Endocrine changes in pregnancy

[Ref: PK1:p345-346; WG21:p452-p453]

Production of hormones by placenta

Peptide hormone

Human chorionic gonadotrophin (hCG)

Production

By syncytiotrophoblast cells

Structure

Made up of alpha and beta subunits
* Like pituitary glycoprotein hormones

Alpha subunit is the same as the alpha subunit of LH, FSH, and TSH

Function

• Primarily luteinising and luteotropic
  * Acts on same receptor as LH
• Very little FSH activity

Thus,

Maintains corpus luteal oestrogen and progesterone production in 1st trimester
--> Maintains pregnancy until the placenta takes over

Levels

hCG level peaks at 10-12 weeks of pregnancy then declines to term

Other notes

Detection

• Can be detected in blood as early as 6 days after conception
• Can be detected in urine as early as 14 days after conception

Secretion in other situation

• Small amounts of hCG are also secreted by some GIT and other tumours
• Foetal liver and kidney also produce small amounts of hCG

 

Human placental lactogen (hPL)

aka human chorionic somatomamotropin (hCS)

Production

By syncytiotrophoblast cells

Amount produced is proportional to the size of the placenta

Structure

Very similar to human growth hormone

hPL, growth hormone and prolactin may come from a common progenitor hormone [WG21:p453]

Function

• Lipolysis
• Antagonise actions of insulin
  * i.e. Decrease glucose utilisation
• K+, nitrogen, and Ca2+ retention
• May also inhibit maternal growth hormone secretion

NB:

• Lipolysis and glucose-sparing divert glucose to the foetus

Levels

hPL level rises throughout the pregnancy and peaks near term

Steroid hormone

i.e. Oestrogen and progesterone

Placenta produces enough oestrogen and progesterone from maternal and foetal precursors to take over the function of corpus luteum after the 6th week of pregnancy

 

Other hormones produced by placenta

• GnRH and inhibins
  --> Possible paracrine function to regulate hCG secretion
  * GnRH stimulates and inhibin inhibits
• Prolactin
• Alpha subunits of hCG which cannot combine with beta subunit
  --> Unknown function
• Endorphin and enkephalin [PK1:p352]

 

Changes in pituitary hormones

Increase in

• Prolactin
• ACTH (adrenocorticotrophin)
• MSH (melanocyte-stimulating hormone)

 

Decrease in

• Growth hormone
  * Possibly by hPL (which also has growth hormone activity)
• Gonadotrophin
  * By increased level of oestrogen and progesterone

 

Other changes

Adrenal hormones

All increase
* ???? By oestrogen and progesterone [PK1:p346]

• Cortisol (both free and total)
• Aldosterone
  * Due to natriuretic effect of progesterone
• Renin and angiotensin

Thyroid hormones

• Both T3 and T4 synthesis increase

However,

• Thyroid binding globulin also increase
  --> Free plasma level of T3 and T4 are unchanged

Parathyroid hormone

• PTH increase due to increased utilisation of free Ca2+
• Increase in PTH
  --> Increase GIT absorption of Ca2+

Prostaglandins

• Prostaglandin A increase 300% during 1st trimester
  --> Systemic vasodilation
• Prostaglanding E only increases during 3rd trimester

Corpus luteum

• Fails to regress when fertilisation occurs
• Enlarges due to hCG
• Enlarged corpus luteum of pregnancy secretes oestrogen, progesterone, and relaxin
• Corpus luteum starts to decline after 8 weeks of pregnancy, but persists throughout pregnancy