Stages of Pregnancy, Parturition and Lactation

Question 1

Describe these terms with respect to delivery:

• At term
• Pre-term
• Post-term

Answer 1

• Term
  
  • Between 37 and 42 completed weeks
• Pre-term
  
  • Before 37 completed weeks
• Post-term
  
  • Beyond 42 completed weeks

Question 2

What are the stages of pregnancy?

Answer 2

• First trimester - up to 12 weeks.
• Second trimester - 12-27 weeks.
• Third trimester - 28 weeks to term.

Question 3

Describe the development of the foetal genitalia and explain how sex is established.

Answer 3

• All early embryos have 2 primitive duct systems; Wolffian and Mullerian ducts have the potential to form male or female tracts.
• In males:
  
  • ​Wolffian ducts become reproductive tract
  • Mullerian ducts degenerate
• In females:
  
  • Mullerian ducts become reproductive tract
  • Wolffian ducts degenerate
• Development into male or female depends upon hormones secreted by foetal testes – testosterone (stimulated by human chorionic gonadotrophin from placenta) and Mullerian Inhibiting Factor Iinduces regression of Mullerian ducts).
• Without stimulus of male testicular hormones, Wolffian ducts regress, Mullerian ducts develop and foetus will develop female characteristics.

Question 4

What is the role of softening of the cervix during labour?

How does it happen?

Answer 4

• Caused by prostaglandins and possibly relaxin.
• Breakdown of cervical collagen fibres allows cervix to dilate during labour.

Question 5

What are the stages of cervical effacement?

Answer 5

Question 6

How is labour initiated?

Answer 6

• Increased oestrogen synthesis by placenta produces dose-dependent changes in the uterus.
• Oestrogen increases the density of oxytocin receptors in the myometrium.
• Maternal oxytocin does not rise - parturition is triggered by increased sensitivity to oxytocin.

Question 7

Describe the 1st stage of labour.

Answer 7

• Onset of regular contractions to fully dilated cervix.
• May take many hours
• ‘Latent phase’
  
  • Onset of painful contractions 5-10 minute intervals.
  • Cervical ripening and effacement.
  • Cervix slowly dilating up to 3-4cm.
• ‘Active phase’
  
  • From cervix 3-4cm dilated, more rapidly 0.5-1 cm / hour.
  • Progressive increase in frequency and strength of contractions.
  • Descent of the presenting part.
  • Cervix dilates to 10cm to accommodate the baby’s head.
  • Breech birth – another part of body approaches cervix first.
  • Rupture of amniotic sac (maybe) - “waters break” - lubricates birth canal.

Question 8

Describe the 2nd stage of labour.

Answer 8

• Fully dilated cervix to birth.
• Takes ~30-90 minutes.
• Cervix fully dilated (10 cm).
• Contractions stronger at 2-5 minute intervals.
• Presenting part descends.
• Urge to bear down.
• Baby moves through cervix to vagina.
• Stretch receptors in vagina trigger contraction of abdominal wall to augment uterine contractions.
• Mother can voluntarily contract abs also (“push”).
• Ferguson reflex – stretching of the perineum/pelvic floor in late labour seems to stimulate oxytocin release.
• After birth, the baby is freed from placenta by cutting umbilical cord.
  
  • Cord is tied and forms umbilicus.

Question 9

What is the Ferguson reflex?

Answer 9

A positive feedback loop.

Question 10

Desribe the 3rd stage of labour.

Answer 10

• Expulsion of the plancenta and membranes.
• Separation due to forceful uterine contraction and reduces size of placental bed which reduces bleeding
• Takes a few minutes to an hour or so (depending on management).
• Placenta separates from myometrium and uterine contractions cause it to be expelled– “afterbirth”.
• Myometrium contracts and prevents haemorrhage by constricting uterine blood vessels at site of placental attachment.

Question 11

Describe post-partum involution.

Answer 11

• Shrinkage of uterus to pre-pregnancy size. ​
• Takes 4-6 weeks.
• Induced by:
  
  • Fall in oestrogens and progesterone levels after placenta is lost.
  • Oxytocin which is released in response to breast feeding.
• Other physiological adaptations return to pre-pregnancy state in the days and weeks that follow birth.

Question 12

Describe the structure of the breast.

Answer 12

• Breast has a glandular structure:
  
  • Each duct terminates in a lobule
  • Lobule is made of milk producing glands - alveoli
  • Milk is secreted from epithelial cells into lumen

Question 13

Describe the breast development associated with pregnancy.

Answer 13

• At birth, the breast consists of lactiferous ducts without any alveoli (also male breast).
• At puberty, under influence of oestrogens, the ducts proliferate and masses of alveoli form at the ends of the branches.
• During pregnancy under the influence of oestrogen, progesterone and prolactin the glandular portion of the breast undergoes hypertrophy replacing adipose tissue.
• From week 16 the breast tissue is fully developed for lactation but is quiescent awaiting activation.
• After parturition the breast produces colostrum before mature milk production begins.

Question 14

What are the triggers for breast development during pregnancy?

Answer 14

• During pregnancy:
  
  • ↑ oestrogen ➔ duct development
  • ↑ progesterone ➔ lobule formation
• Prolactin (anterior pituitary hormone) & human chorionic somatomammotropin (placental hormone) ➔ synthesis of enzymes for milk production.
• Prolactin also stimulates milk production after parturition.
• Stimulatory action of prolactin is blocked in later stages of pregnancy by high levels of oestrogen and progesterone.
• Immediately after parturition oestrogen & progesterone levels fall, allowing prolactin to induce milk production.

Question 15

What are the important cells types in lactation?

Answer 15

• Secretory alveoli / acini cells - produce milk, stimulated by prolactin.
• Contractile myoepithelial cells - surround each alveolus, stimulated by oxytocin.

Question 16

Summarise lactation and explain how it is controlled.

Answer 16

• Consists of milk production and milk letdown.
• Initiated by precipitous drop in oestrogen and progesterone after delivery.
• Prolactin surges each time mother nurses baby due to nerve impulses from nipples to hypothalamus.
  
  • Without nursing stimulation, no prolactin surge and loss of milk production.
• When not nursing, hypothalamus produces prolactin inhibitory hormone.
• Lactation inhibits FSH and LH and thus lactation interferes with reproductive function.

Question 17

What are the effects of suckling?

Answer 17

• Suckling triggers:
  
  • Neuroendocrine reflex - leads to secretion of:
    
    • Prolactin ➔ milk production
    • Oxytocin ➔ milk ejection
      
      • Oxytocin stimulates contraction of myoepithelial cells.
      • Oxytocin hastens involution.
      • Oxytocin suppresses LH & FSH secretion – suppresses menstrual cycle

Question 18

Describe the role of prolactin in the suckling reflex.

Answer 18

• Suckling stimulus inhibits the hypothalamic release of dopamine (PIF) and prolactin is released in proportion to the strength and duration of the suckling.

Question 19

Describe the role of oxytocin in the milk ejection reflex.

Answer 19

• Suckling stimulates neurones in the hypothalamus to synthesise oxytocin which is carried to posterior pituitary.
• The release of oxytocin into the blood stream acts on myo-epithelial cells in the alveoli – causing the ‘let down’ of milk.
• Conditioned reflex:
  
  • Let down in response to the cry of the baby
  • Oxytocin release is inhibited by catecholamines - stress can inhibit the reflex.

Question 20

Which is the mode of secretion of breast milk?

Describe this.

Answer 20

• Apocrine secretion.
• Synthesised milk fat moves through the cell to the surface membrane. Enclosed lipid droplet is pinched off into the duct lumen.
• Milk protein passes through the Golgi apparatus and is released by exocytosis.
• Composition varies within a feed and within the lactation period.

Question 21

How much milk is produced per day (approximately)?

What is the energy content of this milk?

Answer 21

~800mL / day with an energy content of 70kCal / 100mL (this will vary throughout lactation).

Question 22

Describe the mechanism of breast milk production.

Answer 22