Let’s begin with something you already know. When a baby is ready to be born, the mother’s body doesn’t just “decide” one day — it follows a precise, hormone-driven sequence. Think of it like a carefully timed relay race: one hormone passes the baton to the next, and each step triggers the next until labour begins.
The everyday intuition
Imagine you are waiting for a train. The station master doesn’t just blow the whistle randomly. First, the train arrives at the platform, then the guard checks the signal, then the whistle blows, and only then does the train move. Parturition — the process of giving birth — works the same way. The baby’s body sends the first signal, the mother’s body receives it, and a chain of hormonal commands follows. No signal, no movement.
What parturition means
Parturition is the medical term for childbirth — the expulsion of the fully developed baby from the mother’s uterus. In NCERT Class 12 Biology (Chapter: Human Reproduction), it is described as the process that begins with the onset of labour and ends with the delivery of the baby and the placenta.
The key point: parturition is not a sudden event. It is the climax of a hormonal cascade that starts weeks earlier.
The hormonal relay: step by step
The entire process is controlled by a feedback loop between the baby’s brain, the mother’s hypothalamus, and her uterus. Here is the sequence as NCERT presents it:
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The baby’s brain starts it. As the foetus grows, its hypothalamus matures and releases corticotropin-releasing hormone (CRH). This hormone travels to the foetal pituitary gland.
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Foetal pituitary secretes ACTH. ACTH (adrenocorticotropic hormone) stimulates the foetal adrenal glands to produce cortisol.
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Cortisol from the foetus acts on the placenta. The placenta, which connects mother and baby, responds by producing oestrogen in increasing amounts. At the same time, progesterone levels fall.
The shift from high progesterone to high oestrogen is the critical switch. Progesterone keeps the uterus relaxed (quiet). Oestrogen makes the uterine muscle sensitive to contraction signals. Without this shift, labour cannot begin.
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Oestrogen stimulates the uterus to produce oxytocin receptors. The mother’s posterior pituitary gland also releases oxytocin — the hormone that causes strong, rhythmic contractions of the uterine muscles.
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Oxytocin creates a positive feedback loop. Each contraction pushes the baby’s head against the cervix. This stretching of the cervix sends nerve signals back to the mother’s brain, which releases more oxytocin. More oxytocin → stronger contractions → more stretching → even more oxytocin. This cycle continues until the baby is delivered.
Why this matters
- It explains why labour cannot be induced artificially without medical intervention — the baby’s own hormones must first signal readiness.
- It also explains why stress or illness in the mother can delay labour: if the foetal cortisol signal is weak, the oestrogen shift may not happen on time.
- In some cases, doctors use synthetic oxytocin (Pitocin) to start or strengthen labour — this mimics the natural hormone.
A quick summary of the key hormones
| Hormone | Source | Role in parturition |
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| CRH | Foetal hypothalamus | Starts the cascade |
| ACTH | Foetal pituitary | Stimulates foetal adrenal glands |