Blood Volume

There is an overall increase in plasma, red blood cells (RBCs) and total blood volume. Plasma volume increases by 15% during the first trimester; accelerates in the second trimester; peaks at around 32 weeks, reaching up to 50% above non-pregnant levels; and stays elevated until term. It returns to non-pregnant levels by 6 days post-delivery. There is often a sharp rise of up to 1 litre in plasma volume within the maternal circulation at 24 hours after delivery.

Red Blood Cell Volume

RBC volume falls during the first 8 weeks of pregnancy, increasing back to non-pregnant levels by 16 weeks and then rising to 30% above non-pregnant levels by term. The relatively smaller increase in RBC compared with plasma results in haemodilution and ‘physiological anaemia’ of pregnancy.

Coagulation and Fibrinolysis in Pregnancy

Plasma levels of factors VII, VIII, IX and XII, together with fibrinogen and fibrin degradation products, increase during pregnancy (fibrinogen from 2.5 to 4 g/L). Levels of factors XI and III decrease. These changes overall increase coagulability and make pregnancy a ‘hypercoagulable’ state.

Platelets

Pregnancy is associated with enhanced platelet turnover. Thrombocytopenia(platelets < 100 × 10⁹/L) occurs in 0.8%–0.9% of normal pregnant women, while increases in platelet factor and β-thromboglobulin suggest elevated platelet activation and consumption. Since there is no change in platelet count in the majority of pregnant women, there is probably an increase in platelet production to compensate for the increased consumption.

Heart

The heart is pushed upwards and rotated forwards, with lateral displacement of the left border. All heart sounds are louder and the first sound is split. A systolic ejection murmur is normal and is due to turbulence secondary to increased blood flow through normal heart valves. A diastolic murmur is heard occasionally. Cardiac output is increased as a result of increased heart rate, reduced systemic vascular resistance and increased stroke volume. Heart rate is increased above non-pregnant values by 15% at the end of the first trimester. This increases to 25% by the end of the second trimester, but there is no further change in the third trimester.

Stroke volume is increased by about 20% at 8 weeks and up to 30% by the end of the second trimester, and then remains level until term.

Blood Pressure

Systolic blood pressure does not show a significant drop in pregnancy. It may drop slightly, by 6%–8%. However, there is a marked drop in diastolic pressure. It is reduced in the first two trimesters by up to 20%–25% and returns to the non-pregnant level at term. This is due to the placenta acting as an arteriovenous shunt, together with peripheral vasodilating factors such as oestrogen, progesterone and increased endothelial synthesis of prostaglandin E2 and prostacyclins. Both blood pressure and cardiac output are reduced during epidural analgesia. In a supine position, 70% of mothers have a fall in blood pressure of at least 10%, and 8% have decreases of between 30% and 50%.

ECG Changes During Pregnancy

The following changes are of no clinical significance:

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  Sinus tachycardia and atrial and ventricular ectopics. Rotation of the electrical axis of the heart to the left

  
  
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  ST segment depression and T-wave inversion in inferior and lateral leads

Changes in echocardiograph during pregnancy:

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  Left ventricular hypertrophy by 12 weeks

  
  
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  50% increase in left ventricular mass at term

  
  
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  12%–14% increase in aortic, pulmonary and mitral valve sizes

Anatomical Changes

Capillary engorgement of the nasal and pharyngeal mucosa and larynx begins early in the first trimester. This may explain why many pregnant women complain of difficulty in nasal breathing, experience more episodes of epistaxis and experience voice changes. The thoracic cage increases in circumference by 5–7 cm because of the increase in both the anteroposterior and transverse diameters from flaring of the ribs. The level of the diaphragm rises by about 4 cm early in pregnancy even before it is under pressure from the enlarging uterus. This would account for the decrease in residual volume, since the lungs would be relatively compressed at forced expiration.

Physiology

During pregnancy minute ventilation increases by about 40%, from 7.5 to 10.5 L/minute and oxygen consumption increases by about 18%, from 250 to 300 mL/minute. Tidal volume increases gradually from the first trimester by up to 45% at term. Functional residual capacity is decreased by 20%–30% at term due to reductions of 25% in expiratory reserve volume and 15% in residual volume.

Blood Gases

PaCO₂ decreases to 3.7–4.2 kPa by the end of the first trimester and remains at this level until term. Metabolic compensation for the respiratory alkalosis reduces the serum bicarbonate concentration to about 18–21 mmol/L, the base excess by 2–3 mmol/L and the total buffer base by about 5 mmol/L. PaO₂ in upright pregnant women is in the region of 14.0 kPa, higher than that in non-pregnant women. This is due to lower PaCO₂ levels, a reduced arteriovenous oxygen difference and a reduction in physiological shunt. Pregnant women maintain a normal arterial pH of 7.4–7.45 (Table 1.1).

Table 1.1 Normal arterial blood gas values in pregnancy

pH	7.40–7.45
PaCO2	3.7–4.2 kPa
PaO2	13.0–14.0 kPa
HCO3−	18–21 mmol/L

Glucose Metabolism

Pregnancy is associated with an insulin-resistant condition, similar to that of type 2 diabetes. Early in pregnancy, increasing oestrogen and progesterone levels, which lead to pancreatic β-cell hypertrophy and insulin excretion, alter maternal carbohydrate metabolism. Secretion of other hormones such as human placental lactogen, prolactin, cortisol, oestrogen and progesterone induce insulin resistance. These hormones are found to be in significantly greater levels in pregnant women.

Thyroid Gland

There is increased synthesis of thyroxine-binding globulin (TBG) by the liver in pregnancy. This increase leads to a compensatory rise in serum concentrations of total thyroxine (T4) and triiodothyronine (T3). There is, however, no change in the amount of free circulating thyroid hormones. There is iodine deficiency as a result of loss through increased glomerular filtration and decreased renal tubular absorption. Active transport of iodine to the fetoplacental unit and fetal thyroid activity also deplete the maternal iodide pool further from the second trimester.

Pituitary Gland

There is significant enlargement of the pituitary gland during pregnancy. The growth is a result of an increase in the number of prolactin-secreting cells, with the proportion of lactotrophs increasing from 1% to 40%. This results in elevated prolactin levels to up to 10–20 times those of normal, non-pregnant values. These return to normal by 2 weeks postpartum, unless the woman breastfeeds. Gonadotropin levels are suppressed by the high concentrations of oestrogen and progesterone and are undetectable during pregnancy. Levels of basal growth hormone and antidiuretic hormone remain unchanged during pregnancy.

Adrenal Gland

Plasma corticosteroid binding globulin (CBG) concentrations increase during pregnancy. Levels of both free and bound cortisol also increase and levels of serum and urinary free cortisol increase three-fold by term. Adrenocorticotropic hormone (ACTH), which influences steroid secretion from adrenal cortex, remains within the normal range for non-pregnant women.

Skin

During pregnancy the skin undergoes a number of changes, mainly thought to be due to hormonal changes.

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  Hyperpigmentation: This occurs in up to 90% of women during pregnancy. This begins in the first trimester and is prominently noticed in areas of normal hyperpigmentation such as nipples, areola, perineum and vulva. Both oestrogens and progesterone, which have melanogenic stimulant properties, are thought to be responsible for this hyperpigmentation.

  
  
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  Linea nigra: This appears as an area of pigmentation extending from the symphysis pubis to xiphisternum. Although the pigmentation fades after delivery it rarely returns to pre-pregnancy levels.

  
  
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  Melasma: Develops in up to 70% of women, mainly in the second half of pregnancy. It appears as patches of light-brown facial pigmentation usually over the forehead, cheeks, upper lip, nose and chin.

  
  
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  Spider naevi: These present as a central red spot and reddish extensions which radiate outwards like a spider’s web and occur on the face, the trunk and arms. Most appear in early pregnancy and regress following delivery, although in up to 25% of women they may persist. Recurrences are known to occur at the same site in subsequent pregnancies.

  
  
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  Striae gravidarum: These appear perpendicular to skin tension lines as pink linear wrinkles. They fade and become white and atrophic, although never disappear completely.

  
  
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  Palmar erythema: Palmar erythema is reddening of the palms at the thenar and hypothenar eminences. This is thought to be due to high levels of oestrogen in pregnancy and is seen in up to 70% of women by the third trimester and fades within 1 week of delivery.