2. Every maternal organ adapts to pregnancy, each at a different time and in a different
way. Maternal systems adapt as pregnancy progresses to accommodate the increasing
demands of fetal growth and development.
Management of both healthy and diseased pregnancy necessitates knowledge of the
physiology of normal pregnancy.
Understanding these adaptations enable clinicians to identify abnormal changes that
lead to complications , as well as recognize changes that mimic disease , and
understand altered responses to stress.
3. Volume homeostasis
Maternal blood volume expands during pregnancy;
To allow adequate perfusion of vital organs , including the placenta & fetus,
To anticipate blood loss associated with delivery.
The rapid expansion of blood volume begins at 6–8 weeks gestation and
plateaus at 32–34 weeks gestation.
4. While there is some increase in intracellular water, the most marked expansion
occurs in extracellular fluid volume, especially circulating plasma volume.
This expanded extracellular fluid volume accounts for between 8 and 10 kg of the
average maternal weight gain during pregnancy.
Overall, total body water increases from 6.5 to 8.5 L by the end of pregnancy.
5. Changes in blood volume are key to other physiological adaptations; predominantly
increases in cardiac output and in renal blood flow.
The interpretation of hematological indices in normal pregnancy is also affected,
for example the larger increase of plasma volume relative to erythrocyte volume
Results in haemodilution and a physiologic anaemia.
The mechanisms responsible for fluid retention and changes in blood volume are
unclear.
7. Blood
Haematology
Maternal haemoglobin levels are decreased because of the discrepancy between the
1000 to 1500 mL increases in plasma volume and the increase in erythrocyte mass,
which is around 280 mL.
Transfer of iron stores to the fetus contributes further to this physiological anaemia.
The mean haemoglobin concentration falls from13.3 g/dL in the non-pregnant state to
10.9 g/dL at the36th week of normal pregnancy.
8. Pregnant women require increased amounts of iron, and absorption of dietary iron
from the gut is increased as a result.
Despite this adaptation , women who do not take supplementary iron during
pregnancy show a reduction in iron in the bone marrow as well as a progressive
reduction in mean red cell volume and serum ferritin levels.
The latter are still lower at six months after delivery than in early pregnancy ,
suggesting that pregnancy without iron supplementation leads to depletion of iron
stores.
9. Haemostasis and coagulation
Pregnancy is a hypercoagulable state, which returns to normal around 4 weeks after
delivery.
Changes in the hemostatic system are presumed to occur in preparation for delivery .
Almost all procoagulant factors, including factors VII, VIII, IX, X and XII and
fibrinogen, are increased during pregnancy.
Fibrinogen is increased by 50 per cent, from a mean of 300 mg/dL in the non-
pregnant state to a mean of 450 mg/dL in pregnancy.
10. Hematologic Changes
Blood volume increases 50-100%
RBC increases 25-40% leading to relative anaemia
Increased heart rate by 40%.
Increase cardiac output with peak increase at 3rd stage of labor.
Hypercoagulable state with increased fibrinogen ,clotting factors e.g. factor 2,
7, 8, 9, 10.
Fall in factor 6, 11.
Increased WBC.
11. Decreases in:
• hemoglobin concentration;
• hematocrit;
• plasma folate concentration;
• protein S activity;
• plasma protein concentration;
• creatinine, urea, uric acid.
12. Increases in:
• erythrocyte sedimentation rate;
• fibrinogen concentration;
• activated protein C resistance;
• factors VII, VIII, IX, X and XII;
• D-dimers;
• alkaline phosphatase.
13. Cardiovascular changes
↑ Heart rate (10–20 %).
↑ Stroke volume (10 %).
↑ Cardiac output (30–50 %).
↓ Mean arterial pressure (10 %).
↓ Pulse pressure.
↓ Peripheral resistance (35 %).
Murmurs > 96% of women (systolic murmur)
ECG changes e.g. left axis deviation, prominent Q wave, flat or inverted T wave.
14. In normal pregnancy, cardiac output increases as early as 5 weeks gestation and rises to
around 40 per cent above the pre-pregnancy baseline by 24 weeks, i.e. from about 5.0
to 7.0 L/min when at rest .
The increase in cardiac output is caused partly by an increase in heart rate, which is
detected first as early as 5 weeks, and partly by an increase in stroke volume.
The cardiac output is elevated at the onset of labor to over 7.0 L/min, rising further
within labour .
This increase is due to the uterine contractions each of which squeezes 300–500 mL to
blood into the maternal circulation.
15. At delivery, a shift of blood from the empty uterus into the maternal circulation –
called autotransfusion – causes an increase of 10–20 % in the cardiac output.
Stroke volume, heart rate and cardiac output remain elevated for the first 2 days
postpartum.
Within the first 2 weeks after delivery the cardiac output falls rapidly , at 6 weeks
postpartum it is halfway between pregnant and non-pregnant values .
At 24 weeks after delivery the cardiac output falls to below 5.0 L/min, (Cardiac out
put in non pregnant adult female is 4.5 L/min).
16. Signs & Symptoms of Normal Pregnancy that may Mimic Heart Disease
Signs: Peripheral edema, Jugular vein dilatation
Symptoms: Palpitation , reduced exercise tolerance, and dyspnea
Auscultation: Systolic ejection murmur
Chest x-ray: Change in heart position & size
ECG: Non specific ST-T wave changes, and axis deviation
17. Respiratory Tract
Air way
The neck, oropharyngeal tissues, breasts and chest wall are all affected by weight
gain during pregnancy. This, as well as breast engorgement and airway oedema , can
compromise the airway leading to difficulty with visualization of the larynx during
tracheal intubation.
The vascularity of the respiratory tract mucosa increases and the nasal mucosa can
be both edematous and prone to bleeding. During pregnancy this is often perceived
as congestion and rhinitis.
18. Ventilation
Ventilation begins to increase significantly at around 8 weeks of gestation, most
likely in response to progesterone-related sensitization of the respiratory center to
carbon dioxide and the increased metabolic rate.
As pregnancy progresses, the diaphragm is elevated 4 cm by the enlarging uterus,
and the lower ribcage circumference expands by 5 cm. The increased relaxin levels of
pregnancy allow the ligamentous attachments of the ribcage to relax, increasing the
ribcage subcostal angle.
Respiratory muscle function remains unaffected in pregnancy, as do the maximum
inspiratory and expiratory pressures.
However, lung volumes change slightly as a result of the reconfiguration of the chest
wall and the elevation of the diaphragm.
There are also increases in pulmonary blood flow in pregnancy.
20. Blood gas and acid–base changes
• ↓ pCO2.
• ↑ pO2.
• pH alters little.
• ↑ Bicarbonate excretion.
• ↑ Oxygen availability to tissues and placenta.
21. Reproductive Organs
Uterus
Uterine blood flow increases 40-fold to approximately 700 mL/min at term , with 80%
of the blood distributed to the inter villous spaces of the placentae , and 20% to the
uterine myometrium.
Estrogens mediates the adaptation of the uterine smooth muscle to pregnancy.
High levels of maternal estradiol and progesterone induce both hyperplasia and
hypertrophy of the myometrium, increasing the weight of the uterus from 50–60 g
prior to pregnancy to 1000 g by term.
22. The growing size of the uterine contents is an important stimulus , with individual
muscle fibers increasing in length by up to 15- fold.
By the third trimester, the uterus is described in lower and upper segments.
The lower segment is the part of the uterus and upper cervix which lies between
the attachment of the peritoneum of the utero vesical pouch superiorly and the
level of the internal cervical os inferiorly. It is thinner, contains less muscle and
fewer blood vessels and is the site of incision for the majority of caesarean
sections.
23. Cervix
The cervix is described as looking bluer during pregnancy, which is due to its
increased vascularity. It becomes swollen and softer during pregnancy under the
influence of progesterone and oestradiol; the latter also stimulates growth of the
columnar epithelium of the cervical canal. This becomes visible on the ectocervix and
is called an ectropion.
Under the influence of estrogens , the vaginal epithelium becomes more vascular
during pregnancy, and there is increased desquamation resulting in increased vaginal
discharge. This discharge has a more acid pH than non-pregnant vaginal secretions
(4.5–5.0) and may protect against ascending infection.
24. Breasts and lactation
Deposition of fat around glandular tissue occurs, and the number of glandular
ducts is increased by oestrogen , while progesterone and human placental
lactogen increase the number of gland alveoli.
Prolactin is essential for the stimulation of milk secretion and during pregnancy
prepares the alveoli for milk production.
Although prolactin concentration increases throughout pregnancy , it does not
then result in lactation since it is antagonized at an alveolar receptor level by
oestrogen.
25. The rapid fall in oestrogen concentration over the first 48 hours after delivery
removes this inhibition and allows lactation to begin.
Towards the end of pregnancy, and in the early puerperium , the breasts produce
colostrum, a thick yellow secretion rich in immunoglobulins.
Lactation is initiated by early suckling, which stimulates the anterior and posterior
pituitary to release prolactin and oxytocin, respectively.
