Download File - PBL Group 14

Nutrition and Metabolism in Pregnancy
1. Understand that the fetus derives nutrients from the maternal body and so
maternal nutrient requirements for energy and oxygen during pregnancy are
increased.
2. Discuss, using iron as an example, how fetal nutritional requirements may
deplete maternal stores of nutrients and vitamins.
The developing fetus is totally dependent on maternal organ systems for nourishment, respiration,
and waste removal. Maternal systems perform these functions in addition to their normal
operations. For example, the mother must absorb enough oxygen, nutrients, and vitamins for
herself and for her fetus, and she must eliminate all the wastes that are generated. Although this is
not a burden over the initial weeks of gestation, the demands placed on the mother become
significant as the fetus grows. The fetus is 0.64g by the end of the 2nd trimester, and 3.2kg by the
end of the 3rd, showing when the demands on the mothers energy and oxygen levels are the
highest. The uterus itself has grown from 30-40g to 1100g and contains 2L fluid, aside from the
weight of the fetus & placenta.
For the mother to survive under these conditions, maternal systems must compensate for changes
introduced by the fetus. In practical terms, the mother must breathe, eat, and excrete for two., and
pregnant women tend to have increased hunger sensations. Total energy requirements of a normal
single pregnancy over 9 months = 350,000KJ: which can be met with an increase in energy intake
by the mother of about 1200 KJ/day in the second and third trimesters. This energy increase is
needed to support foetal growth, development of the uterus, breasts, placenta, increase in
maternal fat stores and blood volume. There is an increase in basal metabolic rate in mothers that
is partly due to increase energy needed to move a larger body and rapid growth of foetus and
some maternal tissues.
The major changes that occur in maternal metabolic systems include the following:
Maternal Respiratory Rate Goes Up and Tidal Volume Increases.
As a result, the mother’s lungs deliver the extra oxygen required, and remove the excess carbon
dioxide generated by the fetus. Cardiac Output increases 30% during 1st trimester, and is
accommodated by reduction in peripheral resistance by up to 30%, and there is only a slight
blood pressure increase. Pulmonary ventilation increases by 40%, with the foetus requiring O2 in
continuous supply. The placenta itself is very active and consumes 40-60% of the glucose/O2
supplied by the mother. O2 diffuses readily across placental interface, as does CO2 generated by
the fetus.
Maternal Blood Volume Increases.
This increase occurs because blood flowing into the placenta reduces the volume in the rest of the
systemic circuit, and because fetal metabolic activity both lowers blood PO2 and elevates PCO2.
The latter combination stimulates the production of renin and erythropoietin, leading to an
increase in maternal blood volume (see below on iron requirement). By the end of gestation,
maternal blood volume has increased by almost 50 percent. Typical levels of blood loss during
subsequent birthing are therefore not problematic to the mother. Nb blood begins to form in the
liver of the neonate in the 2nd month, and continues in the bone marrow by the 3rd month, and the
spleen by the 4th month.
Maternal Requirements for Nutrients Climb 10–30%.
The main ingredient in fetal growth is carbohydrate, with 50% of the calories needed for growth
& metabolism coming from glucose, then AA & lactate formed from glucose in the placenta. A
pregnant woman becomes resistant to the effects of insulin – so the overall rate of glucose
removal from the blood into maternal muscle and adipose tissue is slowed, resulting in increased
glucose available for the foetus. Between meals, maternal glucose levels are lower than normal.
This deficit in maternal energy requirements is met by oxidising fat at a higher rate. Some of the
fat is converted to ketone bodies (which can also be used by the fetus).
Protein: requirements increase in pregnancy from normal (45-50g/day) to 60g/day. The protein is
required for foetal, placental and uterine growth. Rather than gain these from the diet, evidence
suggests that intermediary metabolism of amino acids appreas to increase – thus urea excretion
falls during pregnancy, suggestins that the same intake of proteins is being used more efficiently.
Fatty Acids: Used by the foetus for membrane synthesis, leukotriene and prostaglandin synthesis
and as an energy store in adipose tissue. Fatty acids are not a major source of energy until after
birth when the enzymes required for metabolism are developed.
Calcium RDI in pregnancy increases from 800mg/day to 1200mg/day. The increase is required
for foetal skeletal development, particularily in the 3rd trimester. Vitamin D increases calcium
absorption from the gut, parathyroid hormone decreases calcium loss in the urine while both
increase bone resorption. Calcitonin stimulates deposition of calcium into bone. The three
hormones balance to create a net increase in plasma calcium.
Folate is an essential B group vitamin (B10) required for DNA synthesis. Intake during pregnancy
increased to 400mg/day reduces risk of neural tube defects by 50% and 800mg/day reduces risk
by 70%. The first few weeks of pregnancy is the most important time for adequate folate
supplementation thus supplementation is recommended before becoming pregnant. Vitamin B12,
a cofactor in folate metabolism and in the metabolism of some fatty acids & amino acids, is also
required in pregnancy, but as both are provided at the expense of maternal stores, fetal deficiency
is unlikely.
Iron requirements increase in pregnancy by ~1g due to increased maternal blood volume,
increased erythropoiesis and fetal blood supply. Maternal anaemia is a risk for the foetus so
supplementation in the 2nd-3rd trimester is usually recommended – see below ‘feature text box’.
Fetal blood contains iron at 2-3 times the concentration of maternal blood, accumulated as
unbound iron by the placental transport system. Additional iron is required to provide av. 300mg
to the fetus, 50mg to the placenta, 200mg in blood loss after labour, and 500mg to increase
maternal haemaglobin mass. Iron absorption is enhances from 10% (1st trimester) to 30% (3rd
trimester).
Maternal Glomerular Filtration Rate Increases by Roughly 50%.
This increase, which corresponds to the increase in blood volume, accelerates the excretion of
metabolic wastes generated by the fetus. Because the volume of urine produced increases and the
weight of the uterus presses down on the urinary bladder, pregnant women need to urinate
frequently.
Maternal anaemia:
Classified as haemoglobin level < 11g/dL
Most common cause is iron deficiency (20-30% of pregnancies without iron supplement)
All pregnant women should have Hg elevated
Women are usually asymptomatic, unless Hgb < 6-7 g/dL
RBC Mass and plasma both increase, but the plasma increase (40-60%) is much more than the
RBC (15-30%)
Iron supplementation prevents low haemoglobin at birth, and at 6wks postpartum.
Folate supplementation is associated with a reduction in the proportion of women with low
haemoglobin level in late pregnancy and megaloblastic erythropoiesis. Anaemia will affect CV,
Respirations, GIT, endocrine, immune and metabolic functions of the mother and are associated
with increased perinatal morbidity and mortality.
Infant of anaemic mother has an increased risk of foetal death, abnormalities, premature death,
decreased birth weight and anaemia in infancy