Obsetry and Gynecology : Maternal Physiology

 

Maternal physiology undergoes many changes during pregnancy. These changes, which are largely secondary to the effects of progesterone and estrogen, begin as early as 4 week gestation and are progressive. In the First 12 weeks of pregnancy progesterone and estrogen are produced predominantly by the ovary and thereafter by placenta. These change both enable the fetus and placenta to grow and prepare the mother and baby for childbirth. These condition will makes a physiologic adaptations in the mother body system, such as hematology system, cardiovascular system, respiratory system, endocrine system and the others.

CARIOVASCULAR SYSTEM

Estrogen and progesterone mediated relaxation of vascular smooth muscle in pregnancy cause vasodilatation reducing the peripheral vascular resistance by 20%. consequently systolic and diastolic blood fall (figure1.1). A reflex increase in heart rate by 25% together with a 25% cardiac output (figure 1.2). Activating of renin-angiotensin system result in increased circulating angiotensin II which encourages sodium and water retention (leading to a 40% increase in blood volume) and directly constricts the peripheral

vasculature.

Figure 1.1 blood pressure changes throughout pregnancy, falling during the first trimester, until approximately 20 weeks gestation, increasing towards of slightly above normal levels at 40 weeks gestation. (ABC of Antenatal Care, 4th edn, Blackwell Publishing. 2002)

Figure 1.2 cardiac output increase during the first trimester or pregnancy, remaining elevated throughout gestation. (ABC of Antenatal Care, 4th edn, Blackwell Publishing. 2002)

 

HEMATHOLOGY SYSTEM

In pregnancy the mean plasma volume increase by 40% to 60% for a single gestation between 12 and 36 weeks of gestation. Red cell mass, white cell count, and platelet production are all increase during pregnancy(figure 3). Total red cell volume will increase progressively to values about 15% above the normal pregnant state woman in woman who do not take iron. in supplement iron is prescribed then the volume may increase to over 30% above normal the non-pregnant woman’s. the hematocrit tends to fall and at about 36 weeks is 5% below the non-pregnant state. the hemoglobin concentration of the red cells decreases some 10% in non-supplemented woman and about 2% in supplemented ones. This gives rises to a hypochromic anemia. the erythrocyte count decrease to a nadir between 26 and 30 weeks which is about 12% below the non-pregnant state.

Increased concentration of progesterone and estrogen directly act on the kidney causing the release of renin. This activation leading to renal sodium retention and an increase in total body water. Plasma volume increase by 45%.

During the 3rd, maternal hemoglobin concentration fall from 150 g per liter pre-pregnancy to 120 g per liter. This is termed physiological anemia of pregnancy.

the increase circulating volume offers protection for mother and fetus from the effects of haemorrhage at delivery by increased circulating levels of factors 1 (fibrinogen), VII, VIII,IX and X. But, this action will predispose to thromboembolism.

 

RESPIRATORY SYSTEM

Respiration adaptation during pregnancy are designed to optimized maternal and fetal oxygenation, and to facilitate transfer of CO₂ waste from fetus to mother. Changes in respiratory system  during pregnancy is increasing in ventilations (figure 1.4), functional residual capacity (FRC) decrease by 20% and increasing O₂ consumption by 20% at term (labor 60%).

Figure 1.4 changes in respiratory physiology during pregnancy, showing increase respiratory rate throughout pregnancy, and increasing tidal volume, minute and alveolar ventilation throughout pregnancy (the surgeon-journal of the royal colleges of surgeon of edinburgh and ireland)