Hormones in pregnancy and labour and how they influence birth

Labour is regulated by a complex interplay of hormones that coordinate uterine contractions, cervical ripening, and fetal readiness for birth. These hormones change levels and activity as pregnancy progresses, triggering the onset of labour.

Key Hormones and Their Roles:

Oxytocin:

Produced by the posterior pituitary gland.

Stimulates strong, rhythmic uterine contractions during labour.

Promotes milk ejection postnatally.

Its release is stimulated by cervical stretch (Ferguson reflex) such as in a stretch and sweeps and nipple stimulation (colostrum harvesting), creating a positive feedback loop that intensifies contractions.

Prostaglandins:

Produced locally in the uterus, especially PGE2 and PGF2α.

Promote cervical ripening by softening and dilating the cervix.

Increase uterine contractility, working together with oxytocin.

Clinically used to induce labour or ripen the cervix.

Oestrogen:

From the ovaries and placenta.

Near term, levels rise significantly.

It Increase the number of oxytocin receptors on uterine muscles.

Stimulate prostaglandin synthesis.

Promote cervical softening and prepare the uterus for contractions.

Progesterone:

From the ovaries (early pregnancy) and placenta (later pregnancy).

Maintains pregnancy by relaxing uterine muscles and preventing contractions.

Keeps the cervix closed and firm.

Near term, a functional withdrawal occurs—either via decreased receptor sensitivity or local metabolism—allowing contractions to begin.

Fetal Cortisol:

Produced by the fetal adrenal glands.

Matures fetal lungs and other organs. Can be given artificially to speed up fetal lung maturation in premature labour (steroids injections)

Acts as a critical signal for initiating labour by stimulating the placenta to produce prostaglandins, promoting cervical ripening and uterine contractions.

Relaxin:

Secreted by the placenta and corpus luteum.

Softens the cervix and relaxes pelvic ligaments to facilitate passage of the fetus.

The Hormonal Cascade to Labour:

During pregnancy:

High progesterone keeps the uterus relaxed and the cervix closed.

Approaching labour:

Rising oestrogen levels increase oxytocin receptor density and prostaglandin production.

Fetal cortisol levels increase, stimulating prostaglandin synthesis.

Onset of labour:

Functional withdrawal of progesterone allows the uterus to become more contractile.

Increased oxytocin and prostaglandins lead to stronger, coordinated contractions.

Cervix softens, effaces, and dilates in response.

Progression:

Positive feedback between contractions, oxytocin release, and prostaglandin activity sustains labour until delivery.

Summary:

During pregnancy, progesterone maintains uterine quiescence.

Near term, increased oestrogen, fetal cortisol, prostaglandins, and oxytocin work together to initiate and progress labour.

The decline in progesterone activity is essential for the transition from pregnancy to active labour.

As you can see this is a delicate balance that can easily be disrupted by fear, anxiety, bright lights, loud environments. Travelling to hospital can easily disrupt this process due to uncertainty of what will happen, uncomfortable car ride. At the hospital bright lights, procedures not being explained properly, not understanding medical jargon, medical professionals not seeking true consent, feeling out of control.

Independent midwives are very good at seeking consent, we will always ask permission and make sure you understand before any procedure. To us no means no. Likewise if you ask for a procedure such as vaginal examination, we will ensure you are aware of the benefits and risk.

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