Maternal
physiology
Sindhu Srinivas, MD, MSCE
Division of Maternal Fetal Medicine
Goals
To understand the normal changes associated with pregnancy
Body Water

TBW increases from 6.5L to 8.5L




At term water content of fetus, placenta and AF is 3.5L
BV, PV, RBC, extravascular, intracellular
Pregnancy is a condition of chronic volume overload
Water retention exceeds Na retention-decreased plasma
osmolality (Na dec by 3-4)
To recognize physiologic and pathologic states during pregnancy
Hematology – Blood volume



Increases progressively from 6 to 8 weeks’
gestation
maximum volume at 32 weeks - 45% increase
possibly due to estrogen stimulation of reninangiotensin-aldosterone system
(Inc Prog, NO->Dec SVR->Dec MAP->Inc
Na retention)
Hematology – RBC mass



Red blood cell mass increases by 250450 cc by term
Increased production
Possibly hormonally mediated
Hematology - Iron
 Maternal
 normal
requirement is 1000mg
pregnant woman needs to absorb
about 3.5 mg/day of iron
 the goal of iron supplementation is to
prevent maternal iron deficiency
 iron is actively transported to the fetus
Hematologic changes

IMPLICATIONS
 The
increase in plasma volume and rbc mass
translates into a 45% increase in circulating
blood volume
 may protect from hemodynamic instability
 may serve to dissipate fetal heat production
and provide increase renal filtration
 physiologic anemia of pregnancy
 may
function to decrease blood viscosity
 may improve intervillous perfusion?
Hematology

LEUKOCYTES
 Peripheral wbc rises progressively during pregnancy





1st ∆ – mean 9500/mm3 (3000-15,000)
2nd and 3rd ∆ – mean 10,500 (6000-16,000)
Labor – may rise to 20-30,000
Rise is due to increase in pmns (demargination)
PLATELETS
Platelets experience a progressive decline but should
remain within normal range
 Likely due to increased destruction

Hematology

COAGULATION FACTORS
 Increased
levels
 Fibrinogen
(Factor I)
 Factors VII through X
 No
change in prothrombin (Factor II),
Factors V and XII
 Decline in platelet count, Factors XI and XIII
 Bleeding
time and clotting time are unchanged in
normal pregnancy
Cardiovascular – Cardiac output

Maternal cardiac output increases about 30-50%
during pregnancy (mean 33%)
pregnancy maximum of 6 L/min
 CO remains maximal until delivery
 Earliest rise in CO is due to increase in SV
 As pregnancy progresses

Gradual increase in mat HR (15-20 bpm rise)
 SV declines to near non-pregnant levels
 increase HR is what maintains the elevated CO

Cardiovascular – Cardiac output

CO is position dependent

Lower when supine

IVC compression by the uterus reduces venous return to
the heart
At 38-40 weeks, there is a 25-30% fall in CO when
turning from the side to the back
 Fall in CO is compensated by a rise in peripheral
vascular resistance


supine hypotensive syndrome (1-10% patients)
Cardiovascular – Cardiac output

Distribution of CO

First trimester and non-pregnant state


Uterus receives 2-3%
By term
Uterus receives 17%
 Breasts 2%

Reduction of the fraction of CO going to the
splanchnic bed and skeletal muscle
 CO to the kidneys, skin, brain and coronary arteries
does not change

Cardiovascular – Arterial BP

BP varies with position

Peripheral vascular resistance falls during pregnancy


Progesterone’s smooth muscle relaxing effect
?heat production by the fetus  vasodilatation

The reduction in PVR may lead to a progressive fall in
systemic arterial bp during the first 24 weeks of
pregnancy

Gradual rise after 24 weeks non-pregnant levels by
term
Cardiovascular – Venous system

Venous compliance increases during pregnancy
decrease in flow velocity and stasis
 ?progesterone effects on smooth muscle
 Forearm venous pressure increases by 40-50%
 Calf venous pressures are always higher


due to the enlarging uterus
Cardiovascular - LV function

Left ventricular dimensions and volume increase
during pregnancy

most parameters of LVF are the same as in the nonpregnant state


Ejection fraction, rate of internal diameter shortening,
percentage of fractional shortening, and ventricular wall
thickness
Bottom line: preservation of myocardial
function
Signs and Symptoms of Normal Pregnancy

Symptoms
reduced exercise tolerance
 dyspnea


Signs
peripheral edema
 distended neck veins
 point of maximal impulse displaced to the left

Signs and Symptoms of Normal Pregnancy

Auscultation
increased splitting of the first and second heart
sound
 S3 gallop
 SEM along the left sternal border
 Continuous murmurs

Signs and Symptoms of Normal Pregnancy

CXR
straightening of left heart border
 heart position more horizontal – may appear as
cardiomegaly on cxr
 increased vascular markings in lungs


ECG
left axis deviation
 non-specific ST-T wave changes

Cardiovascular - Labor


First stage of labor: 12-31% rise on CO due to
an increase in SV
Second stage of labor: 34% increase in CO
Not only pain-related
 UCs result in the transfer of 300-500 cc of blood
from the uterus to the general circulation

Enhanced venous return to the heart
 Increase in CO by 10-15%

Cardiovascular - Postpartum

Immediate pp period: 10-20% rise in CO
release of obstruction of venous return
 extracellular fluid mobilization


Rise in CO associated with reflex bradycardia

SV increases  this may persist for one to two
weeks after delivery
QUESTION

During which of the following states is the
blood pressure lowest?
a)
b)
c)
d)
First trimester
Second trimester
Third trimester
Non pregnant
QUESTION

Increased cardiac output immediately
postpartum is due to:
a)
b)
c)
d)
Increased HR
Release of obstruction of venous return
Reduced mobilization of extracellular fluid
Reduced stroke volume
Respiratory system

UPPER RESPIRATORY TRACT

Hyperemic mucosa of nasopharynx





Estrogen-mediated
nasal stuffiness and epistaxis
Polyposis of nose and sinuses may occur and regress after delivery
“chronic cold”
MECHANICAL CHANGES

Configuration of thoracic cage changes early in pregnancy


Increase in subcostal angle, transverse diameter and circumference of chest
With advancing gestation, the level of diaphragm is pushed up
Changes in pulmonary function tests
during pregnancy
Serial
Serialmeasurements
measurementsofoflung
lungvolume
volumecompartments
compartmentsduring
duringpregnancy.
pregnancy.Functional
Functional
residual
residualcapacity
capacitydecreases
decreasesapproximately
approximately2020percent
percentduring
duringthe
thelatter
latterhalf
halfofof
pregnancy,
pregnancy,due
duetotoa adecrease
decreaseininboth
bothexpiratory
expiratoryreserve
reservevolume
volumeand
andresidual
residualvolume.
volume.
Redrawn
Redrawnfrom
fromProwse,
Prowse,CM,
CM,Gaensler,
Gaensler,EA,
EA,Anesthesiology
Anesthesiology1965;
1965;26:381.
26:381.
Respiratory system

LUNG VOLUME AND PULMONARY
FUNCTION

30-40% increase in tidal volume (Amount of air I
and E with each breath)

30-40% increase in minute ventilation (likely P4 mediated)
ERV falls by 20%
 Vital capacity and inspiratory reserve volume remain
unchanged

Respiratory system

LUNG VOLUME AND PULMONARY
FUNCTION
Respiratory rate is unchanged
 Due to elevation of the diaphragm

Total lung volume decreases (diaphragm) by 5%
 Residual volume decreases (RV) by 20%
 FRC is reduced 20%


No change in FEV1 or the ratio of FEV1 to forced
vital capacity
Respiratory system

GAS EXCHANGE


Minute ventilation rises 30-40% by late pregnancy
O2 consumption increases only 15-29%



Results in higher PAO2 (alveolar) and PaO2 (arterial)
Normal PaO2: 104-108 mmHg
Fall in PACO2 and PaCO2 levels

Normal PaCO2 level: 27-32 mmHg


Increases gradient of CO2 facilitating transfer from fetus to mother
Arterial pH remains unchanged

Increased bicarbonate excretion via kidneys
Respiratory system

DYSPNEA OF PREGNANCY

Common complaint
60-70% of patients
 late first or early second trimester


Likely due to various factors
reduced PaCO2 levels
 awareness of increased tidal volume of pregnancy

QUESTION

Which of the following is increased in
pregnancy?
a)
b)
c)
d)
FRC
ERV
RV
TV
Renal system

ANATOMY

Kidney enlargement


increased renal vascular and interstitial volume, R>L
Ureteral and renal pelvis dilatation by 8 weeks

Right > left



mechanical compression by uterus and ovarian venous plexus
smooth muscle relaxation by progesterone
Implications



Increased incidence of pyelonephritis
difficulty in interpreting radiographs
interference with studies
Renal system

RENAL HEMODYNAMICS

Effective renal plasma flow (ERPF) and GFR
increase

Filtration fraction falls


Returns to normal by late third Δ
Endogenous creatinine clearance increases

Begins by 5 weeks
Renal system

METABOLITES



increased GFR decline in serum urea and creatinine
BUN – 8-9 mg/dl by end 1st Δ
Decline in serum creatinine
 0.7 mg/dl by end 1st Δ


0.5-0.6 mg/dl by term
Early decline in serum uric acid levels


nadir at 24 weeks
same as nonpregnant level at end of pregnancy due to increased
reabsorption of urate
Renal system

SALT AND WATER METABOLISM

Plasma osmolality begins to decline by 2 weeks after
conception


Sodium loss during pregnancy




reduction in serum sodium and other anions
50% rise in GFR
Progesterone: natriuresis
Renal tubular reabsorption of Na+ increases (aldosterone,
estrogen and deoxycorticosterone)
Sodium homeostasis
Renal system

NUTRIENT EXCRETION

Increase in glucose excretion

1-10 g glucose excretion per day


implications



inability to use urine glucose
susceptibility of pregnant women to UTI
Increase in amino acid excretion during gestation


Due to 50% increase in GFR
no increased protein loss (100-300 mg/24 hr)
Increased urinary loss of folate and vitamin B12
QUESTION

All of the following are increased in pregnancy
except:
a)
b)
c)
d)
Renal plasma flow
GFR
Serum creatinine
Tubular sodium resorption
Gastrointestinal - Appetite

Increase early 1st Δ

Increase intake 200 kcal by end 1st Δ

RDA: 300 kcal/day during pregnancy

Sense of taste may be blunted

Pica

check for poor weight gain and refractory anemia



South - clay or starch (laundry or cornstarch)
UK – coal
Also soap, toothpaste and ice pica
Gastrointestinal - Mouth

Unchanged pH or production of saliva


Saliva production is unaltered
Ptyalism – usually in women with HEG




Gums – edematous and soft


due to inability to swallow
Can lose up to 1-2 L of saliva per day
Decreasing starchy foods might help
May bleed after brushing
Epulis gravidarum


regress 1-2 mos after delivery
excise if persistent or excessive bleeding
Gastrointestinal - Stomach

Decreased tone and motility




Conflicting info about delayed gastric emptying
Reduced tone of the gastroesophageal junction
sphincter


progesterone
possibly due to decreased levels of motility
Increased intraabdominal pressure leads to acid reflux
Lower incidence of PUD
 may be due to decreased gastric acid secretion
delayed emptying, increase in gastric mucus, and
protection of mucosa by prostaglandins
Gastrointestinal - Small bowel
 Reduced
motility of small bowel
 increased
transit time in the third
trimester and postpartum
 Enhanced
 as
iron absorption
a response to increased iron needs
Gastrointestinal - Colon

Constipation




Mechanical obstruction by the uterus
Reduced motility (p4)
Increased water absorption
Portal venous pressure is increased

Dilation of gastroesophageal vessels


issue in those with preexisting esophageal varices
Dilation of hemorrhoidal veins

hemorrhoids
Gastrointestinal - Gallbladder

Fasting and residual volumes double in 2nd and
3rd Δ


Slower rate of emptying
Biliary cholesterol saturation increases and
chenodeoxycholic acid decreases

increased risk gallstone formation
Gastrointestinal - Liver


Liver does not enlarge
Hepatic blood flow remains unchanged


Spider angiomata and palmar erythema


CO to the liver decreases by ~35%
elevated estrogen levels
Lab data





Drop in serum albumin
Rise in serum alkaline phosphatase
 placental production and some hepatic production
Rise in serum cholesterol, fibrinogen, ceruloplasmin, binding proteins for
corticosteroids, sex steroids, thyroid hormones, and vitamin D
No change in serum bilirubin, AST, ALT, protime and 5’ nucleotidase
Rise in GGT is controversial
Gastrointestinal system

NAUSEA AND VOMITING



Morning sickness complicates 70% of pregnancies
Onset 4-8 weeks up to 14-16 weeks
Cause?



Relaxation of smooth muscle of stomach, elevated levels of steroids
and hCG
Rx – supportive: reassurance, support, and avoiding triggers…
HEG



weight loss, ketonemia, electrolyte imbalance and dehydration
possible renal or hepatic damage
IVF, antiemetics


NPO
continue IV
Conclusion



Understanding maternal physiology is crucial in
understanding the changes and clinical scenarios
associated in pregnancy
This knowledge will help us distinguish the
physiologic and pathologic processes during
pregnancy
This knowledge will also improve patient’s
education about their pregnancy
Endocrine - Thyroid


The normal pregnant woman is euthyroid
Changes in thyroid morphology and lab indices





Serum TSH decreases early in gestation


role of hCG stimulating the thyroid
Rise in TBG leads to rise in total T4 and total T3


rises to pre-pregnancy levels by end of first Δ
T4 increases early in gestation


Estrogen-induced increase in TBG
Decreased circulating extrathyroidal iodide
Thyroid enlargement usually not detected by exam
Normal thyroidal uptake of iodide
active hormones free T4 and free T3 are unchanged
Free T4 is the most reliable method of evaluating thyroid function in
pregnancy
Endocrine - Adrenal glands

Expansion of the zona fasciculata


Plasma corticosteroid-binding globulin (CBG) rises



increased production and delayed clearance
Plasma DOC (deoxycorticosterone) rises


due to enhanced liver synthesis
Free plasma cortisol rises


site of glucocorticoid production
fetoplacental unit
DHEAS (dehydroepiandrosterone) decreases
Testosterone is slightly elevated

Increased SHBG and androstenedione
Endocrine - Pancreas


Hypertrophy and hyperplasia of the B cells
Fasting associated with accelerated starvation



maternal hypoglycemia, hypoinsulinemia and hyperketonemia
due to diffusion of glucose by the fetoplacental unit
Feeding response



hyperglycemia, hyperinsulinemia, hypertriglyceridemia and
reduced tissue sensitivity to insulin
glucose response greater during pregnancy
peripheral resistance to insulin: diabetogenic effect of pregnancy.
 hPL and cortisol mediated
 greater insulin resistance as the pregnancy advances
Endocrine - Pancreas

Fetus primarily depends on glucose
 Facilitated
diffusion
 carrier-mediated
process
but not energy dependent

Active transport of amino acids to the
fetus

Ketones diffuse freely across the placenta
Endocrine - Pituitary

The pituitary gland enlarges in pregnancy
 proliferation
of chromophobe cells on the
anterior pituitary
 stalk remains midline
Skin

Spider angiomata (face, upper chest, and arm)
and palmar erythema
 elevated estrogen levels
 both regress after delivery

Striae gravidarum

Increased eccrine sweating and sebum excretion
Skin


Hyperpigmentation
Melasma: “mask of pregnancy”


Nevi may darken, enlarge or show increased activity


rapidly changing nevi should be excised
Hairs in telogen phase decrease in late pregnancy




elevated e2 and p4
increases after delivery
hair loss 2-4 mos pp
re-growth in 6-12 mos
Masculinization of the skin rarely occurs

evaluate for possible luteomas of pregnancy (which regress
after delivery)
Breasts

Early change


tenderness, tingling and heaviness
vascular engorgement leads to enlargement





Ductal growth due to e2
Alveolar hypertrophy due to p4
Enlargement and pigmentation of areolae
Colostrum may be expressed later in pregnancy
Milk production


E2, p4, prolactin, hPL, cortisol and insulin
Lactation likely due to drop in estrogen and progesterone
after delivery
Skeleton

Lordosis
keep center of gravity over the legs
 back pain…


Relaxin

relaxation of the pubic symphysis and sacroiliac
joints


facilitates vaginal delivery but may lead to discomfort
Implications

unsteadiness of gait and trauma from falls
Skeleton

Total serum calcium declines throughout pregnancy
until 34-36 weeks


Serum ionized calcium is constant and unchanged


due to the fall in serum albumin
“Physiologic hyperparathyroidism”
 increased gut absorption
 decreased renal losses
 no bone loss seen in bone density studies
 preservation due to calcitonin?
Rate of bone turnover and remodeling increases
throughout pregnancy

twice as great at term
Eye
 Increased
thickness of cornea due to
fluid retention (contact lens
intolerance)
 Decreased
intraocular pressure