Physiological Changes In Pregnancy

oPatientPlus articles are written by UK doctors and are based on research evidence, UK and European Guidelines. They are designed for health professionals to use, so you may find the language more technical than the condition leaflets.

Pregnancy is associated with normal physiological changes that assist fetal survival as well as preparation for labour. It is important to know what 'normal' parameters of change are in order to diagnose and manage common medical problems of pregnancy, such as hypertension, gestational diabetes, anaemia and hyperthyroidism.

See separate articles on Antenatal Care and Minor Symptoms of Pregnancy.

See also the separate article on Gestational Diabetes.

Pituitary

  • FSH/LH fall to low levels.
  • ACTH and melanocyte-stimulating hormone increase.
  • Prolactin increases.

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Thyroid and parathyroid[1]

  • Thyroxine-binding globulin (TBG) concentrations rise due to increased oestrogen levels.
  • T4 and T3 increase over the first half of pregnancy but there is a normal to slightly decreased amount of free hormone due to increased TBG-binding.
  • TSH production is stimulated, although in healthy individuals this is not usually significant. A large rise in TSH is likely to indicate iodine deficiency or subclinical hypothyroidism.
  • Serum calcium levels decrease in pregnancy, which stimulates an increase in parathyroid hormone (PTH).
  • Colecalciferol (vitamin D3) is converted to its active metabolite, 1,25-dihydroxycolecalciferol, by placental 1α-hydroxylase.

Adrenal and pancreas[2]

  • Cortisol levels increase in pregnancy, which favours lipogenesis and fat storage.
  • Insulin response also increases so blood sugar should remain normal or low.
  • Peripheral insulin resistance may also develop over the course of pregnancy and gestational diabetes is thought to reflect a pronounced insulin resistance of this sort.
  • Progesterone reduces systemic vascular resistance by about 20% early in pregnancy. Postural hypotension may result.
  • Diastolic and systolic blood pressure tend to fall during mid pregnancy and then return to normal by week 36.
  • Venous return in the inferior vena cava can be compromised in late pregnancy if a woman lies flat on her back. This is relieved by lying in the left lateral position.
  • Increased circulating angiotensin II encourages water and sodium retention, leading to an increased plasma volume (to 50% by 30 weeks) and predisposing to oedema. This enables increased uterine blood flow to meet growing nutritional and oxygenation needs of the fetus. It also enables blood loss (average 500 ml) at delivery to be met without physiological decompensation.
  • Advise women not to take up unaccustomed, vigorous exercise in pregnancy as there is a risk of diversion of uterine blood flow to the skeletal muscles.
  • Blood flow to kidneys, skin and mucosa increases.
  • Cardiac output increases by 30-50% with 15% increase in heart rate and 25-30% increased stroke volume. Much of this adjustment occurs prior to 12 weeks of gestation and so impaired cardiac function is likely to present problematically in early pregnancy or with the sudden increase in pre-load in the third stage of labour.
Cardiac examination in pregnancy:
  • Many women have a third heart sound after mid-pregnancy.
  • Diastolic murmurs should be considered potentially pathological.
  • Systolic flow murmurs are common.
  • ECG - left axis deviation is common, sagging ST segments and inversion or flattening of the T wave in lead III may also occur.
  • Tidal volume increases by about 200 ml, increasing vital capacity and decreasing residual volume. In later stages of pregnancy, splinting of the diaphragm may occur with some decrease in tidal volume. Respiratory rate does not alter significantly.
  • Increased oxygen consumption by approximately 20%.
  • State of compensated respiratory alkalosis - arterial pCO2 drops, arterial pO2 remains unchanged and decrease in bicarbonate prevents pH change. Lower maternal pCO2 facilitates oxygen/carbon-dioxide transfer to/from the fetus.
  • Many women complain of feeling short of breath in pregnancy without explanatory pathology. The mechanism of this is not fully understood
  • Appetite is usually increased, sometimes with specific cravings.
  • Progesterone causes relaxation of the lower oesophageal sphincter and increased reflux, making many women prone to heartburn.
  • Gastrointestinal motility is reduced and transit time is consequently longer. This allows increased nutrient absorption. Constipation is common.
  • The gallbladder may dilate and empty less completely. Pregnancy also predisposes to the precipitation of cholesterol gallstones.
  • Gums become spongy, friable and prone to bleeding. Good dental care is important.
  • The increased blood volume and cardiac output during pregnancy cause a 50-60% increase in renal blood flow and glomerular filtration rate (GFR). This causes an increased excretion and reduced blood levels of urea, creatinine, urate and bicarbonate.
  • Mild glycosuria and/or proteinuria may occur because the increase in GFR may exceed the ability of the renal tubules to reabsorb glucose and protein.
  • Increased water retention causes a reduction of plasma osmolality.
  • The smooth muscle of the renal pelvis and ureter become relaxed and dilated, kidneys increase in length and ureters become longer, more curved and with an increase in residual urine volume.
  • Bladder smooth muscle also relaxes, increasing capacity and risk of UTI.
  • Approximately 5% of pregnant women have bacteriuria, often asymptomatic, and there is a greater risk of developing pyelonephritis in pregnancy.
  • Dilutional anaemia is caused by the rise in plasma volume. Elevated erythropoietin levels increase the total red cell mass by the end of the second trimester but haemoglobin concentrations never reach pre-pregnancy levels.
  • A modest leukocytosis is observed.
  • A normal pregnancy creates a demand for about 1000 mg of additional iron. This equates to 60 mg elemental iron or 300 mg ferrous sulfate per day.
  • Serum iron falls during pregnancy whilst transferrin and total iron binding capacity rise.
  • Levels of some clotting factors (VII, VIII, IX and X) and fibrinogen increase whilst fibrinolytic activity decreases. These changes protect from haemorrhage at delivery but also make pregnancy a hypercoagulable state with increased risk of thromboembolism. See also the separate article on Venous Thromboembolism in Pregnancy.
  • One study found that during early pregnancy: antithrombin activity remained unchanged, protein S activity decreased significantly and there was a potentially biologically significant increase in protein C activity (see the separate article on Thrombophilia).[6]
  • Serum alkaline phosphatase increases during pregnancy - due to placental production.
  • Serum albumin decreases.
  • Changes in energy requirements in pregnancy remain controversial - healthy levels of fat deposition and variation in women's physical activity levels cause uncertainty as to the recommendations that should be made for this time.[7]
  • The basal metabolic rate increases slowly over the course of pregnancy, by 15-20%.
  • In women with normal BMIs, energy requirement does not increase significantly during the first trimester, increases by about 350 kcal/day in the second trimester and 500 kcal/day in the third.[7]
  • Active energy expenditure tends to fall over pregnancy.
  • Normal weight gain is approximately 12.5 kg (usually at a rate of 0.5 kg per week for the final 20 weeks). 5 kg is the fetus, placenta, membranes and amniotic fluid and the rest is maternal stores of fat and protein and increased intra- and extra-vascular volume.
  • Hyperpigmentation of the umbilicus, nipples, abdominal midline (linea nigra) and face (chloasma) are common due to the hormonal changes of pregnancy.
  • Hyperdynamic circulation and high levels of oestrogen may cause spider naevi and palmar erythema.
  • Striae gravidarum ('stretch marks') are common.
  • Increased ligamental laxity caused by increased levels of relaxin contribute to back pain and pubic symphysis dysfunction.
  • Shift in posture with exaggerated lumbar lordosis leading to the typical gait of late pregnancy.[8]
Interpreting blood test results in pregnancy[9]
  Trend in normal pregnancy (compared to non-pregnant state) Pregnancy normal values
(ALWAYS USE LOCAL REFERENCE RANGES)
Abnormalities and possible interpretations
Haemoglobin Decreased 10.5-13.5 g/dL Consider dilutional anaemia of pregnancy.
White cell count Increased 8-18 x109/L Always consider in the light of the patient's clinical status.
Platelets Unchanged/slightly increased 200-600 x109/L Always consider in the light of the patient's clinical status.
Sodium Slightly decreased 132-140 mmol/L Always consider in the light of the patient's clinical status.
Potassium Slightly decreased 3.2-4.6 mmol/L Always consider in the light of the patient's clinical status.
Urea Decreased 1.0-3.8 mmol/L Increased in dehydration, hyperemesis, late stages of pre-eclampsia and renal impairment.
Creatinine Decreased 40 - 80 μmol/L Increased in renal impairment and the late stages of pre-eclampsia.
Fasting glucose Unchanged 3.0-5.0 mmol/L Increased in gestational diabetes.
Total calcium Decreased 2.0-2.4 mmol/l Increased in primary hyperparathyroidism.
Magnesium Unchanged 0.6-0.8 mmol/L Decreased if there is vomiting or hyperemesis gravidarum.
Albumin Decreased 24-31 g/L Decreased further if there is malnutrition, recurrent vomiting or hyperemesis gravidarum.
Bilirubin Decreased 3-14 μmol/L Increased in obstetric cholestasis, HELLP, the late stages of pre-eclampsia, acute fatty liver, viral hepatitis. See the separate article on Jaundice in Pregnancy.
ALT Unchanged/slightly decreased 1-30 U/L As for bilirubin.
AST Unchanged/slightly decreased 1-21 U/L As for bilirubin.
ALP Increased 125-250 U/L Increased further in metabolic bone disorders or rare pregnancy-associated conditions - eg, chronic histiocytic intervillositis.

 

TSH
Slight decrease in the first trimester, normal in the second trimester, slightly raised in the last trimester 0.1-4.0 IU/L Less than 0.05 in Graves' disease or hyperemesis gravidarum.
fT4 Unchanged 10-25 pmol/L Increased in Graves' disease or hyperemesis gravidarum.
fT3 Unchanged 3.5-6 pmol/L Increased in Graves' disease or hyperemesis gravidarum.

Further reading & references

  1. Lazarus JH, Premawardhana LD; Screening for thyroid disease in pregnancy; J Clin Pathol. 2005 May;58(5):449-52.
  2. Butte NF; Carbohydrate and lipid metabolism in pregnancy: normal compared with gestational diabetes mellitus.; Am J Clin Nutr. 2000 May;71(5 Suppl):1256S-61S.
  3. Thornburg KL, Jacobson SL, Giraud GD, et al; Hemodynamic changes in pregnancy.; Semin Perinatol. 2000 Feb;24(1):11-4.
  4. Chesnutt AN; Physiology of normal pregnancy.; Crit Care Clin. 2004 Oct;20(4):609-15.
  5. Physiological changes of pregnancy; Anaesthesia UK
  6. Said JM, Ignjatovic V, Monagle PT, et al; Altered reference ranges for protein C and protein S during early pregnancy: Implications for the diagnosis of protein C and protein S deficiency during pregnancy. Thromb Haemost. 2010 May;103(5):984-8. doi: 10.1160/TH09-07-0476. Epub 2010 Feb 19.
  7. Butte NF, Wong WW, Treuth MS, et al; Energy requirements during pregnancy based on total energy expenditure and energy deposition. Am J Clin Nutr. 2004 Jun;79(6):1078-87.
  8. Foti T, Davids JR, Bagley A; A biomechanical analysis of gait during pregnancy. J Bone Joint Surg Am. 2000 May;82(5):625-32.
  9. Tran H; Biochemical tests in Pregnancy, Australian Prescriber 2005;28:98-101

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. EMIS has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.

Original Author:
Dr Chloe Borton
Current Version:
Peer Reviewer:
Dr John Cox
Document ID:
740 (v26)
Last Checked:
25/01/2013
Next Review:
24/01/2018