Physiological Changes During Pregnancy

Training pregnant clients safely requires understanding the significant physiological changes that occur during pregnancy. These changes affect virtually every body system and have direct implications for exercise capacity, safety, and programming.

This chapter provides essential knowledge about pregnancy physiology. Note that as a fitness professional, your role is to understand these changes for safe program design—medical advice and clearance should always come from the client's healthcare provider.

Cardiovascular System Changes

Blood Volume and Composition

Pregnancy dramatically affects the cardiovascular system:

Blood volume:

• Increases 40-50% by week 32-34
• Plasma volume increases more than red blood cells
• Results in "physiological anemia" (lower hemoglobin concentration)
• Returns to normal 6-8 weeks postpartum

Heart changes:

• Cardiac output increases 30-50%
• Heart rate increases 10-20 bpm at rest
• Stroke volume increases
• Heart physically shifts position as uterus grows

Hemodynamic Implications

Effects on exercise:

• Higher resting heart rate means relative intensity is different
• Traditional heart rate zones don't apply
• Blood pressure may fluctuate
• Increased blood flow to uterus and skin

Important consideration - supine hypotensive syndrome:

• After first trimester, lying flat on back can compress the inferior vena cava
• Reduces blood return to heart
• Can cause dizziness, nausea, and reduced fetal blood flow
• Avoid supine exercises after ~16-20 weeks

Blood Pressure Changes

Blood pressure typically follows this pattern:

• First trimester: May decrease slightly
• Second trimester: Usually at lowest point
• Third trimester: Gradually returns toward pre-pregnancy levels

Training implications:

• Monitor for signs of low blood pressure (dizziness, lightheadedness)
• Position changes should be gradual
• Stay hydrated
• Report unusual blood pressure symptoms to healthcare provider

Respiratory System Changes

Anatomical Changes

Diaphragm and ribcage:

• Diaphragm elevates 4-5 cm as uterus grows
• Ribcage expands (increased circumference)
• Decreased space for lung expansion

Airway changes:

• Upper airway swelling possible
• Increased nasal congestion common

Ventilation Changes

Breathing patterns:

• Increased tidal volume (amount of air per breath)
• Respiratory rate may slightly increase
• Minute ventilation increases 30-50%
• Feeling of breathlessness is common

Why breathlessness occurs:

• Increased oxygen demand
• Progesterone stimulates breathing
• Mechanical restriction from growing uterus
• This is normal, not dangerous

Exercise Implications

Breathing during exercise:

• Breathlessness may occur at lower intensities
• Use RPE rather than heart rate
• Allow adequate rest between exercises
• Maintain conversation ability during exercise

Warning signs to stop:

• Severe breathlessness
• Inability to speak during moderate activity
• Chest pain
• Feeling faint

Musculoskeletal Changes

Postural Changes

Center of gravity shift:

• Anterior shift as pregnancy progresses
• Increased lumbar lordosis (lower back curve)
• Forward head posture common
• Wider stance for balance

Joint loading changes:

• Increased load on lower back
• Greater stress on hip joints
• Knee alignment may change
• Foot arch may flatten

Hormonal Effects on Joints

Relaxin hormone:

• Secreted throughout pregnancy
• Increases joint laxity
• Affects all joints, not just pelvis
• Peaks in first trimester and around delivery
• Persists several months postpartum

Implications of increased laxity:

• Greater range of motion (not always beneficial)
• Increased injury risk, especially with ballistic movements
• Joint instability possible
• Need for more controlled movements

Diastasis Recti

What it is:

• Separation of rectus abdominis muscles at linea alba
• Common in pregnancy (affects majority of women by third trimester)
• Can persist postpartum

Contributing factors:

• Abdominal wall stretching from growing uterus
• Hormonal softening of connective tissue
• Multiple pregnancies
• Poor loading strategies during pregnancy

Training considerations:

• Avoid exercises that cause visible "doming" of abdomen
• Modify traditional abdominal exercises
• Focus on core control rather than core strength
• Will address recovery strategies in postnatal chapter

Common Discomforts

Lower back pain:

• Affects 50-70% of pregnant women
• Related to postural changes and weight gain
• Appropriate exercise can help reduce pain

Pelvic girdle pain:

• Pain in pubic symphysis and/or sacroiliac joints
• Affects walking, standing, turning in bed
• May require significant exercise modification

Round ligament pain:

• Sharp pain in lower abdomen/groin
• Usually with sudden movements
• Benign but uncomfortable

Metabolic Changes

Energy Metabolism

Caloric needs:

• Increased energy requirements (especially 2nd/3rd trimester)
• Additional ~300-500 calories/day in later pregnancy
• Individual variation based on activity level

Blood glucose:

• Insulin sensitivity changes throughout pregnancy
• Gestational diabetes risk increases
• Blood sugar regulation important

Temperature Regulation

Thermoregulation changes:

• Basal metabolic rate increases
• Heat production increases
• Core temperature regulation affected
• Fetus cannot regulate own temperature

Training implications:

• Avoid exercising in hot environments
• Stay well hydrated
• Don't overheat
• Core temperature shouldn't exceed 39°C (102.2°F)
• Warning signs: excessive sweating, feeling overheated, dizziness

Weight Gain

Normal weight gain patterns:

• First trimester: 0.5-2 kg (1-4 lbs)
• Second trimester: ~0.5 kg (1 lb) per week
• Third trimester: ~0.5 kg (1 lb) per week
• Total: varies based on starting weight (typically 11-16 kg / 25-35 lbs for normal BMI)

Exercise implications:

• Don't train for weight loss during pregnancy
• Increased body weight affects exercise capacity
• Joint stress increases with weight gain
• Balance affected by changing body composition

Neurological and Psychological Changes

Cognitive Changes

"Pregnancy brain":

• Some women report memory and concentration changes
• Research is mixed on actual cognitive effects
• May be related to sleep disruption, stress, hormonal changes

Training implications:

• Be patient with instruction comprehension
• Use clear, simple cues
• Don't introduce overly complex new movements
• Focus on familiar exercises

Mood and Energy

Common experiences:

• Fatigue, especially first and third trimesters
• Mood fluctuations
• Anxiety about pregnancy and delivery
• Variable motivation for exercise

Trainer considerations:

• Energy levels vary day to day
• Be flexible with programming
• Exercise can improve mood
• Create supportive, non-judgmental environment

Trimester-by-Trimester Summary

First Trimester (Weeks 1-12)

Key changes:

• Hormonal surge (nausea, fatigue common)
• Blood volume beginning to increase
• Relaxin levels rising
• Usually no visible changes yet

Exercise considerations:

• Fatigue and nausea may limit training
• Can generally continue pre-pregnancy routine
• Avoid overheating
• Good time to establish healthy habits

Second Trimester (Weeks 13-27)

Key changes:

• Energy often returns
• Visible pregnancy beginning
• Center of gravity shifting
• Cardiovascular changes progressing

Exercise considerations:

• Often called the "golden period" for exercise
• Begin modifying exercises as needed
• Avoid supine positions after ~20 weeks
• Balance training becomes important

Third Trimester (Weeks 28-40)

Key changes:

• Significant size increase
• Breathing more difficult
• Fatigue returns
• Joints at maximum laxity
• Preparing for delivery

Exercise considerations:

• Significant modifications likely needed
• Focus on maintenance, not progression
• Reduce intensity as needed
• Prioritize comfort and safety
• Some women reduce training significantly; others continue modified exercise

Medical Conditions Requiring Referral

Absolute Contraindications to Exercise

Do not train pregnant clients with these conditions—refer to healthcare provider:

• Ruptured membranes (water broken)
• Premature labor
• Unexplained persistent vaginal bleeding
• Placenta previa after 26 weeks
• Pre-eclampsia
• Incompetent cervix
• Intrauterine growth restriction
• High-order multiple pregnancy (triplets or more)
• Uncontrolled Type 1 diabetes, hypertension, or thyroid disease
• Other serious cardiovascular, respiratory, or systemic disorders

Relative Contraindications

These require medical consultation before exercise:

• History of spontaneous abortion
• History of premature birth
• Mild/moderate cardiovascular or respiratory disease
• Anemia
• Malnutrition or eating disorder
• Twin pregnancy after 28 weeks
• Other significant medical conditions

Key Takeaways

1. Cardiovascular changes affect heart rate, blood pressure, and exercise response
2. Avoid supine positions after the first trimester
3. Respiratory changes cause normal breathlessness—use RPE, not heart rate
4. Joint laxity increases injury risk—avoid ballistic and extreme range movements
5. Prevent overheating through hydration and appropriate environment
6. Modifications increase as pregnancy progresses
7. Medical clearance is essential; know when to refer
8. Individual variation is significant—adjust to each client's experience