HUBS3511 · Human Reproduction And Pregnancy
Labour and Birth
For nine months progesterone keeps the uterus quiescent (silent). Labour is the controlled switch from quiescent to contractile, and the University of Newcastle frames it as a 4-driver cascade that converges on myometrial activation: progesterone functional withdrawal → oestrogen priming → oxytocin → prostaglandins. At its heart is the textbook example of positive feedback — the oxytocin loop — in which contraction increases its own stimulus so contractions escalate to delivery, and the loop breaks only when the baby is born (the Ferguson reflex). HUBS3511 examines the four drivers in order, why the oxytocin loop is positive (not negative) feedback and what breaks it, the three stages of labour and what defines each (full dilation = 10 cm; stage 3 = placenta), the functional-vs-absolute progesterone distinction, that oestrogen primes but does not directly contract, and tocolytic vs uterotonic. Once the baby breathes, a second switch flips the circulation from fetal to newborn.
What this chapter covers
- 017.1 The onset of labour: the UON 4-driver cascade
- 02Progesterone functional (not absolute) withdrawal in humans
- 03Oestrogen priming: ↑ oxytocin receptors + gap junctions (primes, not contracts)
- 047.2 The oxytocin positive-feedback loop (Ferguson reflex)
- 05Negative vs positive feedback compared
- 067.3 The three stages of labour (10 cm; stage 3 = placenta)
- 07Tocolytic vs uterotonic (Syntocinon)
- 087.4 The switch at birth: ↓PVR, ↑SVR → shunts close
Worked example: why is the oxytocin labour loop positive feedback?
- +1Contraction → stretch: a uterine contraction pushes the fetal head onto the cervix, stretching it.
- +1Stretch → oxytocin: cervical stretch receptors signal the brain; the posterior pituitary releases oxytocin into the blood.
- +1Oxytocin → stronger contraction: oxytocin reaches the uterus → a stronger contraction → MORE cervical stretch → MORE oxytocin — the loop amplifies.
- +1Why positive: the response (contraction) increases its own stimulus (stretch → oxytocin), so the output escalates toward an end-point rather than settling at a set-point. Calling it negative feedback is the classic error.
- +1What stops it: when the baby is delivered, cervical stretch ceases, the stimulus is gone, and the loop breaks. It is self-limiting only because delivery removes the stretch.
Key terms
- Progesterone functional withdrawal
- Near term, progesterone's action falls (driven by inflammation, placental senescence, uterine distension and stress) so myometrial excitability rises. In humans this is a functional/relative withdrawal — not necessarily a drop in blood progesterone level (contrast the absolute drop seen in many animals).
- Oestrogen priming
- Late-pregnancy oestrogen increases myometrial oxytocin receptors and gap junctions so the muscle can respond to oxytocin and contract as a coordinated sheet. Oestrogen primes the uterus; it does not directly contract it.
- Oxytocin loop (Ferguson reflex)
- The positive-feedback loop of labour: contraction → cervical stretch → oxytocin release → stronger contraction. It amplifies until delivery removes the stretch and breaks it.
- Tocolytic vs uterotonic
- A tocolytic STOPS contractions (to delay preterm labour); a uterotonic CAUSES contractions — e.g. synthetic oxytocin (Syntocinon) and prostaglandins to induce/augment labour and to prevent postpartum haemorrhage.
- Stages of labour
- Stage 1 = onset to full cervical dilation (10 cm); stage 2 = full dilation to delivery of the baby; stage 3 = delivery of the placenta. Failed stage 3 risks retained placenta and postpartum haemorrhage.
Labour and Birth FAQ
Why is the oxytocin loop positive feedback rather than negative?
Because the response increases its own stimulus: contraction → cervical stretch → oxytocin → stronger contraction. Negative feedback opposes a change to hold a set-point; here the output escalates to an end-point (delivery), which is what then breaks the loop.
Does oestrogen contract the uterus?
No — oestrogen primes the uterus by adding oxytocin receptors and gap junctions; it does not directly contract it. Naming oestrogen as the contractile agent loses the mark.
What defines full dilation and stage 3?
Full cervical dilation is 10 cm (the end of stage 1, not the moment of birth). Stage 3 is delivery of the placenta, not the baby. If stage 3 fails you get retained placenta and postpartum haemorrhage, usually from uterine atony — which is why oxytocin is given to contract the empty uterus.
What happens to the circulation at birth?
The first breath inflates the lungs so pulmonary vascular resistance (PVR) drops, and cord clamping removes the low-resistance placenta so systemic vascular resistance (SVR) rises. Left atrial pressure now exceeds right, so the foramen ovale closes; rising O₂ and falling prostaglandins close the ductus arteriosus and ductus venosus.
Exam move
Lead with the UON 4-driver cascade as an ordered list (progesterone functional withdrawal → oestrogen priming → oxytocin → prostaglandins) and pre-empt its two traps (functional not absolute withdrawal; oestrogen primes, does not contract). The single most examinable concept is that the oxytocin loop is POSITIVE feedback — be able to walk one turn of the loop and say it stops only because delivery removes the stretch. Memorise the three stages by their defining boundary (10 cm; baby; placenta) and keep tocolytic vs uterotonic the right way round. Finally, rehearse the birth switch as a two-trigger mechanism (↓PVR from the first breath, ↑SVR from cord clamp → pressures reverse → shunts close) since it bridges straight into the fetal chapter.
