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

First Trimester

  • 21 y/o G1P0 at 8w0d by L=6 week US at the (WVC) 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
  • PSxH: 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:
    • Constipation:
      • Mechanism: ferrous-sulfate in the pre-natal vitamins slows GI motility
      • Tx: give colace
    • Nausea and Vomiting: 
***beta-HCG (peaks 10-12 weeks)
***progesterone: a smooth muscle relaxant; causes decreased bowel movement which can lead to increased time for water reabsorption and thus constipation
***decreased GI motility
    • weight gain:
      • average Weight Gain
        • Normal Weight for Height: about 20 lbs
        • Underweight Women: about 30 lbs
        • Overweight Women: about 16 lbs


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.
  • 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.
  • She was given a GI cocktail in triage and she was still miserable.


  • Differential Diagnosis:
    • Constipation
      • Small bowel obstruction
    • Gallstones
    • Pancreatitis
    • Appendicitis


First / Second Trimester GI Issues

*Estrogen / Progesterone Gallstones
  • Recall that progesterone is a smooth muscle relaxant.
  • Recall that progesterone is at high levels throughout pregnancy to maintain the uterine environment.
  • 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
    • “Full stomach” sensation


  • During pregnancy, the appendix is moved medially such that appendicitis pain will present mid-quadrant.

Second / Third Trimester GI Issues

  • 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.
  • 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, UA (urinary analysis), CMP (complete metabolic panel), Coags (coagulation studies)
What is ABG?


  • 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


  • Are all these values normal or abnormal?

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
  • Normal

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 (compared to a normal of...)
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.

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).

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%

ABG in pregnanc

  • Pt's ABG was ?
  • PH: 7.44
  • pCO2: 30
  • BiCarb: 21
  • paO2: 103


  • Acidic?
  • Basic


  • Hyperventilate?
  • Hypoventilate?


Respiratory System in pregnancy

  • ABGs: PH= 7.40, PCO2=26-32, BiCarb= 18-21, PO2= 101-106 
*Pertinence: Relative Hyperventilation:
    • O2 consumption increases by 15-20%, minute ventilation increases to a greater degree 
*Pertinence: Relative Hyperventilation
    • Leading to decreased pCO2 
*Relative Respiratory Alkalosis (26-32)
    • PH is usually slightly alkaline (7.40-7.46) due to renal excretion of HCO3 (serum 18-21 (+/-) 1.6 lowers the serum bicarb)
    • Hyperventilation and increased O2 carrying capacity, pO2 is slightly elevated (106-108)
  • Pertinence: A normal pO2 in a pregnant patient is frequently abnormal


  • Changes in ABG
Measure Non-pregnant Pregnant Change
pH 7.38 - 7.42 7.4 - 7.46 Looser
pCO2 (mmHg) 38 - 45 26 - 32 Lower
pO2 (mmHg) 70 - 100 101 - 106 Lower
HCO3- (mEq / L) 24 - 31 18 - 21 Higher
O2 saturation (%) 95 - 100 95 - 100 None


  • Importance:
    • A normal pregnant woman has a compensated respiratory alkalosis and a diminished pulmonary reserve.
    • Respiratory Alkalosis with a compensatory metabloic acidosis.
  • Cause:
    • Progesterone
    • 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

Coagulation in pregnancy

  • In pregnancy the pt is hypercoaguable
  • Procoagulant’s are increased: I, VII, VIII, IX, X and Fibrinogen.
  • Prevents Peripartum Hemorrhage


Renal System in pregnancy

  • Bun & Serum Creatinine decreases by about 25%
  • 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


  • Anatomic:
    • Kidney size increases
    • Kidney weight increases
    • Ureteral dilation (right side greater than left side)
    • 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


  • 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.

Caring Family

  • The patient’s husband lies her flat in an effort to keep her comfortable. 
*The fetus’s FHT’s drop to 80.
  • But, the ROCK STAR medical student wedges the patient quickly on her left side and the FHT’s recover quickly!


  • What happened?
  • Gravid Uterus placed pressure on the IVC that in turn decreased “utero-placental blood flow.”
  • Regional Blood Flow Redistribution:
    • 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 with a leftward deviation
    • Systolic Ejection murmur
    • Dyspnea
    • Anemia
    • Hypercoaguable
    • NORMAL for Pregnancy

Maternal Physiology Review

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