Nearly every body system goes through some degree of change during pregnancy to ensure the well-being of the growing foetus, sometimes at the expense of the mother’s comfort.

Cardiovascular changes

Cardiovascular changes begin at week 5 of gestation and persist up to a year post partum.

  • Resting heart rate (HR) increases by 15 – 20 beats per minute
  • Cardiac output (CO = amount of blood pumped out by the heart per minute) increases by 30 – 50%. This is due to both the increased HR and an increased stroke volume (SV = amount of blood pumped in each heart beat) which increases by 10% by the end of the first trimester
  • Blood flow to the pregnant uterus is markedly increased compared to the non pregnant state
  • The kidneys are triggered by hormones to retain water and salt which results in a 40% increase in blood volume
  • The kidneys also increase their filtration rate by 50% to get rid of waste more efficiently, which means more toilet breaks for the mother
  • Blood volume increases but at first this is only an increase in the fluid part of blood and not the red blood cells so there is relative anaemia (dilutional anaemia)
  • The kidneys then increase their production of EPO to signal to the body to produce more red blood cells. Although this leads to a higher number of red blood cells, the composition of the blood never fully corrects back to normal ratios
  • The body also increase production of clotting factors which helps prevent serious blood loss at delivery but puts the pregnant woman at an increased risk of DVT during the pregnancy

Respiratory changes

Respiratory changes occur that are both anatomical and functional to ensure the growing foetus receives enough oxygen.

  • The diaphragm rises by approximately 4cm due to a flaring of the lower ribs
  • There is an increase in the amount of air per breath (from 450mL to 650ml) but not an increase in the rate of breathing
  • Despite this many women still feel out of breath due to the effects of the pregnancy hormone progesterone

Gastrointestinal changes

Gastrointestinal changes occur to help the foetus but they may make the mother more uncomfortable in the process.

  • Progesterone causes smooth muscle to relax which delays gastric emptying and causes a increase in reflux
  • Insulin resistance increases in a diabetic like state ensuring there is more glucose in the blood available for use by the foetus
  • These high insulin levels and other hormones also lead to storage of extra body fat in early pregnancy for use later

Musculoskeletal changes

Musculoskeletal changes occur to accommodate the growing foetus and help facilitate delivery at term but these aren’t always in the best interests of the mother.

  • The growing breasts, uterus and foetus and an increased lumbar lordosis result in a shift in the mother’s centre of gravity which may cause balance problems
  • Hormonal changes cause ligament laxity to help soften the pubic symphysis and prepare for delivery
  • Other joints are also affected by this ligamentous laxity which can increase the risk of sprains and strains
  • Increased intra-abdominal pressure caused by the growing uterus can put excessive strain on the pelvic floor muscles and connective tissue

How much weight gain is normal during pregnancy?

  • This depends on your starting weight
  • A mother with a normal pre-pregnancy weight (BMI 18.5 – 25) should gain 11.5 – 16kg during pregnancy
  • This equates to 0.5 – 2kg in the first trimester and 0.5kg/week in the 2nd and 3rd trimesters
  • Underweight women (BMI < 18.5) should gain 12.5 – 18kg during pregnancy
  • Overweight women (BMI 25-30) should aim to gain 7 – 11.5kg during pregnancy (0.25kg/week)
  • Obese women (BMI > 30) should gain 5-9kg during pregnancy

Where does the weight go?

  • Foetus = 3.2 – 3.6kg
  • Fat stores (for later use) = 2.7 – 3.6kg
  • Increased blood volume = 1.4 – 1.8kg
  • Increased other fluid volume = 0.9 – 1.4kg
  • Amniotic fluid = 0.9kg
  • Breast enlargement = 0.45 – 1.4kg
  • Uterine hypertrophy = 0.9kg
  • Placenta = 0.7kg