Maternal Adaptations to Pregnancy

Part 3: Hematologic Changes
 

• There is a physiologic anemia of pregnancy.
  • Total blood volume increases about 40% during pregnancy. It’s even higher in multiple fetus pregnancies.
  • However, the plasma volume increases about 50% and the red blood cell volume only increases 25-30%.
  • This causes a physiologic anemia and it is actually a selective advantage. When an anemic person bleeds, they lose less red blood cells per unit volume than a person with normal blood.
    • For patients that need surgery, but are against blood transfusions you can actually create this anemia by diluting the blood with saline as a protective measure.
  • Iron supplements can decrease this anemia, which is good now that pregnant women aren’t at quite the risk of bleeding to death.
• There is an increase in white blood cell counts.
  • This is predominantly an increase in granulocytes (a left shift if you remember back to IHR).
• Pregnant women are in a hypercoagulable state.
  • Not only do they have stasis of blood, but the liver makes more of certain clotting factors.
    • For the detail obsessed, those factors are I, VII, VIII, IX, and X.
    • All other factors are unchanged.
  • This hypercoagulable state causes and 2-3x increase in the risk of thrombosis.
  • At delivery the risk rises to 5x.
    • This is another selective advantage. Although clotting doesn’t play a huge role in the prevention of hemorrhage after delivery (its mostly uterine contraction) it still helps.

Part 4: Pulmonary Changes
 

Anatomic changes

• Mucosal hyperemia due to increased blood flow and increased blood volume.
• There is an increased chest circumference due to the pressure effect of the growing uterus.
• There is an increase in the diaphragmatic excursion of about 4cm.

Functional changes

• The tidal volume increases about 40% (it seems like lots of stuff increase by 40%, but maybe that’s just me).
• Inspiratory capacity increases by 5%. Dr. Pompeo says that this is not a significant change.
• There is no change in respiratory rate.
• The functional residual capacity decreases.
• The minute ventilation increases (this means deeper breaths at the same rate as a normal breath because the rate doesn’t change).
  • This is hyperventilation and it is perceived as dyspnea by patients.
  • This starts early on in pregnancy and occurs in 70-80% of pregnancies.
• The values on an arterial blood gas also change, which makes sense since the respiration is changing significantly.
  • There is an increase in oxygen content and a decrease in CO2.
    • The kidneys show a compensatory loss of bicarbonate in the kidney to maintain the pH.
  • The decrease in CO2 is important to keep in mind if your pregnant patient is asthmatic as well.
    • If you see this patient’s CO2 levels returning to “normal” (non-pregnant) values this is cause for alarm. The values should be low so if they are increasing to normal that is a sign of impending respiratory failure.

Part 5: Renal Changes
 

Anatomic Changes

• Because of increased blood flow and interstitial volume the kidneys increase in size by about 1cm.
• The renal pelvis and ureters dilate because of increased smooth muscle dilatation and the pressure effect of the uterus.
  • This seems to occur more on the right side than the left because more often, the uterus is dextro-rotated rather than levo-rotated.
• The bladder has a decreased emptying and urinary stasis leading to an increased risk of urinary tract infection.
  • This then means a pregnant woman has a higher risk of developing pyelonephritis secondary to a lower UTI.

Functional changes

• Renal blood flow increases about 75% and GFR increases approximately 50%.
  • Creatinine clearance is much higher in pregnant woman.
  • You have to keep this in mind when determining the dosages of drugs for pregnant women.
  • Because of higher clearance rates, you need to give more of a drug more often in order to maintain therapeutic levels in the blood.
• More glucose is present in the blood during pregnancy (more on this later). This increased glucose leads can overwhelm the transport systems in the kidney.
  • Some glucosuria is normal, particularly after a large meal.
  • A very high level of glucose in the urine is not normal and is indicative of diabetes (more on this tomorrow).
• BUN and Creatinine blood levels will be decreased because of the increased clearance.
  • Keep this in mind if you see a creatinine greater than 1.0. While still technically within the normal range, this is not normal for a pregnant woman unless she has underlying kidney disease.
• The renin-angiotensin system shows increased activity during pregnancy.
  •  However, the pregnant woman seems to resist the changes normally brought about by this system.
  • The only exception is preeclamptics who have high blood pressure (again, more tomorrow).
  • No one really knows how or why this all works.

Part 6: Gastrointestinal Changes
 

• There is a decrease in GI motility, due once again to progesterone’s effect of smooth muscle relaxation.
  • Transit time increases by 15-30%. This increase leads to higher water absorption causing…you guessed it, constipation.
• Progesterone also relaxes the smooth muscle of sphincters, particularly the gastroesophageal one.
  • As pressure from the uterus increases, food will take the path of least resistance…back up into the esophagus leading to GERD.
• Again due to progesterone, the gall bladder empties less. Bile remains in the gall bladder. This is called cholestasis.
  • Cholestasis leads to gallstones.
  • There is an easy way to remember those at highest risk for gallstones. It’s called the 4 F’s.
    • Are you (or your patient) Fat? Female? Fertile? Over Forty?
    • Any or all of these increase your risk.
• The liver goes into overdrive, producing all sorts of good stuff.
  • There are increases in binding proteins and clotting factors.
  • There is decreased albumin production.
  • Cholesterol and triglycerides are increased.
    • This extra cholesterol is used by the fetus to synthesize hormones. It is not abnormal.
    • Dr. Pompeo doesn’t even bother checking cholesterol and triglycerides in a pregnant patient because they will always be elevated.
    • Sometimes a sharp increase in cholesterol could be your first indication that a patient is pregnant.
• Nausea and vomiting are very, very common, particularly early in pregnancy.
  • It is thought to be hormonally related, but no one is really sure what hormone is to blame or how it works.
  • There have been correlations between high beta-HCG and more severe nausea and vomiting.
  • Patients are not hospitalized for emesis unless they show signs of weight loss or electrolyte abnormalities.

Return to Section 1 of Maternal Adaptations to Pregnancy 

Continue to Section 3 of Maternal Adaptations to Pregnancy 

Back to the Reproductive System Index