OBGYN - Maternal-Fetal Physiology

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Contents

Maternal-Fetal Physiology

Objectives

  • List pertinent physiologic changes in various maternal systems
  • Recognize that signs and symptoms in a pregnant patient are often more difficult to interpret
  • Non-pregnant lab values and measurements are frequently abnormal in the pregnant state

Case: First Trimester

  • 21 y/o G1P0 at 8w0d by L=6 week US at the WVC (women's visit center at Wishard) presents for her OB registration appointment.
    • She denies any LOF (leaking of fluid), VB (vaginal bleeding).
    • Pt with C/C of cramping and dark “poop”, nausea and vomiting with heart burn.
  • PMH: Denied
  • PSocialH: Denied
  • OB: G1
  • Gyn: Denied
  • Social Hx: Denied x 3
  • Allergies: NKDA
  • Medications: PNV (Prenatal Vitamin)
  • VS: BP: 110/80 P: 80 T: 98.6 Weight: 65Kg Height: 5’1
  • UA: WNL
  • PE: WNL


GI Issues in the First Trimester

  • Constipation:
    • Iron (ferrous-sulfate) in the pre-natal vitamins slows GI motility and causes constipation.
    • Progesterone: a smooth muscle relaxant; causes decreased bowel movement which can lead to increased time for water reabsorption and thus constipation
    • Tx: give colace
    • Could be as bad a month!
    • There is fear of miscarriage throughout pregnancy so anything seeming abnormal (like not pooping) starts a fear of miscarriage.


  • Nausea and Vomiting:
    • beta-HCG causes n/v
      • beta-HCG peaks at 10-12 weeks.
    • {rogesterone: a smooth muscle relaxant; causes decreased bowel movement which can lead to n / v.


  • Weight gain:
    • can occur early or late, ideal is a progression.
    • vomitting can lead to weight loss over the first weeks, but mom and baby will be ok
    • Normal Weight for Height: about 20 lbs
    • Underweight Women: about 30 lbs
    • Overweight Women: about 16 lbs
    • Only an addition 300 calories / day are necessary to feed a fetus.
      • 500 cal for twins

Case: Second Trimester

  • 21 y/o is now 20 weeks pregnant
  • Presents to triage at Wishard with horrible heartburn after eating a deep fried twinkie at the state fair.
  • She threw-up so much in early pregnancy that she is making up for it now.
    • She has gained 25 lbs (from 8 weeks to second trimester).
    • That is a lot of weight gain.
  • She denied any LOF (leaking of fluid), denied VB (vaginal bleeding) or cramping.
  • She stated that she had some of the chalkie things and it didn’t make it better this time.
    • Tums.
  • She was given a GI cocktail in triage and she was still miserable.


  • Differential Diagnosis:
    • Constipation
      • Small bowel obstruction
      • Ask when they last deficated.
    • Gallstones
      • Increased estrogen and progesterone
      • Slow emptying of the stomach
      • Often already at high risk (four "F"s: fat, forty, fertile, female)
      • Can look really, really, bad.
    • Pancreatitis
    • Appendicitis
      • Appendix moves from right lower quadrant to midquadrant
      • Can get tucked up behind the uterus

First / Second Trimester GI Presentations

  • High levels of estrogen / progesterone in pregnancy lend themselves to gallstone formation
    • Especially when woman gets pregnant a second time without having gallstones from a previous pregnancy removed.


  • Recall that progesterone is a smooth muscle relaxant.
  • Recall that progesterone is at high levels throughout pregnancy to maintain the uterine environment.
    • Progesterone is really rampinig up during the first trimester.
  • High circulating levels of progesterone can lead to:
    • Increased GERD from relaxation of GE sphincter
    • Constipation from delayed transit through large bowel
    • Increased N/V from delayed gastric emptying
    • Stomach can still be full of content 8 hours later.
    • “Full stomach” sensation


  • Tx is often to keep pt from eating for 8 to 12 hours and then to switch to a little bit, throughout the day type consumption pattern.

Case: Second / Third Trimester

  • 21 y/o G1P0 at 28w0d
  • Presents to triage at St. Francis Beech Grove.
  • Patient has been brought in by her family “Doc I feel horrible. I am dizzy. I was sitting at the high-school football game, I stood up to cheer, my heart started racing and I felt like I was going to pass-out! My ankles are swollen and I feel horrible."
  • Denies LOF (leaking of fluid), VB (vaginal bleeding), CTXs (contractions).
  • Endorses +FM (fetal movement).
  • ED doctor noticed that she is breathing deeply.


  • Pertinent Vitals:
    • BP: 100/60
    • P: 90
    • T: 98.6
    • RR: 16
    • O2: 96% RA
    • Weight: 80kgs


  • Labs ordered:
    • CBC w/platelets
    • CXR (chest xray)
    • ABG (arterial blood gas)
    • UA (urinary analytes)
    • CMP (complete metabolic panel)
    • Coags (coagulation studies)


  • PE:
    • Lungs: CTA B
    • CV: Systolic murmur
    • Abd: Gravid, NT
    • Extrem: +1 pitting edema


  • Lab results:
    • WBC: 10,000 (slightly elevated)
    • Hgb / HCT: 10.5/31.7% (decrease)
    • Platelets: 200,000 (slight decrease)
    • Fibrinogen: 600 (increased)
    • Bun / Cr: 3 / 0.6 (decreased)
    • pH: 7.44
    • pCO2: 30
    • BiCarb: 21
    • paO2: 103

Physiological Changes in Pregnancy

Blood pressure in pregnancy

  • Pt blood pressure: 100/60
  • Both systolic and diastolic BP decrease in 2nd trimester
  • Systalic decreases 5-10 mmHg
  • Diastolic decreases 10-15 mmHg
  • We even take severe hypertensive pts off their meds.
  • Normal


  • Monitor from baseline!
    • Athletes will have low baseline BP and therefore it will be a challenge to keep their BP up during pregnancy.

Pulse in pregnancy

  • Pt's pulse: 90
  • Increases 20 beats (peak at 32 weeks, middle of 3rd trimester)
  • Normal

Cardiac output in pregnanc

  • Recall that cardiac output (CO) is stroke volume (SV) multiplied by the heart rate (HR).
  • CO increases at 8 weeks and peaks at 20 weeks.
  • CO increases by 30-50%
  • CO rises to 4.5-6.0 L / min 
How do we know her stroke volume?
  • Systolic ejection murmur is common (90% of women).
  • S3 gallop is common (90% of women).
  • Peripheral vascular resistance falls during pregnancy, too.
    • Hence the swelling.
    • Wear flipflops, wear compression socks
    • Especially medical residents.

Respiratory rate in pregnancy

  • Recall your standard respiratory volumes (see image below).
  • Note that O2 demand (consumption) is increased by 15-20% in pregnancy.
  • These volumes are affected by the displacement of the diaphragm rostrally.
  • Note that respiratory rate should remain unchanged in normal pregnancy.
    • This is possible because the tidal volume increases (because the minute ventilation increases).
  • Functional residual capacity decreases by 20% because of displacement the uterus.


004f.gif 001f.gif


  • So dyspnea is normal with pregnancy.
Volume (mL) Definition Non-pregnant Pregnant Change
Total lung capacity Vital capacity + residual volume 4200 4000 -4%
Vital Capacity Total lung capacity - residual volume 3200 3200 0%
Inspiratory Capacity Vital capacity - expiratory reserve volume 2500 2650 +6%
Tidal volume Volume moved in and out with each normal breath 450 550 +33%
Expiratory Reserve Volume Vital capacity - inspiratory capacity 700 550 -20%
Inspiratory Reserve Volume Inspiratory capacity - total volume 2050 2050 0%
Residual volume Total volume - vital capacity 1000 800 -20%
Functional Residual Capacity Residual volume + expiratory reserve volume 1700 1350 -20%

Arterial blood gas (ABG) in pregnancy

  • NB: ABG should never be "normal" in pregnancy!
    • Should have a respiratory alkalosis from blowing off their CO2 at higher rates.
    • Should have a metabolic acidosis (compensatory).
  • Many pregnant women will hyperventalate.
    • Give them a bag in which to breath.
    • Very hard to settle them down so important to give the bag quickly and pt will often become syncopous.


  • Pt's ABG was:
    • pH: 7.44 (
    • pCO2: 30
    • BiCarb: 21
    • paO2: 103


  • Recall that O2 consumption increases in pregnancy.
    • 15-20%
  • Recall that minute ventilation (the amount of air breathed in over the course of one minute) is increased relatively more than the O2 consumption.
    • This means that more air is breathed in and out than is necessary for the rate of oxygen consumption.
    • This is the definition of hyperventilation!
  • Recall the chemical equation: CO2 + H20 <=> H2CO3 <=> HCO3 + H
  • Recall that hyperventilation gives off CO2, dragging the equation to the left.
    • That is, pCO2 will be decreased.
  • Therefore pregnant, hyperventilating women become alkalotic (decreased H+).
    • That is, pH will be high (less H+).
    • This is a respiratory alkalosis.
  • Recall that the kidneys can excrete HCO3 to compensate for loss of CO2 (because of hyperventilation) and thus draw the equation back to the right.
    • Thus we expect the serum HCO3 to be low.
  • Recall that in pregnancy there is an increased oxygen carrying capacity (facilitated by RBC volume increased by 50%).
    • Thus we expect the pO2 to be high.


  • Given this respiratory alkalosis and metabolic compensation, we expect:
    • the pCO2 to be low: 26-32 mmHg (compared to non-pregnancy 38-45)
    • the pH to be high (alkaline): 7.40-7.46 (compared to non-pregnancy 7.38-7.42)
    • the HCO3 to be low: 18-21 mEq / L (compared to non-pregnancy 24-31)
    • the pO2 to be high: 106-108 mmHg (compared to non-pregnancy 70-100)


  • A high pCO2 is cause for alarm in the asthmatic pregnant woman.
  • Also, pCO2 should be monitored carefully in the acutely infected pregnant woman because they are immune compromised and have very little respiratory reserve.
Measure Non-pregnant Pregnant Change Reason
pH 7.38 - 7.42 7.4 - 7.46 Higher Because of the balance of respiratory alkalosis and (compensatory) metabolic acidosis.
pCO2 (mmHg) 38 - 45 26 - 32 Lower Because minute ventilation increases more than oxygen consumption (CO2 breathed off faster than oxygen consumed).
pO2 (mmHg) 70 - 100 101 - 106 Lower Because RBC volume is increased.
HCO3- (mEq / L) 24 - 31 18 - 21 Lower Because the kidneys are excreting HCO3- in an attempt to compensate for respiratory alkalosis.
O2 saturation (%) 95 - 100 95 - 100 None


  • Importance:
    • A normal pregnant woman has a compensated respiratory alkalosis (with a compensatory metabloic acidosis) and a diminished pulmonary reserve.
  • Cause:
    • Progesterone: relaxes the smooth muscle
    • Diaphragm raised 4cm

CBC in pregnancy

  • Plasma Volume and RBC Mass
    • Plasma volume increases by about 50%
    • RBC volume increases by about 30%
  • Result:
    • “Physiologic anemia of pregnancy”
    • The mean Hgb is about 11.5 g/dl
  • At 28 weeks, get new labs.
  • Will likely need to start iron.
    • Start at or below 10.
    • Yucky tasting, makes woman feel horrible.
    • Necessary for proper oxygenation.

Coagulation in pregnancy

  • In pregnancy the pt is hypercoaguable
    • This is the body's way to not die from bleeding when the parasite is ejected / ripped from the internal surface of the uterus.
  • DIC is the number 1 cause of death in laboring women around the world.
    • Not so much here in the US.
  • Procoagulant’s are increased: I, VII, VIII, IX, X and Fibrinogen.
  • Prevents Peripartum Hemorrhage
  • Can lose 1500mL without blinking.
  • Over 2000 mL lost, start worrying b/c coag factors are becoming deficient.

Renal System in pregnancy

  • Bun & Serum Creatinine decreases by about 25%
    • Low because of increased blood volume, increased GFR, increased blood flow to the kidneys
  • Plasma osmolarity decreases by about 10 mOsm / kg H20
  • Increase in tubal reabsorption of sodium
  • Marked increase in renin and angiotensin levels
    • But markedly reduced vascular sensitivity to their hypertensive effects
  • Increased glucose excretion
    • Fine during labor, a coping mechanism.
    • Labs at 28 weeks detect gestational diabetes.


  • Anatomic:
    • Kidney size increases
    • Kidney weight increases
    • Ureteral dilation (right side greater than left side)
      • Because of the angle of the uterus
    • Bladder becomes an intra-abdominal organ


  • Hemodynamic:
    • GFR increase by 50%
    • Renal plasma increased by flow by 75%
    • Creatinine Clearance increases to 150-200 cc / min
      • Most important kidney function test in pregnancy


  • Swollen Ankles:
    • Recall that the Plasma Volume Increases 50% in pregnancy.
    • Recall that there is decreased Systemic Vascular resistance in pregnancy.
    • Recall that there is renal retention of Na and H20 in pregnancy.
      • Leads to a total increase of 6 – 8 L in total body H20!
      • 2 / 3rd extracellular
      • 1 / 3rd intravascular
      • Intravascular increase is put in the "dependent" (affected by gravity) areas like the lower venous system.
    • Hard to move fluids through the venous system.

Case: Caring Family

  • The patient’s husband lies her flat in an effort to keep her comfortable.
  • The fetus’s FHTs (fetus heart tones) drop to 80.
  • But, the ROCK STAR medical student wedges the patient quickly on her left side and the FHT’s recover quickly!
    • Turns her off the vena cava.
    • Aids in venous return to the heart and therefore to oxygenation of blood for mom and baby.


  • What happened to the cared-for mother?
  • Gravid Uterus placed pressure on the IVC which resulted in decreased “utero-placental blood flow.”
    • Cardiac output falls
    • Blood is shunted to the brain and
    • Blood is shunted away from the uterus and placenta
    • This causes a reflex fetal bradycardia

Importance of Physiology

  • All can be abnormal in the Non-Pregnant state:
    • Decreased BP
    • Swollen ankles
    • CXR: cardiomegaly, leftward deviation
    • Systolic Ejection murmur
    • Dyspnea
    • Anemia
    • Hypercoaguable
    • NORMAL for Pregnancy

Maternal Physiology Review

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