Regular exercise is encouraged during and after pregnancy. However there is no doubt that many anatomical and physiological changes occur during pregnancy, affecting different body systems including musculoskeletal, cardiovascular and respiratory. This article will review the following adaptations that take place:

  1. Musculoskeletal Adaptations
  2. Nutritional Requirements
  3. Cardiovascular Adaptations
  4. Respiratory Adaptations
  5. Thermoregulatory Control
  6. Fetal Responses to Maternal Exercise

1. Musculoskeletal Adaptations

  • Weight gain in pregnancy –> increased force/stress placed on weight-bearing joints such as hips and knees (up to 100%) during weight-bearing exercises such as walking, jogging, running
  • Lumbar lordosis (sway back) is common due to the growing tummy –> high prevalence of low back pain (50%), shift of centre of gravity forward, changes in posture, loss of balance, increased risk of falling
  • Ligamentous laxity due to increased levels of oestrogen and relaxin –> more prone to strains and sprains
  • Uterine activity – inconclusive whether exercise causes an increase in uterine contractions or whether strenuous activity causes preterm labour

2. Nutritional Requirements

  • After the 1st trimester, approx an extra 300 kcal (or 1.2 MJ) per day is required to meet metabolic requirements of pregnancy
  • Energy requirements are matched to energy expenditure (eg. increased energy use through exercise –> more energy required from nutrition)
  • Adequate carbohydrate intake is important as pregnant women metabolise carbohydrates at a faster rate both at rest and during exercise

3. Cardiovascular Adaptations

  • Many haemodynamic changes (to provide adequate nutrients and oxygen to both mother and fetus) including:
    • Increase in blood volume, heart rate (20% greater during 2nd/3rd trimesters), stroke volume (10% greater by end of 1st trimester), cardiac output (30-50% greater by mid-pregnancy)
    • Decrease in systemic vascular resistance
  • Body position influences blood flow:
    • After 1st trimester, supine (lying on back) –> obstruction of venous return –> decrease in cardiac output (this position should therefore be avoided as much as possible during rest and exercise)
    • Stationary standing –> decrease in cardiac output
  • Exercise increases heart rate, therefore strenuous or high intensity aerobic exercise which may cause an excessive rise in heart rate could affect blood flow to the fetus and should be avoided

4. Respiratory Adaptations

  • Increase in tidal volume and minute ventilation (~50%)
  • Increase in resting oxygen requirements, oxygen uptake and baseline oxygen consumption (~10-20%)
  • Increased work of breathing due to increased pressure placed on the diaphragm by the growing uterus
  • Decreased oxygen availability for aerobic exercise –> maximum exercise performance reduced

5. Thermoregulatory Control

  • Increased basal metabolic rate and heat production during pregnancy
  • Increase in body core temperature during exercise is directly related to the intensity of the activity
  • During moderate intensity aerobic exercise, core temperature of non-pregnant women increases ~1.5 deg C (during first 30 mins) and plateaus
  • Exercising in hot, humid conditions or undertaking very high intensity exercise will cause the core temperature to continue to rise
  • Fetal body core temperature are ~1 deg C > maternal temp

6. Fetal Responses to Maternal Exercise

  • During or after maternal exercise, a minimum or moderate increase in fetal heart rate (by 10-30 bpm over baseline) has been shown
  • A link between strenuous physical activity, deficient diet and developing intrauterine growth restriction (IUGR) has been reported
  • Birth weight does not appear to be affected by exercise in women who have adequate energy intake (eg. very little risks have been reported on continuing physical training in pregnant athletes)
  • Other reports have suggested that mothers whose work involves standing or repetitive, strenuous, physical work such as lifting tend to deliver earlier and have smaller gestational age infants

Summary

In general, it is safe for pregnant mothers to perform moderate intensity exercise. However it is important to be aware of the anatomical and physiological changes that take place during pregnancy in order to exercise smartly whilst still meeting the nutritional and metabolic requirements of the growing fetus.

Other related articles you may be interested in:

Exercise during pregnancy

Exercise after pregnancy: looking after yourself

Round ligament pain in pregnancy

Reference

Artal and O’Toole (2003) Guidelines of the American College of Obstetricians and Gynecologists for exercise during pregnancy and the postpartum period. British Journal of Sports Medicine 37: 6-12.