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CHANGES IN MATERNAL
PHYSIOLOGY DURING
PREGNANCY
Dr .Steve Sandler PhD DO
• INTRODUCTION
• During pregnancy great changes occur to
the maternal physiology
– To give the foetus the nutrition for growth
– To give the mother the energy she needs to
sustain this growth both for labour, and for
lactation
• INTRODUCTION
• In the first two months of pregnancy when
the placenta is at a very early stage of
development, most hormone changes
come from the corpus luteum of
pregnancy.
• The corpus luteum of pregnancy develops
under the influence of HCG secreted by
the developing blastocyst.
Introduction
• The changes in the beginning are in the ovary
and uterus and are designed to encourage
implantation and embryonic growth.
• In the earliest stages of pregnancy she may not
even realise she is or has been pregnant.
• I say has been because you are not pregnant
until you have had a positive pregnancy test!!
Introduction
• How many women whose period arrives a day
early or two days late or whose period is slightly
heavier than usual have been pregnant and lost
the baby and never knew it?
• If we consider the millions and millions of
changes in the cells of the developing
blastocyst, and which structures are going to be
laid down first in the embryo, the CNS and
vascular systems, then if one of these goes
wrong the body somehow realises it and the
pregnancy aborts without her even knowing she
conceived.
Introduction
• The systems involved in change in the
mother include the following :– O2 supplies and CO2 dispersal
– Fluid and electrolyte balance
– Nutrient balance
– Defence and waste disposal
– Temperature regulation
– The genitals and breasts
– The musculo skeletal system
Oxygen supplies and Carbon
Dioxide disposal
• The supply of oxygen to the growing
foetus is protected by changes occurring
in the mother in
– Ventilation
– Number of red cells
– Circulation
Oxygen supplies and Carbon
Dioxide disposal
• Ventilation
– The need for oxygen increases progressively
during pregnancy with the growth of the
mother and the foetus.
– At term resting oxygen consumption is up by
15% over non pregnant levels
Oxygen supplies and Carbon
Dioxide disposal
• Ventilation
– More oxygen is also needed for the extra
energy expended in daily activities because of
the mothers weight gain .
– Total 12.5 kg on average = 20% of body
weight.
Oxygen supplies and Carbon
Dioxide disposal
• Ventilation
– Progesterone increases the sensitivity of the
respiratory control centres in the
hypothalamus to CO2, so that ventilation is
greater at any particular level of arterial CO2
then in the non pregnant state.
– The depth of breathing increases but the
number of breaths per minute does not
change
Oxygen supplies and Carbon
Dioxide disposal
• Ventilation
– As a result of the increased ventilation, the
partial pressure of oxygen in alveolar air
increases, with the result that the maternal
arterial PO2 increases and PCO2 falls by
about 10mmHg.
– This increases the rate of diffusion of gasses
across the placenta improving foetal oxygen
uptake and carbon dioxide excretion.
Introduction
Oxygen supplies and Carbon
Dioxide disposal
• Red Cell numbers
– Red cell production by bone marrow is
stimulated by erythropoietin leading to a 20%
increase in the total number of red cells in the
circulation.
– However as the plasma volume increases by
an even larger amount the red cell count
actually falls due to haemodilution.
– This is the physiological anaemia of
pregnancy
Introduction
Oxygen supplies and Carbon
Dioxide disposal
• Circulation
– The growth of maternal tissues causes an
increase in the number of blood vessels in the
circulation notably in the placenta.
– Progesterone levels increase causing a
relaxation of vascular smooth muscle leading
to a fall in peripheral resistance.
– There is also a decreased response to
Angiotensin II .
Oxygen supplies and Carbon
Dioxide disposal
• Circulation
– Circulatory pressure is maintained by
• An expansion of blood volume
• An increase in cardiac output
Oxygen supplies and Carbon
Dioxide disposal
• Circulation
– The expansion of blood volume is made up by
• an increase in plasma volume of about 1 litre.
• a progressive increase in the number of red cells
throughout the pregnancy
Oxygen supplies and Carbon
Dioxide disposal
• Circulation
– Cardiac output increases in early pregnancy
reaching 40% above the non pregnant state
by 12 weeks gestation.
– It remains that way until term
– Heart rate and stroke volume increase by
15% and 20% respectively
Oxygen supplies and Carbon
Dioxide disposal
• Circulation
– As a result of these changes in volume and
cardiac output, arterial blood pressure is
normally fairly constant .
– Towards the end of the pregnancy there is a
small decrease in diastolic pressure of about
10mmHg.
Weight Gain in Pregnancy
• 2kg in the first 20 weeks
• 0.5kg per week thereafter
• This is approx. 12Kg in total.
Weight Gain in Pregnancy
• This is made up as follows
– Increased blood volume
– Interstitial fluid
– Breasts
– Fat
– Placenta
– Foetus
– Amniotic fluid
– Uterus
1.5Kg
1.0Kg
0.5Kg
3.5Kg
0.6Kg
3.4Kg
0.6Kg
0.9Kg
Fluid and Electrolyte Balance
•
•
•
•
•
Expanding Blood Volume
+
Expanding interstitial fluid volume
=
Increase in extra cellular fluid volume by 2
to 3 litres.
• This will require a change in renal function
and an altered sensitivity of the control
mechanisms
Fluid and Electrolyte Balance
• Renal changes
– Blood flow and glomerular filtration rate
increase by 50% in early pregnancy.
– This will lead to an increased amount of
solutes to be recovered
– Salt and water transport and increased by the
actions of Aldosterone and ADH.
– The transport of glucose and amino acids
should not change very much ( it may even
decrease).
Fluid and Electrolyte Balance
• Later in the pregnancy when the load of
glucose and amino acids delivered to the
kidney exceeds the tubular transport
mechanisms, they appear in the urine.
• This is glycosuria and amino acidurea of
pregnancy .
• Care is needed that this is not the signs of
a diabetic pregnancy.
Fluid and Electrolyte Balance
• In the later stages of pregnancy when the
foetal skeleton is growing fastest, there is
an increase in absorption of calcium by the
renal tubules and more calcium is
recovered from the filtrate.
• This is stimulated by parathormone
• The maternal blood calcium falls and can
lead to weak teeth and nails.
Fluid and Electrolyte Balance
• The Renin-Angiotensin system is
stimulated leading to increased
Aldosterone secretion from the adrenal
cortex.
• Also enhanced by increased secretion of
ACTH by the anterior pituitary gland.
• This gland enlarges by 40% in pregnancy!
Nutrient Balance
• The maintenance of good nutrient balance
in pregnancy occurs with the adjustment of
3 mechanisms:– Food Intake
– Metabolism
– Function of the digestive system
Increased recommended increase
in dietary daily intake in pregnancy
• Nutrient
•
•
•
•
•
•
Calcium
Folate
Zinc
Iodine
Protein
Iron
% increase
+140
+100
+30
+25
+11
+8
• If the mother does not meet these requirements
then the foetus feeds from her own body stocks
parasitically.
Nutrient Balance
• If patients want to find good sources of
these dietary components then a “Google”
search on the internet will reveal many
pages for them to read.
Nutrient Balance
• Apetite is stimulated in early pregnancy by
the action of progesterone on the
hypothalamus.
• In early pregnancy the intake of food
exceeds needs which means the excess is
laid down as fat to be used later as the
foetus grows at the end of the pregnancy.
Function of the Gastrointestinal
tract
• Pregnant women commonly suffer from
– Heartburn
– Nausea especially morning sickness in early
pregnancy
– Constipation
– Haemorrhoids
Function of the Gastrointestinal
tract
• The secretion of gastric acid decreases in the
first half of the pregnancy.
• This and the slower transit of food materials
enhances the absorption of iron and calcium in
the upper part of the intestine
• The action of the hormone Calcitol adds to this
so that by six months pregnant the calcium
absorption is twice that of the non pregnant
women.
Waste Disposal
• The foetus produces many metabolic
waste products including
– CO2
– Urea, creatinine and uric acid
– Unconjugated bilirubin
Waste Disposal
• All of these are eliminated via the placenta
and after diffusing across they are
eliminated via the mothers excretory
systems.
• Bilirubin is formed by the breakdown of red
blood cells . It’s production may increase if
foetal red cells are destroyed by maternal
antibodies ( rhesus incompatibility).
Waste Disposal
• In early pregnancy water excretion by the
kidney is increased and causes frequency
of micturition which can be misdiagnosed
as an UTI.
• Frequency in late pregnancy is due to the
pressure of the gravid uterus on the
bladder.
Waste Disposal
• Smooth muscles of the renal pelvis,
ureters and the bladder relax under the
influence of progesterone, leading to
dilatation of the renal pelvis .
• Sometimes there is a kinking of the ureters
which if severe can lead to blockage and
there is now a risk of urinary stasis on that
side.
Temperature regulation
• The basal body temperature will rise by
0.5 degrees C after ovulation.
• If conception occurs it remains raised until
mid pregnancy
• Blood flow to the skin in particular the
hands and feet increases in pregnancy
and this helps to dissipate the heat.
Changes to the genitals and
breasts
• The most obvious and important changes
will take place within the uterus.
– The lining or decidua becomes thicker and
very heavily vascular under the influence of
progesterone and oestrogen produced by the
corpus luteum of pregnancy.
– This is particularly important at the fundus and
the upper body of the uterus because this is
where the placenta ideally is going to implant.
Changes to the genitals and
breasts
• Changes to the uterus
– After conception the upper part of the uterus
begins to enlarge due to the effects of
oestrogen.
– The uterus changes to a globular or tear drop
shape to anticipate foetal growth and to
accommodate increasing amounts of licor and
placental tissue.
6 weeks
10 weeks
Changes to the uterus
in the early weeks
of pregnancy
16 weeks
Changes to the genitals and
breasts
• Changes to the uterus
– 12th week of pregnancy
• The uterus is no longer anteverted and anteflexed .
It has risen out of the pelvis and upright often
rotating to the right because of the pressure of the
left colon pushing it away.
• At 12 weeks the fundus may be palpated
abdominally above the pubic symphasis.
Changes to the genitals and
breasts
• Changes to the uterus
– 20th week of pregnancy
• The uterus is now pear shaped and has a thicker
and more rounded fundus.
• The fallopian tubes being restricted by attachment
to the broad ligaments become progressively more
vertical.
Changes to the genitals and
breasts
• Changes to the uterus
– 30th week of pregnancy
• The lower uterine segment can be identified .
• It lies above the internal os and is where the
midwife is going to try to palpate the head of the
baby ( ballotment).
Changes to the genitals and
breasts
• Changes to the uterus
– 36th week of pregnancy
• The uterus now reaches the level of the
xiphisternum.
• The softening of the tissues of the pelvic floor
together with the good tone of the uterus
encourages the foetus to sink into the lower pole of
the pelvis.
• The head engages in primiparous mothers but not
often in mutigravid women.
Changes to the genitals and
breasts
• Changes to cervix
– The cervix acts as an effective barrier against
infection throughout the pregnancy ;it also retains the
pregnancy.
– Under the influence of progesterone it secretes a
thick viscous mucus ,the so called mucus plug which
has to be shed at the start of the labour.
– In late pregnancy prostaglandin secretion softens the
cervix and labour starts. But no one really knows just
how and when labour starts.
Changes to the genitals and
breasts
• Once the placenta is formed it starts to produce
it’s own hormones.
– Now the corpus luteum of pregnancy is no longer
maintained by HCG and so it atrophies to become the
corpus albicans just as in a normal menstrual cycle.
– The dates of the 3rd and 4th missed periods are
important because of the changes that take place to
the hormone levels and to the placenta at
implantation. These are the 12th and 16th weeks of
pregnancy respectively.
Changes to the genitals and
breasts
• The myometrium
– Oestrogen is responsible for the growth of
uterine muscle. For the first 20 weeks the
uterine muscle hypertrophies and the size of
the muscle fibres increase.
– After 20 weeks it grows by simply stretching.
Changes to the genitals and
breasts
• Increase in weight of the uterus
– From 60grams to 900 grams
• Increase in size from 7.5x5x2.5cm to
30x23x20cm
• This increase is also possible because
progesterone encourages the growth of
smooth muscle.
Changes to the genitals and
breasts
• Uterine blood supply
– The blood supply to the uterus has to increase to
keep pace with the growth and also to meet the
needs of the functioning placenta.
– Oestrogen causes development of new blood vessels
. Initially they form a twisted network throughout the
uterine walls, but as the uterus grows and stretches
they become straightened until after the birth when
the uterus involutes ands shrinks when they become
tortuous again.
Changes to the genitals and
breasts
Changes to the genitals and
breasts
• Changes to the breasts
– All breast changes are as a result of increased
hormone activity.
– Oestrogen develops the duct system and
progesterone the glandular tissues.
– The areola changes from pink to dark brown in
preparation for lactation.
– Prolactin stimulates colostrum production ( a
forerunner of breast milk) .
– The breasts will enlarge due to increase in growth,
vascularity and fat deposition.
Changes to the genitals and
breasts
• Changes to the uterus
– The lining or decidua of the uterus becomes
thicker and more vascular at the upper part of
the uterus. It’s job is to provide a glycogen
rich environment for the blastocyst until the
cells of the trophoblast are able to form the
placenta.
Changes to the genitals and
breasts
• Changes to the uterus
– The Myometrium
• Oestrogen is responsible for the growth of uterine
muscle.
• Progesterone encourages relaxation of smooth
muscle as it grows.
• During labour the uterus contracts from above
downwards in waves of contractile pulses to push
the presenting part towards the internal os and
encourage it to open.
Summary
• The changes to the mother during her
pregnancy are designed then to
– Enable her to change so as to support and
nurture the developing foetus
– Enable her to accommodate the growth and
expansion of the uterus with minimal
discomfort and disturbance to her own
physiology and homoeostatic systems.
– Thus the child can develop and thrive.