Pregnancy

Overview 

• Fertilization and implantation 
  • ovum is fertilized in ampulla of Fallopian tube several hours after ovulation
  • blastocyst implants in endometrium 6 to 7 days after ovulation
    • syncytiotrophoblast cells secrete hCG that rescues corpus luteum
  • hCG
    • in first trimester,hCG sustains corpus luteum in presence of falling LH levels
      • corpus luteum produces progesterone
        • progresterone is necessary for maintenance of pregnancy
    • hCG levels detectable in blood (1 week post-conception)
    • hCG levels detectable in urine (2 weeks post-conception)
      • e.g., home pregnancy test
  • in second and third trimesters, placenta produces estrogen and progesterone
    • corpus luteum degenerates
  • hCG levels elevated in certain pathologies
    • hydatidiform moles
    • choriocarcinoma
• Lactation 
  • post-delivery, ↓ progesterone levels, prolactin promotes lactation
    • initiation of milk production by alveolar cells
• Suckling
  • stimulates sensory nerves that carry suckling signal from breast to hypothalamus via spinal cord, a process that mediates prolactin and oxytocin release

• Prolactin
  • suckling sends signal from breast (nipple) to hypothalamus
    • suckling input in arcuate nucleus of hypothalamus inhibits dopamine release
      • dopamine travels via hypothalamic-portal system to anterior pituitary to inhibit prolactin (PRL) release by lactotrophs
      • suckling inhibits release of dopamine and causes ↑ PRL
  • prolactin promotes lactation and maintains lactation once established
  • prolactin → (-) hypothalamus → ↓ GnRH → ↓ FSH and ↓ LH
    • prolactin inhibits reproductive function 
• Oxytocin
  • suckling sends signal from breast (nipple) to hypothalamus
    • suckling input in supraoptic and paraventricular nuclei of hypothalamus triggers oxytocin release via posterior pituitary
  • oxytocin promotes contraction of myoepithelial cells → promotes milk letdown
  • oxytocin may promote uterine contractions in childbirth 
• Relaxin 
  •  leads to sacroiliac joint laxity and widening of the pubic symphysis in preparation for delivery
• Physiologic changes   
  • increased red blood cell mass
  • increased plasma volume
  • increased cardiac output
  • decreased vascular resistance
  • mild respiratory alkalosis due to increased tidal volume
  • hypercoagulability
  • increased glomerular filtration rate 
  • supine positioning can result in hypotension 
    • due to compression of the IVC by the gravid uterus