Physiological Changes in Pregnancy
Changes occur in the mother’s body to provide for fetal needs, and to prepare the maternal body for labour & delivery. Here are some of those changes according to the bodily systems:
With the upward displacement of the diaphragm, the apex is moved anteriorly & to the left:
- ECG findings: left axis deviation, depressed ST segments & inversion or flattened T-waves in lead III.
Cardiac output increases starting from week 6 – 8 of gestation, reaching a 30 – 50% increase by 32-34 weeks.
- Progesterone & oestrogen acting on the renin-angiotensin & aldosterone pathways: Retaining water & sodium
- Increases stroke volume by 25 – 35%
- Contractility remaining unchanged.
- Left ventricular hypertrophy & dilatation
- This increase enables uterine blood flow to meet growing nutritional & oxygenation needs of the foetus. It also enables blood loss (average 500 ml) at delivery without decompensation.
Oestrogen & progesterone causes vasodilatation:
- Resulting in a fall in peripheral vascular resistance by about 20% early in pregnancy.
- Postural hypotension may occur.
- Consequently heart rate increases by 15 – 25%
- Blood flow to kidneys, skin & mucosa increases.
Diastolic& systolic blood pressures tend to fall during mid pregnancy, which return to normal by week 36.
Renal erythropoietin increases red cell mass by 20-30%
- Which is a smaller rise than that in plasma volume.
- Resulting in haemodilution (physiological anaemia of pregnancy).
Serum iron ↓ falls whilst transferrin & total iron binding capacity rise↑.
↑ white cell count rises & peaks after delivery.
Pregnancy induces a hypercoagulable state:
- ↑ concentration of clotting factors & fibrinogen.
- ↓ fibrinolytic activity decreases.
- Platelet production is ↑.
- Platelets count ↓because of dilution & consumption; function remains normal.
Insulin production & response ↑:
Blood sugar level usually remains normal, or sometimes becomes low.
Sometimes the insulin response is met with resistance caused predominantly by human placental lactogen.
- Follicular stimulating & luteinising hormones ↓
- ACTH & melanocyte-stimulating hormone ↑
- Prolactin ↑
Preparing the mother for breastfeeding.
- Thyroxine-binding globulin (TBG) ↑
- T4 & T3 ↑ over the first half of pregnancy but slightly ↓ during the pregnancy due to ↑ TBG-binding
- Serum calcium levels ↓, stimulating an increased production of parathyroid hormone (PTH↑ ).
- Cortisol levels ↑:
Favouring lipogenesis & fat storage
Appetite is usually ↑, sometimes with specific cravings.
Many women are prone to heartburn because:
- Progesterone causes relaxation of the lower oesophageal sphincter & increases reflux.
- The uterus causes a gradual upward displacement of stomach & intestines, which increases intragastric pressure & changing the angle of the gastroesophageal junction.
Serum alkaline phosphatase increases up to 3 times normal, as a result of placental production.
Serum albumin ↓.
Gastrointestinal motility is ↓& transit time is longer to increase nutrient absorption.
-Therefore, constipation is common.
↓ in the gallbladder contractility:
- Predisposition to forming cholesterol gallstones.
Gums become spongy, friable & prone to bleeding.
The diaphragm is progressively displaced upward by the uterus.
- Moves with greater exertion during breathing.
Breathing is mainly diaphragmatic.
- Total lung capacity ↓slightly
- Inspiratory reserve volume is increased but FEV1 remain unchanged
- Tidal volume ↑
- ↑ vital capacity & ↓ residual volume
Capillary engorgement & oedema of the upper airway down to the pharynx, false cords, glottis & arytenoids (due to hormonal changes).
Respiratory rate usually doesn’t increase.
Increased oxygen consumption by approximately 20%
Airway resistance is ↓due to progesterone-mediated bronchial & tracheal smooth muscle relaxation
State of compensated respiratory alkalosis.
The basal metabolic rate éslowly during pregnancy by 15 – 20%.
In women with normal BMIs, energy requirement:
- Does not increase significantly during the 1st trimester.
- ↑350 kcal/day in the 2nd trimester.
- ↑500 kcal/day in the 3rd trimester.
Normal weight gain is approximately 12.5 kg.
Renal plasma flow & glomerular filtration rate (GFR) éprogressively during pregnancy, reaching 50-60% higher at term:
- Mild glycosuria &/or proteinuria is viewed as normal.
- ↑ clearance of urea, creatinine, urate & excretion of bicarbonate.
Plasma osmolality ↓ because of water retention.
Kidneys increase in length & ureters become longer, more curved & with an increase in residual urine volume.
Pregnant women are more prone to urinary tract infections because of progesterone-mediated smooth muscle relaxation of the renal pelvis, ureters, & bladder:
- 5% of pregnant women have bacteriuria, often asymptomatic.
- Symptomatic or not you have to treat.
Hyperpigmentation of the umbilicus, nipples, abdominal midline (linea nigra) & face (chloasma) due to hormonal changes.
Spider naevi & palmar erythema may be seen.
Striae gravidarum ('stretch marks') are common.
Increased ligamental laxity caused by increased levels of relaxin.
Shift in posture with exaggerated lumbar lordosis leading to the typical gait of late pregnancy
Oakley, Celia, and Carole A Warnes. Heart Disease In Pregnancy. Malden, Mass.: Blackwell Pub./BMJ Books, 2007. Print.
Editorial, AnaesthesiaUK. 'Anaesthesia UK : Physiological Changes Of Pregnancy'. Frca.co.uk. N.p., 2015. Web. 1 Oct. 2015.
Said, J. M. 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 103.5 (2010): 984-988. Web. 1 Oct. 2015.
Written by: Abeer AlKhairy.
Reviewed by: Roaa Amer.
Format editor: Roaa Amer
Web Publisher: Adel Yasky