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