Uterine atony as the commonest cause of primary postpartum haemorrhage
Uterine atony, where the womb muscle fails to contract, is the commonest cause of primary postpartum haemorrhage (heavy bleeding within 24 hours of birth).
Remember
Heavy bleeding soon after birth with a soft, boggy (spongy) uterus points to uterine atony, the commonest cause of primary postpartum haemorrhage. The slack muscle does not squeeze shut the blood vessels at the placental site, so they keep bleeding.
Primary postpartum haemorrhage (PPH) is heavy bleeding within 24 hours of birth. The usual way to organise its causes is the 4 Ts: Tone, Trauma, Tissue and Thrombin. Tone comes first, because uterine atony, in which the uterus fails to contract, is the most common cause.
Once the baby and placenta are out, the myometrium (the thick muscular wall of the uterus) normally contracts firmly. That squeezes shut the large blood vessels that supplied the placenta, which is how the placental site (the patch of uterine lining where the placenta was attached) stops bleeding. If the muscle stays slack, those vessels stay open and bleed freely. They carry a high flow, because they supported the foetus in pregnancy, so the loss can quickly become life threatening.
A slack uterus feels soft and boggy (spongy) instead of firm, and that is the clue. The other three Ts start elsewhere. Trauma means tears or other injury to the birth canal. Tissue means part of the placenta left behind in the uterus, which stops the vessels that supplied it from closing. Thrombin means a clotting defect, which can contribute to primary PPH but is rare; an inherited one is usually identified and managed beforehand.
If the uterus is firm but bleeding continues, look for a tear. Finding atony should not stop you looking for a second cause, because several of the Ts can coexist.
Key facts
- Definition
- primary PPH is heavy bleeding within 24 hours of birth, traditionally 500 ml or more.
- Commonest cause
- uterine atony is estimated to account for about 70 to 80% of cases of primary PPH.
- Mechanism
- contraction of the myometrium squeezes the placental-site vessels shut, and failure leaves them open.
- Clue
- a soft, boggy uterus with heavy bleeding soon after delivery.
- Treatment
- uterotonic drugs (drugs that make the uterus contract), such as oxytocin.
- Firm uterus
- if bleeding persists despite a firm uterus, look for tears or retained tissue.
- Secondary PPH
- bleeding that occurs after the first 24 hours.
Exam traps
- Retained placental tissue holds the placental vessels open, but a complete placenta makes retained tissue less likely, so a soft uterus points to poor tone.
- A tear of the birth canal bleeds from damaged tissue, not from failed contraction, so it does not account for a soft, enlarged uterus.
- An inherited clotting defect can contribute to primary haemorrhage, but it is rare and usually identified and managed beforehand, and when nothing suggests a bleeding disorder, the soft uterus points to poor tone.
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