Pregnancy - physiology

[biomedicool]

Hormones

• hCG - human chorionic gonadotropin - present in blood and urine, produced by blastocyst and placenta
  
• Low levels of hCG could mean miscarriage, ectopic pregnancy, miscalculation of dates
• High levels = molar pregnancy (cells that normally form a baby dysfunction and form cysts instead), multiple pregnancy (twins etc), miscalculation of dates
• Progesterone increases fat deposition in early weeks and stimulates appetite
• Increased oestrogen from corpus luteum promotes mammary gland development (breasts enlarge)
• And inhibits ovulation via negative feedback [OES decreases GnRH release by hypothalamus –> linhibition of FHS and LH –> no ovulation]
• Relaxin (secreted by corpus luteum) softens connective tissue in preparation for labour - not specific, all joints can be affected
• Peaks in early and late pregnancy

Physical changes

Blood pressure

• As early as 4 weeks into pregnancy 
  
• plasma volume increase
  
• caused by the affects of oestrogen and progesterone on the kidneys
  

OES and Prg cause vasodilation 

• less resistance = less pressure
  
• Heart rate increases by 25% to compensate
  
• stroke volume increases 
  
• increased overall cardiac output by 50% in third trimester
  

Direct action of enlarged uterus:

• compresses the descending aorta and inferior vena cava 
  
• decreased venous return - less blood in means less blood out, less blood in ventricle –> reduced pressure on walls –> reduced force to exit –> reduced placental perfusion
• increased aortic pressure
• increased heart rate
• [shouldn’t sleep on back for this reason]
  

Outcomes:

• Fall in blood pressure (can cause collapse if serious)
  
• Can cause foetal hypoxia even without mother symptoms
  

Haematological changes

• red cell mass increase by 20% (renal - increased erythropoietin production in response to detection of blood oxygenation and sodium/water balance)
  
• Plasma volume increases more than RBC count does, causing the impression of decreased haemoglobin (anaemia)
  
• Increased tendency to clot (hypercoagulable)
  
• due to increase in clotting factors and plasma fibrinogen
  
• platelet production increase (however count drops due to increase in activity and consumption) - function remains normal]
  
• WBC count may increase due to an increase in granulocytes
  

Respiratory changes

• Increased chest diameter, diaphragmatic expression and elevation
  
• dyspnoea common (difficulty breathing)
  
• breathing becomes more costal (mouth) than abdominal 
  
• mainly mediated by progesterone levels (cause bronchial and smooth muscle relaxation and hypersensitivity to CO2)
  

Changes to the urinary system

• cardiac output increase –> increase in renal plasma flow and glomerular filtration
  
• increase in urea, creatinine, urate and biocarbonate clearance 
  
• with progesterone, renin and aldosterone up water retention increases
• can lead to urinary stasis and increased risk of UTI
• Any drugs given that are excreted renally must be given in much higher quantities consequently

Gastrointestinal changes

• Appetite increase + cravings 
  
• Gastric reflux sphincter relaxation (increased indigestion, also due to increased intra-gastric pressure (expanding uterus)) 
  
• gallbladder dilated 
  
• GI motility decreased and transit time slower 
  
• albumin and protein decrease 
• cholesterol twice normal value

Muskuloskeletal 

• Calcium reuptake into bloodstream results in mild decalcification of bones 
  
• relaxin softens joints (pubic symphysis + alters gait (waddling))
  
• abdominal muscles stretch to elastic limit 
  
• stretch marks (stria gravidarum) caused by rupture of elastic fibres and small blood vessels 
  

Reproductive changes

• Massive increase in mass
  
• Placenta growth (nutrition, excretion, immunity, endocrine) 
  

Parental blood supply