Growth and Development 4: Labour, foetal circulation and lactation

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What happens when we give birth?

During the last week of pregnancy, oestrogen reaches a peak. This causes myometrial weakness and irritability. Then, weak 'Braxton Hicks' contractions occur, which become uterine contractions stimulated by oxytocin and prostaglandins closer to birth. Emotional and physical stress establishes a positive feedback mechanism with the hypothalamus, increasing oxytocin release.

The stages of labour are summarised below:

  1.   Dilation Stage (8 hours)
  • Onset of labour
  • Cervix dilates
  • Foetus moves towards birth canal
  • Contractions begin and increase in frequency
  • Water eventually breaks
  2.   Expulsion Stage (2-3 hours)
  • Cervix pushed open by foetus
  • Contractions increase in intensity, frequency and length
  • Continues until foetus emerges from vagina (delivery)
  • Surgical intervention possible: episiotomy (cutting of perineal membrane) or caesarean section
  • Newborn health assessed immediately
  3.   Placental Stage
  • Occurs after delivery
  • Muscle tension builds up in partially empty uterus
  • Contracts tear connection between endometrium and placenta
  • Plasma ejected

How does the foetus (in utero) have circulating blood?

The foetus receives its blood supply from the umbilical arteries and veins. By the end of the third week, the embryo has a system of paired vessels (e.g. artery with a vein) and the chambers of the heart have fused. Because the foetus doesn't breath, it has three vascular shunts in its heart, which are occluded at birth:
  • Ductus venous, bypassing the liver
  • Foramen ovale, bypassing pulmonary circulation through opening in interatrial septa
  • Ductus arteriosus, transferring blood from the pulmonary trunk to the aortic trunk
Once carbon dioxide is no longer removed by the placenta, central acidosis occurs. This excites the respiratory centres in the medulla oblongata and pons to trigger the first inspiration. This requires tremendous effort as the airways are tiny and the lungs are collapsed.

Once the lungs inflate, surfactant in the alveolar fluid helps reduce surface tension. When the lungs expand, the patterns of circulation then change due to alterations in blood pressure and flow. The ductus arteriosus closes, isolating the pulmonary and systemic trunks. The foramen ovale closes, creating two separate atria and thus two completely separate circuits.

What happens when a woman is lactating?

Lactation is the production of milk by the mammary glands. Oestrogen, progesterone and lactogen stimulate the hypothalamus to release prolactin-releasing hormone. The anterior pituitary gland responds by releasing prolactin. When a baby latches onto the mother's nipple, the following steps occur:
  1. Stimulation of tactile receptors
  2. Neural impulse transmission to the spinal cord and then brain
  3. Oxytocin secretion in the hypothalamus
  4. Oxytocin release from the posterior pituitary gland
  5. Milk ejection from contracted myoepithelial cells in the lactiferous ducts
Breast milk is very beneficial to the newborn:
  • Contains amino acids, lipids, ions, sugars, salts and mucins
  • Contains IgA, immunoglobulin, complement proteins, lysozyme, interferon and lactoperoxidase
  • Interleukins and prostaglandins are present, preventing overzealous inflammatory responses such as allergies
  • Natural laxatives help cleanse the bowels

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