Prostaglandins in late pregnancy and labour: local hormones that ripen the cervix and stimulate contractions
Prostaglandins are fat-based chemical messengers that act as local hormones; in late pregnancy they soften (ripen) the tough, collagen-rich cervix and stimulate the uterine muscle to contract.
Remember
Prostaglandins are made locally in the uterus and do two jobs together: they soften the cervix and make the uterine muscle contract, which is why vaginal prostaglandins are used to start labour in women who are overdue.
Prostaglandins are local hormones. A systemic hormone travels in the blood to distant organs, but a local hormone acts on nearby tissue. In pregnancy prostaglandins are made in the myometrium (the muscular wall of the uterus) and the decidua (the womb lining as modified during pregnancy), and act close by. Higher oestrogen levels promote prostaglandin release, so production is controlled by the oestrogen to progesterone ratio.
Prostaglandins, once released, do two jobs. The first is cervical ripening. During pregnancy the cervix is tough and rich in collagen (a strong structural protein), which helps it retain the foetus. Prostaglandins, helped by oestrogen and relaxin (a hormone whose levels rise in pregnancy), break down this connective tissue, so the cervix becomes softer and offers less resistance to the baby.
The second job is in the uterine wall. Prostaglandins make the smooth muscle contract more powerfully, because they promote the calcium entry and release that muscle cells need in order to contract. Softening the cervix and squeezing the uterus are the two changes that labour needs.
That is why a prostaglandin E2 pessary (a form of the drug that is placed in the vagina) or gel can be used to start labour, called induction, in women who are overdue. Oxytocin is a different hormone: the posterior pituitary gland releases it into the blood, and it also stimulates the placenta to make more prostaglandins. Progesterone does the opposite job, because it inhibits contractions. The exact trigger of labour is not known.
Key facts
- Where they are made
- the myometrium and the decidua, so they act on nearby tissue.
- Cervix
- with oestrogen and relaxin, prostaglandins break down connective tissue, so the cervix softens.
- Uterine muscle
- stronger, more frequent contractions, helped by calcium entry into the muscle cells.
- Control
- higher oestrogen promotes prostaglandin release, whereas progesterone inhibits contractions.
- Oxytocin
- it comes from the posterior pituitary gland and stimulates the placenta to make prostaglandins.
- Induction
- vaginal prostaglandin E2 is a recognised way to start labour in women who are overdue.
Exam traps
- Prostaglandins do not come from the posterior pituitary gland, because they are made locally by the myometrium and decidua; oxytocin, which is released from the posterior pituitary gland, strengthens contractions and also stimulates the placenta to make more prostaglandins.
- Prostaglandins do not keep the uterus quiet until term, because that is the job of progesterone, which inhibits contractions; near term, rising prostaglandin levels stimulate powerful contraction of the smooth muscle.
- Cervical ripening does not increase collagen: prostaglandins promote it through reduced collagen content, and a rise in the collagen of the cervical connective tissue would maintain firmness.
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