Childbirth
The continuity of pregnancy and subsequent childbirth are both regulated by the secretion of hormones
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Pregnancy is maintained by the female sex hormones (estrogen and progesterone) while childbirth is stimulated by the hormone oxytocin
Pregnancy Hormones
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Pregnancy is maintained by the placenta, which takes over the hormonal role of the ovaries (at ~12 weeks) and begins producing estrogen and progesterone
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Estrogen (specifically oestriol) stimulates the growth of uterine muscles (myometrium) and the development of the mammary glands
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Progesterone maintains the endometrium (preventing menstruation) and inhibits oxytocin production (precluding the early onset of labour)
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Both estrogen and progesterone levels drop near the time of birth
Childbirth Hormones
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When a foetus has grown to a certain size it causes stretching of the uterine walls
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The foetus responds to this uterine stretching by releasing hormones that cause the placenta to stop producing progesterone
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When progesterone levels drop, stretch receptors in the uterine lining can trigger the release of oxytocin from the posterior pituitary gland
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Oxytocin stimulates the uterine muscles to contract, resulting in further stretching which promotes more oxytocin release (positive feedback)
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Contractions will stop when labour is complete and the baby is birthed (no more stretching of the uterine wall)
Childbirth (Positive Feedback)
