MATERNAL CHANGES IN PREGNANCY
The fetus is solely dependent on the mother for its nutrition, waste removal, and respiratory functions. During pregnancy, major changes have to be made in the various organ systems of the mother to adapt to the new demands. Initially, the demands are mini-mal; however, as the fetus grows, the demands greatly increase. First, changes occur in the reproductive or-gans and breasts. Second, the metabolic functions are increased to provide sufficient nutrients to the fe-tus. Third, hormones secreted by the placenta pro-duce their own effects.
An average of 10 kg (22 lb) is gained by the mother during pregnancy. The weight of the fetus contributes about 3.63–3.88 kg (7.5–8.0 lb); the uterus, 0.90 kg (2 lb); placenta and membranes, 0.90 kg (2 lb); breasts, 0.68 kg (1.5 lb); with the remaining weight from fat and extracellular fluid.
The metabolism increases in proportion to the weight gain. Other than supplying the fetus its needs, the energy is utilized for the resultant increase in heart rate, respiratory rate, and liver function. The increase in metabolism is aided by the increase in thyroid hormone secretion caused by the thyroid stimulating hormone from the pituitary gland. Often, the thyroid gland hypertrophies as a result and may appear enlarged in about 70% of individuals.
There is a demand by the fetus for an easily convert-ible source of energy. The mother tries to meet these demands by maintaining a higher level of glucose in the blood. As a result, there is less glucose storage in the muscle and liver as glycogen.
Generally, there is less breakdown of protein during pregnancy. Blood amino acids are rapidly used by the fetus for growth. Plasma proteins that help transport hormones are increased. To combat bleeding, the proteins required for clotting, such as fibrinogen, are also increased.
Fat is the mother’s main form of stored energy. Most of it is stored in the abdominal wall, back, and thighs.
The sex hormones and adrenocortical hormones pro-duced during pregnancy cause the mother to gain weight by fluid retention. There is an increase in the number of red blood cells, as well as plasma, in the blood. As a result, the blood volume increases as much as 1 liter (33.8 oz). This, in turn, increases the amount of blood pumped by the heart by 30%. At the time of la-bor, the mother loses about 200–300 mL (6.8–10 oz) of blood. The changes in the hormonal secretion soon af-ter delivery bring the fluid levels close to normal.
The total amount of electrolytes (ions such as sodium, potassium, and calcium) in the blood is in-creased.
The respiratory rate increases together with the vol-ume of air inhaled with every breath (tidal volume).
Flaring out of the ribs and increased movement of the diaphragm initiates this increase in volume. The an-terior-posterior and transverse diameter of the chest increases by about 2 cm (0.8 in). As the fetus grows and occupies more space in the abdominal cavity late in pregnancy, the mother’s breathing relies more on the movement of the ribs than the diaphragm. The di-aphragm is elevated by about 4 cm (1.6 in) as a result of the abdominal contents, as well as the flaring out of the ribs. The changes in hormonal secretion often al-ter the caliber of the bronchi and, sometimes, moth-ers prone to asthma feel better during pregnancy. All these changes result in an increased intake of oxygen with improved supply to the fetus and a decreased level of carbon dioxide, which enables easier transfer of carbon dioxide from fetal to maternal blood.
Both the increase in maternal metabolism and the growth of the fetus place significant demands on this system. The increase in metabolism requires a paral-lel increase in blood supply to the lungs for gaseous exchange, to the kidneys for excreting the increased waste products, and to the skin to dissipate the in-creased heat produced. Also, blood supply to the pla-centa has to be increased as the fetus grows.
This demand is met by an increase in blood volume by retaining fluid. The total body water increases on an average by 5 liters (5.3 qt) towards the end of preg-nancy. All this increases the workload of the heart. Both the heart rate and the volume pumped with each stroke (stroke volume) are increased. The heart in-creases in size and is located at a higher level as a re-sult of the movement of the diaphragm.
The total number of red blood cells and hemoglo-bin content increases. Because the blood volume in-creases at a more rapid rate than the cells and hemo-globin, the hemoglobin levels may appear as less than normal. The number of platelets and white blood cells also increase significantly.
The blood pressure decreases early in pregnancy, with a slight decrease in systolic pressure and a marked decrease in diastolic pressure. The pressure decreases to its lowest level at midpregnancy, after which it gradually increases to reach its preconcep-tion level by 6 weeks after delivery. The change in blood pressure is mainly a result of the distensibility of the blood vessels.
The pressure exerted on the pelvic veins by the en-larged uterus causes vascular changes, especially in the lower limbs. Varicosities of veins and edema are com-mon. These changes are more prominent during the day when the person is upright and gravity adds to the effect. In some individuals in a supine position, the uterus may press on the pelvic vein and reduce the vol-ume of blood returning to the heart, with resultant fall in blood pressure and dizzy or unconscious spells.
In general, appetite and thirst are increased. Toward the later part of pregnancy, the growing fetus exerts pressure on the gut, reducing the capacity for large meals. In the first trimester, the mother may be nau-seous or may vomit. The increasing levels of proges-terone tend to reduce the motility of the gut and re-laxation of sphincters. As a result, relaxation of the sphincter in the lower end of the esophagus can pro-duce regurgitation of food into the esophagus from the stomach and cause heartburn. The slower move-ment in the small intestine may aid better absorption of nutrients, while the slower activity of the large in-testines aids better absorption of water. However, the latter may be responsible for the constipation often experienced by pregnant women.
The mother’s requirements of vitamins and other nu-trients increase by as much as 10% to 30%. The total energy required by the mother in the advanced stages of pregnancy is about 2,500 kcal/day (as compared with 2,100 kcal/day in a nonpregnant state). During lactation, the requirements increase to about 3,000 kcal/day.
The kidney increases in length by about 1 cm (0.4 in). As a result of the increase in waste production, the fluid filtered in the kidney (glomerular filtration) is significantly increased. In early and late stages of pregnancy, there is frequency of micturition. In early pregnancy, the enlarging uterus is still in the pelvis, compressing the bladder. In late pregnancy, the fetal head descends into the pelvis, irritating the bladder. The ureters appear to be dilated and the sphincter be-tween the bladder and the ureter is more relaxed, re-sulting in reflux of urine into the ureter and predis-posing the individual to urinary tract infection. As a result of fluid retention, the total urine volume ex-creted is less than that of nonpregnant individuals.
The breasts are fully developed by the end of the sixth month of pregnancy as a result of the action of many hormones, such as prolactin, oxytocin, estrogen, progesterone, thyroxin, growth hormone, and other hormones from the placenta. The glands begin to se-crete, and the secretions are stored in the ducts.
Significant changes take place in the uterus. It in-creases in length from 7.6 cm (3 in) of a nonpregnant uterus to about 30.5 cm (12 in) at full term. At term, together with the contents and fluids, it weighs about 10 kg (22.1 lb)! Late in pregnancy, the elongated and hypertrophied smooth muscles contract sponta-neously as a result of rising estrogen, oxytocin, and other hormonal levels. These contractions are irregu-lar and nonpersistent and are indicators of false labor.
Hormones, such as estrogen and relaxin, soften the ligaments of the pelvic joints. This is to increase the capacity of the pelvis and to make it more mobile.