphincter tone, which can
lead to acid reflux decreased colonic motility, which leads to increased
water absorption and constipation
Renal - Increase in kidney and ureter size - Increased
glomerular filtration rate (GFR) by 50%, which subsides around 20 weeks
postpartum
Decreased BUN (blood urea nitrogen) and creatinine, and glucosuria (due to
saturated tubular reabsorption)
Persistent glucosuria can suggest gestational diabetes
Increased renin-angiotensin system, causing increased aldosterone levels
Plasma sodium does not change because this is offset by the increase in
GFR
Endocrine - Increased estrogen, which is mainly produced in the
placenta. Fetal well being is associated with maternal estrogen levels
causes an increase in thyroxine-binding globulin (TBG). Increased human
chorionic gonadotropin (β-hCG), which is produced by the placenta. This
maintains progesterone production by the corpus luteum Human
placental lactogen (hPL) is produced by the placenta and ensures nutrient
supply to the fetus. It also causes lipolysis and is an insulin
antagonist, which is a diabetogenic effect.
Increased progesterone production, first by corpus luteum and later by the
placenta. Its main course of action is to relax smooth muscle. Increased
prolactin. Increased alkaline phosphatase
Musculoskeleton and dermatology - Lower back pain due to a shift in
gravity. Increased estrogen can cause spider angiomata and palmar erythema.
Increase melanocyte-stimulating hormone (MSH) can cause hyperpigmentation
of nipples, umbilicus, abdominal midline (linea nigra), perineum, and face
(melasma or chloasma).
Edema - or swelling, of the feet is common during pregnancy, partly
because the enlarging uterus compresses veins and lymphatic drainage from
the legs. For the sake of comfort, many pregnant women wear larger shoes
or go without. This may have something to do with the origin of the phrase
"barefoot and pregnant".
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