Normal Maternal Physiology: Implications for Prenatal Care

Donald J. Dudley, M.D.
Assistant Professor
Department of OB/GYN
U of U College of Medicine

List pertinent normal physiologic changes in the maternal cardiovascular, respiratory, renal, hematologic, gastrointestinal, and reproductive systems.

Describe the implications for these changes for normal and abnormal pregnancies.
List the nutritional requirements for calories, protein, iron, calcium, and folic acid for a normal pregnancy in a healthy gravid woman.
Describe the medical evaluation at the first prenatal visit and then subsequent visits for a normal pregnant woman.
List at least five routine laboratory tests obtained early in pregnancy and the rationale for each.

Dilutional anemia of pregnancy: lower hematocrits are seen in pregnancy because the expansion of plasma volume is greater than the increase in red blood cell mass

Hypercoagulable state of pregnancy: increased predilection for pregnant women to have venous clotting episodes

Hegar's sign: cervical changes of pregnancy such that the uterine cervix appears bluish and engorged

MSAFP (Maternal serum alpha-fetoprotein): Screening test of maternal blood done in the early second trimester to screen pregnant women for fetal anomalies and chromosomal abnormalities

Estimated delivery date (EDD): the estimated date of delivery based on either dating or ultrasound parameters

Bacterial vaginosis: a bacterial infection of the vagina associated with preterm labor and birth

Glucola: a screening test performed on maternal blood for gestational diabetes

Rhogam: an antibody preparation of anti-Rh factor given to Rh (-) women to prevent Rh isoimmunization

Neural tube defect (NTD): an abnormality in closure of the neural tube, resulting in a spectrum of anomalies from anencephaly (no cranium or cerebrum) to spina bifida

Intrauterine growth restriction (IUGR): pathological condition of abnormal placentation resulting in an undergrown fetus

Small-for-gestational age (SGA): the lower 10% of birthweights

Large-for-gestational age (LGA): the upper 10% of birthweights

Macrosomia: an abnormally large infant (usually > 4000 gm)

Outline

Introduction
Pertinent Changes in Normal Maternal Physiology
Nutritional Considerations in the Normal Pregnancy
Prenatal Care for the Normal Pregnancy

Outline

Introduction

The primary goal of prenatal care is to deliver a healthy term infant without impairing the mothers health and to identify and optimally treat the high-risk parturient.
Pertinent Changes in Normal Maternal Physiology
1. Cardiovascular system
  1. Cardiac
    1. Cardiac output increases about 30-50% (from 4.5 to 6.0 L/min)
    2. Stroke volume increases about 10 to 15%
    3. Pulse increases about 15-20 bpm
    4. Systolic ejection murmur and S3 gallop is common (about 90% of pregnant women)
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  2. Blood pressure
    1. Peripheral vascular resistance falls
    2. There is normally a fall in BP during the second trimester (5-10 mmHg systolic, 10-15mmHg diastolic), and then returns to normal during the third trimester Pertinence: Many of the effects of the altered cardiovascular system mimic heart failure (edema, gallops, dyspnea, distended neck veins, abnormal cardiac silhouette on CXR, EKG changes).
2. Respiratory system
  1. Unchanged: respiratory rate, vital capacity, inspiratory reserve volume
  2. Decreased: functional residual capacity (by 20%), expiratory reserve volume (by 20%), residual volume (by 20%), total lung capacity (by 5%)
    
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  3. Increased: inspiratory capacity (by 5%), tidal volume (by 30-40%)
  4. Arterial blood gasses: pH= 7.44, pCO2=30, bicarbonate=20-25, pO2=>100 Pertinence: A normal pregnant woman has a compensated respiratory alkalosis and a diminished pulmonary reserve.
3. Renal system
  1. Anatomic: increase in kidney size and weight, ureteral dilatation (Right > left), bladder becomes an intra-abdominal organ
  2. Hemodynamics:
    1. GFR increases 50%, renal plasma flow increases by 75%
    2. Creatinine clearance increases to 150-200 cc/min
  3. Metabolic changes
    1. BUN and serum creatinine decreases by about 25%
    2. Plasma osmolarity decreases about 10 mOsm/kg H2O
    3. Increase in tubular reabsorption of sodium
    4. Marked increase in renin and angiotensin levels, but markedly reduced vascular sensitivity to their hypertensive effects
    5. Increase in glucose excretion Pertinence: Pregnant women are more prone to pyelonephritis and bladder rupture during abdominal trauma.
4. Hematologic System
  1. Plasma volume and RBC mass
    1. Plasma volume increases by about 50%
    2. RBC volume increases by about 30%
    3. The result: the "dilutional anemia of pregnancy", such that the mean hemoglobin during pregnancy is about 11.5 g/dl
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  2. WBC and platelets
    1. WBC count increases during pregnancy
    2. Platelet count decreases, but stays within normal limits
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  3. Coagulation system: pregnancy as a "hypercoagulable state"
    1. Increased levels of fibrinogen, factor VII-X
    2. The placenta produces a plasminogen activator inhibitor Pertinence: Blood loss is well-tolerated during labor, but maternal vital signs do not change for blood loss of 1500 cc, so vital signs cannot be trusted as an indicator of blood loss. Also, serious thromboembolic disease is more common during pregnancy.
5. Gastrointestinal System
  1. Decreased motility, probably due to influence of progesterone
  2. Reduced gastric acid secretion Pertinence: A pregnant woman is considered to have a full stomach even if she has had nothing to eat or drink for several hours. Peptic ulceration is rare during pregnancy.
6. Reproductive System
  1. The Uterus
    1. Weight: increases from 70 gm to 1100 gm
    2. Blood flow: increases to about 750 cc/min, or about 10-15% of cardiac output
      
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      Pertinence: Laceration of the uterine arteries can result in massive hemorrhage in a short period of time
  2. The Cervix
    1. increase in water content and vascularity (Hegar's sign)
    2. increase in cervical mucous secretions
Nutritional Considerations in the Normal Pregnancy
1. Weight gain: both weight gain and pre-pregnancy weight are directly related to infant birthweight
  1. Average weight gain (no one knows optimal weight gain)
    1. Normal weight for height: about 20 lbs
    2. Underweight women: about 30 lbs
    3. Overweight women: about 16 lbs
  2. Average weight gain by organ system
    1. Fetus--7 1/2 lbs
    2. Placenta and amniotic fluid--3 lbs
    3. Blood volume--4 lbs
    4. Breasts--1 to 2 lbs
    5. Maternal fat--4 lbs
2. Daily dietary requirements for common nutrients
  1. Calories: increased 15% kcal/day, or you need about 2200 cal/day
  2. Protein: an additional 10 to 30 gm /day (about 75 gm/day total)
  3. Iron: supplement 30 to 60 mg of elemental iron per day
  4. Calcium: 1200 mg needed per day, usually provided by a quart of milk per day (can use 2 Tums day, each have 600 mg of calcium carbonate)
  5. Folate: supplement 200 to 400 g per day (most vitamins have 1 mg)
    1. In women with a prior history of having a baby with a neural tube defect, supplementing with 4 mg per day has been shown to decrease the risk of a recurrence in the next pregnancy
3. The pregnant patient is best served by having a healthy balanced diet with iron and folate supplementation. Only rarely are other vitamin supplements needed.
Prenatal Care for the Normal Pregnancy
1. The first visit--The basic decision: normal vs. high-risk
  1. History
    1. Menstrual history: confirm the pregnancy
      
      (1) Regularity, interval, duration
      (2) Last normal menstrual period (LMP): characteristics and bleeding since then?
      (3) Assign an estimated date of delivery (EDD): it is inappropriate for a patient to be past 20 weeks of pregnancy without a definite EDD
    2. Past obstetric history (if any): for many conditions, if the patient had an abnormality in the first pregnancy, then she is predisposed to a recurrence in subsequent pregnancies
      
      (1) Length of gestation
      (2) Birth weight: low (IUGR/SGA) vs. high (LGA/macrosomia)
      (3) Fetal/neonatal outcome: alive vs. dead, impairments
      (4) Length of labor
      (5) Type of delivery: vaginal vs. cesarean, breech vs. cephalic
      (6) Other complications
      (7) Type of anesthesia used
    3. Past medical history
      
      (1) Significant past illnesses
      (2) Permanent conditions: hypertension, diabetes, seizure disorder, thyroid disease, and so on
      (3) Previous surgeries: C/S, gynecologic/abdominal surgery
      (4) Medications: prolonged therapy
    4. Family history
      
      (1) Look for conditions with familial predilection: hypertension, diabetes, cardiac disease, genetic abnormalities
    5. Social history
      
      (1) alcohol use, smoking, drug abuse
      (2) 8-9% of pregnant women in the Salt Lake Valley have a positive urine for at least one drug of abuse
      (3) occupational hazards
    6. Genetic screening: evaluate from patient and family history the risk for genetic abnormalities (is it above the usual 2-3% of all pregnancies?)
      
      (1) Risk of chromosomal abnormalities increases with maternal age:
      
      Age 35    1/204
      
      Age 38    1/103
      
      Age 40    1/65
      
      Age 42    1/40
      
      Age 44    1/25
  2. Physical examination
    1. Vital signs: are they normal or not?
    2. General physical examination: are there any concurrent undiagnosed medical conditions?
    3. Abdominal exam: scars?, enlarged uterus?, other masses?
    4. Pelvic examination: uterine size (confirm dates), cervical examination, Pap smear, clinical pelvimetry
    (1) uterine size large for dates: think twins or incorrect dates
    (2) uterine size small for dates: think IUGR or incorrect dates
    (3) the next step: ultrasound evaluation of the pregnancy
2. Laboratory data
  1. CBC: make certain the patient is normal for pregnancy
  2. Serology for syphilis: RPR, VDRL, confirm with FTA
  3. Blood type, Rh, and indirect Coomb's test: evaluate for blood group isoimmunization
  4. Rubella titer
  5. Hepatitis screen
  6. Maternal serum alpha-fetoprotein (MSAFP) at 15-18 weeks
    
    (1) If elevated, then the patient should be evaluated with a targeted ultrasound for fetal anomalies, including neural tube defects and abdominal wall defects (gastroschisis and omphalocele)
    (2) If low, then the patient should be offered a genetic amniocentesis to evaluate the fetus for trisomy 21 (Down's syndrome)
  7. Urinalysis and urine culture
  8. Pap smear: abnormal smears must be evaluated during pregnancy
  9. Bacterial vaginosis (BV) screening: wet mount
3. Subsequent visits
  1. Frequency: the usual regimen
    1. Monthly up to 32 weeks
    2. Every two week until 36 weeks
    3. Weekly after 36 weeks until delivery
  2. Interval history
    1. General health and well-being
    2. Presence or absence of contractions
    3. Fetal movement: increased, decreased
    4. Leaking clear fluid: rule out spontaneous rupture of membranes
    5. Vaginal bleeding: all vaginal bleeding after the first trimester mandates an evaluation
  3. Examination
    1. Maternal weight
    2. Blood pressure: get worried if it is much above 120/80
    3. Fundal height, estimated fetal weight, fetal position
    4. Always confirm the presence of fetal heart tones (FHT's)
    5. Urinalysis for protein and glucose: simple inexpensive screens for pre-eclampsia and diabetes
  4. Laboratory evaluation
    1. CBC in the early third trimester: rule out anemia
    2. Glucola (diabetes screen) in the early third trimester
    3. Rhogam at 26-28 weeks if the patient is Rh negative
  5. Ultrasound evaluation: routine vs. indicated?
  6. Preparation for labor
    1. Childbirth education classes
    2. Physician input
4. Some common complaints during pregnancy: the "Discomforts of Pregnancy"
  1. Nausea and vomiting: usually dissipates by 15 weeks or so
  2. Constipation: common throughout pregnancy
  3. Heartburn: often worsens as pregnancy progresses
  4. Vaginitis: treat only if symptomatic
  5. Varicose veins and hemorrhoids: treat symptomatically
  6. Headaches
  7. Edema: lower extremity edema is very common
  8. Backache: lordosis is common with change in the center of gravity
  9. Leg cramps: especially in lower leg
  10. Faintness and light-headedness
  11. Breast tenderness
  12. Carpal tunnel syndrome
5. Common questions for which you will need to have an answer
  1. Activity and exercise: moderation should be encouraged
  2. Sexual activity: no problem as long as pregnancy progresses normally
  3. Diet: a general balanced diet is usually all that is required
  4. Bathing and swimming: no high speed sports or jet skis
  5. Douching: OK if pregnancy is normal, best avoided if possible
  6. Dentition: a dental check-up is recommended, any work is OK
  7. Immunizations: should probably avoid live virus vaccines
  8. Travel: no problems, but should have frequent stops to stretch
  9. Employment: usually no contraindication as long as pregnancy is normal

Take Home Points

The pregnant patient is best served by a confident and caring physician who vigilantly searches for high-risk features and then treats the patient as is appropriate for each high-risk condition. Most patients have completely normal pregnancies, but the high-risk pregnancy mandates changes in the "normal" evaluation of the pregnant patient.
The process of antenatal care is on-going risk assessment:
1. What is the genetic risk? (maternal age, abnormal MSAFP screening, folate administration for prior NTD)
2. What is the risk for preterm birth? (BV screening, history of preterm birth)
3. What is the risk for pregnancy-induced hypertension? (history)
4. What is the risk for IUGR? (past history, small uterine size for dates)
5. What is the risk for blood group isoimmunization? (Rhogam for Rh- women NOT previously sensitized))
After the core curriculum, using this lecture as a basis, you should be able to:
1. List at least five conditions identified by medical history that results in a high-risk pregnancy.
2. List at least six conditions found during physical examination of a pregnant woman that denotes a high-risk pregnancy.

Human Reproduction : Clinical, Pathologic and Pharmacologic Correlations.
Copyright C. Matthew Peterson, M.D., 1997