Maternal Physiology Changes During Pregnancy
The physiologic,
biochemical, and anatomic changes that occur during pregnancy are extensive and
may be systemic or local.
Teleologic alterations
during pregnancy maintain healthy environment for the fetus without
compromising the mother’s health. Although, sometimes determine small
discomfort to the mother.
Gastrointestinal Tract
During pregnancy,
nutritional requirements, including those for vitamins and minerals, are
increased, and several maternal alterations occur to meet this demand.
The mother’s appetite
usually increases, so that food intake is greater, some women have a decreased
appetite or experience nausea and vomiting. These symptoms may be
related to relative levels of human chorionic gonadotrophin (hCG).
·1 Oral Cavity
Salivation may seem to increase due
to swallowing difficulty associated with nausea ,and ,if the pH
of the oral cavity decreases, tooth decay may occur.
Tooth decay during
pregnancy, however, is not due to lack of calcium in the teeth. Indeed, dental
calcium is stable and not mobilized during pregnancy as is bone calcium.
The gums may become
hypertrophic, hyperemic and friable; this maybe due to increased systemic estrogen.
Vitamin C deficiency also can cause tenderness
and bleeding of the gums. The gums should return to normal in the
early puerperium
·2 Gastrointestinal Motility
Gastrointestinal motility may be reduced
during pregnancy due to increased levels of progesterone,
which in turn decrease the production of motilin, a hormonal
peptide that is known to stimulate smooth muscle in the gut. Transit
time of food throughout the gastrointestinal tract may be so much slower
that more water than normal is reabsorbed, leading to constipation.
·3 Stomach and Esophagus
Gastric production of
hydrochloric acid is variable and sometimes exaggerated, especially during the
first trimester. More commonly, gastric acidity is reduced.
·4Production of the hormone
gastin increases significantly, resulting in increased stomach volume and
decreased stomach pH.
·5Gastric production of mucus
may be increased.
·6Esophageal peristalses is
deceased, accompanied by gastric reflux because of the slower emptying time
·7and dilatation or
relaxation of the cardiac sphincter.
Gastric reflux is more prevalent in
later pregnancy owing to elevation of the stomach by the enlarged
uterus. Besides leading to heartburn, all of these
alterations as well as lying in the supine lithotomy position,
make the use of anesthesia more hazardous because of the increased possibility
of regurgitation and aspiration.
·8 Small and Large Bowel and
Appendix
·9The large and small bowel
move upward and laterally,
·10the appendix is displaced
superiorly in the right flank area.
These organs return to the
normal positions in the early puerperium.
As noted previously,
motility is generally decreased an gastrointestinal tone is decreased.
Gallbladder
·11Gallbladder function is also altered
during pregnancy because of the hypotonia of the smooth muscle wall.
·12Emptying time is slowed and
often incomplete.
·13Bile can become thick, and
bile stasis may lead to gallstone formation.
·14Liver
There are no apparent
morphologic changes in the liver during normal pregnancy, but there are
functional alterations.
Serum alkaline phosphatase
activity can double, probably because of increased placental alkaline
phosphatase isoenzimes. Thus, a decrease in the albumin/globulin ratio occurs
normally in pregnancy.
Kidneys and Urinary Tract
Renal Dilatation
During pregnancy, each
kidney increases in length by 1-1.5cm, with a concomitant increase in weight.
The renal pelvis is
dilated. The ureters are dilated above the brim of the bony pelvis.
The ureters also elongate,
widen, and become more curved. Thus there is an increase in urinary stasis,
this may lead to infection and may make tests of renal function difficult to
interpret.
The absolute cause of
hydronephrosis and hydroureter in pregnancy is unknown, there may be several
contributing factors:
1.
Elevated progesterone levels may contribute to hypotonia of the smooth muscle
in the ureter.
2.
The ovarian vein complex in the suspensory ligament of the ovary
may enlarge enough to compress the ureter at the brim of the bony pelvis, thus
causing dilatation above that level.
Renal Function
The glomerular
filtration rate (GFR) increases during pregnancy by about 50% .
The renal plasma
flow rate increases by as much as 25-50%.
Urinary flow and sodium excretion rates in late
pregnancy can be alterated by posture, being twice as great in
the lateral recumbent position as in the supine position.
Even thought the GFR
increased dramatically during pregnancy, the volume of the urine passed each
day is not increased. Thus, the urinary system appears tope even more efficient
during pregnancy.
With the increase in GFR,
there is an increase in endogenous clearance of creatinine.
The concentration of creatinine
in serum is reduced in proportion to increase in GFR, and concentration of blood
urea nitrogen is similarly reduced.
Glucosuria during pregnancy is not
necessarily abnormal, may be explained by the increase in GFR with impairment
of tubular reabsorption capacity for filtered glucose.
Increased levels of urinary
glucose also contribute to increased susceptibility of pregnant women
to urinary tract infection.
Proteinuria changes little during
pregnancy and if more than 500mg/24h is lost, a decease process should be
suspected
Levels of the enzyme
rennin, which is produced in kidney, increase early in the first trimester, and
continue to arise until term. This enzyme acts on its substrate
angiotensinogen, to first form angiotensin1 and then angiotensin2, which acts
as a vasoconstrictor.
Normal pregnant are
resistant to the pressor effect of elevated levels of angiotensin2 but those
suffering from preeclampsia are not resistant, this is one of the some theories
to explain this decease.
Bladder as the uterus enlarges,
the urinary bladder is displaced upward and flattened in the
anterior-posterior or diameter. Pressure from the uterus leads to increased in
urinary frequency.
Bladder vascularity
increases and muscle tone decreases, increasing capacity up to 1500ml.
· Hematologic System
Blood Volume `
Perhaps the most striking
maternal physiologic alteration occurring during pregnancy is the increase in
the blood volume. The magnitude of the increases varies according to
·the size of woman,
·the number of pregnancies
she has had,
·the number of infants she
has delivered, and
·whether there is one or
multiple fetuses.
The increases in blood
volume progress until term; the average increase in volume at term is 45-50%.
The increase is needed for
·
extra blood flow to the uterus,
·
extra metabolic needs of fetus,
·
and increased perfusion of others organs, especially kidneys.
·
Extra volume also compensate
·
for maternal blood loss delivery.
The average blood loss with
vaginal delivery is 500-600ml, and with cesarean section is 1000ml.
Red Blood Cells
The increase in red
blood cell mass is about 33%. Since plasma volume increases early in
pregnancy and faster than red blood cell volume, the hematocrit falls
until the end of the second trimester, when the increase in the red blood cells
is synchronized with the plasma volume increase. The hematocrit
then stabilizes or may increase slightly near term.
Iron
With the increase in
red blood cells, the need for iron for the production of hemoglobin, naturally
increases. If supplemental iron is not added to the diet,
iron deficiency anemia will result.
Maternal requirements can reach 5-6mg/d
in the latter half of pregnancy. If iron is not readily available, the fetus
uses iron from maternal stores. Thus, the production of fetal hemoglobin is
usually adequate even if the mother is severely iron deficient.
Therefore anemia in the
newborn is rarely a problem; instead, maternal iron deficiency more
commonly may cause
·preterm labour and
·late spontaneous abortion,
·increasing the incidence of
infant wastage and morbidity.
White Blood Cells
The total blood leukocyte
count increases during pregnancy from a prepregnancy level of 4300-4500/mL to
5000-12000/mL in the last trimester, although counts as high as 16000/mL have
been observed in the last trimester. Counts as high as 25000-30000/mL have been
noted in a normal patient during labor.
Lymphocyte and monocyte
numbers stay essentially the same throughout
pregnancy; polymorphonuclear leukocytes are the
primary contributors to the increase.
Clotting Factors
During pregnancy, levels of
several essential coagulation factors is increase. There are marked
increases in fibrinogen and factor8.
Fibrinolytic activity is depressed during
pregnancy and labor, although the precise mechanism is unknown. The placenta
may be partially responsible for this alteration in fibrinolytic status.
Plasminogen levels increase
concomitantly with fibrinogens levels, causing an equilibration of clotting and
lysing activity.
Clearly, coagulation and
fibrinolytic systems undergo major alterations during pregnancy.
Understanding these
physiologic changes is necessary to manage two of the more serious problems of
pregnancy: hemorrhage and thromboembolic decease, both caused by
disorders in the mechanism of hemostasis.
Cardiovascular System
Position and Size of Heart
As the uterus enlarges
and the diaphragm becomes elevated, the heart is displaced
upward and somewhat to the left with rotation on its long axis,
so that the apex beat is moved laterally.
Cardiac capacity increases by 70-80mL; this
may be due to increased volume or hypertrophy of cardiac muscle. The
size of the heart appears to increase by about 12%
Cardiac Output
Cardiac output increases approximately 40%
during pregnancy, reaching its maximum at 20-24 week’s gestation
and continuing at this level until term.
Cardiac output is very sensitive to
changes in body position. This sensitivity increases with lengthening
gestation, presumably because the uterus impinges upon the inferior vena
cava, thereby decreasing blood return to the heart.
Blood Pressure
Systemic blood
pressure declines slightly during pregnancy. There is a little change
in systolic blood pressure, but diastolic pressure is reduced
(5-10mmHg) from about 12-26 weeks. Diastolic pressure
increases thereafter to prepregnancy levels by about 36 weeks.
The obstruction posed
by the uterus on the inferior vena cava and the pressure of fetal
presenting part on the common iliac vein can result in decreased
blood return to the heart. This decreases cardiac output, leads to a fall in
blood pressure, and causes edema in the lower extremities.
Peripheral Resistance
Peripheral resistance
equals blood pressure divided by cardiac output.
Because blood pressure
either decreases or remain the same during pregnancy and cardiac output
increases appreciably, there is good evidence that peripheral resistance declines
markedly.
The elevated venous
pressure returns toward normal if the woman lies in the lateral
recumbent position.
Effects of the Labor on the Cardiovascular System
When a patient is the
supine position, uterine contractions can cause a 25% increase in maternal
cardiac output, a 15% decrease in heart rate, and a resultant 33% increase in
stroke volume.
However when the laboring patient is in the later
recumbent position, the hemodynamic parameters stabilize , with only a 7.6%
increase in cardiac output, a 0.7% decrease in heart rate, and a 7.7% increase
in stroke volume.
These significant differences are attributable to
inferior vena cava occlusion caused by the gravid uterus.
During contractions, pulse
pressure increases 26% in the supine position but only 6% in the lateral
recumbent position.
Central venous pressure
increases in direct relationship to the intensity of uterine contraction and
increased intra abdominal pressure. Additionally, cardiopulmonary blood volume
increases 300-500mLduring contractions. At the time of delivery, hemodynamic
alterations vary with the anaestesic used.
Pulmonary System
Anatomic and Physiologic Changes
Pregnancy produces anatomic
and physiologic changes that affect respiratory performance.
Early in pregnancy, capillary
dilatation’s occurs throughout the respiratory tract, leading to engorgement of
the nasopharinx, larynx, trachea, and bronchi. This causes the voice to
change and makes breathing though the nose difficult.
Respiratory infections and preeclampsia aggravate
these symptoms.
Chest X-rays reveal increased vascular makings
in the lungs.
As the uterus enlarges, the
diaphragm is elevated as much as 4cm, and the rib cage is displace
upward and widens, increasing the lower thoracic diameter by 2cm and the
thoracic circumference by up to 6cm.
Elevation of the diaphragm
does not impede its movement.
Lung Volumes and Capacities
Dead volumes increases
owing to relaxation of the musculature of conducting airways.
Tidal volumes increases
gradually(35-50%)as pregnancy progresses. Total lung capacity is reduced (4-5%)
by the
elevation of the diaphragm.
Functional residual
capacity, residual volume, and respiratory reserve volume all decrease by about
20%.Larger tidal volume and smaller residual volume cause increased alveolar
ventilation (about 65%) during pregnancy. Inspiratory capacity increases 5-10%.
Functional respiratory
changes include a slight increase in respiratory rate, a 50% increase in minute
ventilation, a 40% increase in tidal volume, and a progressive increase in
oxygen consumption of up to 15-20% above nonpregnant levels by term.
With the increase in
respiratory tidal volume associated with a normal respiratory rate, there is an
increase in respiratory minute volume of approximately 26%.
As the respiratory minute
volume increases, “hyperventilation of pregnancy” occurs, causing a decrease in
alveolar CO2 . This decrease lowers the maternal blood CO2 tension; however alveolar
oxygen tension is maintained within normal limits.
Maternal hyperventilation
is considered a protective measure that prevents the fetus from the exposure to
excessive levels of CO2.
Effects of Labour on the Pulmonary System
There is a further decrease
in functional residual capacity (FRC) during the early phase of each uterine
contraction, resulting from redistribution of blood from the uterus to the
central venous pool.
Metabolism
As the fetus and placenta
grow and place increasing demands on the mother, phenomenal alterations in
metabolism occur. The most obvious physical changes are weight gain and altered
body shape.
Weight gain is due not only
to the uterus and its contents but also to increase breast tissue, blood and
water volume in the form of extravascular and extracellular fluid.
Deposition of fat and
protein and increased cellular water are added to the maternal stores. The
average weight gain during pregnancy is 12.5Kg.
During normal pregnancy,
approximately 1000g of weight gain is attributable to protein. Half of this is
found in the fetus and the placenta, with the rest being distribute as uterine
contractile protein, breast glandular tissue, plasma protein, and hemoglobin.
Plasma albumin levels are
decreased and fibrinogen levels increased.
Total body fat increases
during pregnancy, but the amount varies with total weight gain.
During the second half of
pregnancy, plasma lipids increase , but triglycerides, cholesterol and
lipoproteins decrease soon after delivery.
The ratio of low density
lipoproteins to high density lipoproteins increases during pregnancy.