Causes of atonic PPH

Any condition that interferes with uterine contraction, such as a retained placenta, remnants of placental tissue, or retained amniotic membranes or blood clots, increases the risk of excessive bleeding. If the placenta has separated but is still, even partially, in the uterus, it can prevent the uterus from contracting. Even a small piece of placenta or a blood clot left inside the uterus can keep it in the atonic condition. When the uterus is not contracted, the mother's blood vessels continue to pump blood out and the woman will quickly lose blood.

The real problem with atonic PPH is that you cannot predict who will bleed excessively after the birth, and this is because two-thirds of women who develop atonic PPH have no known risk factors. This is why it is important to remember that all women must be considered at risk and prevention of PPH must be a part of every birth. The most important known risk factors are summarised below.

Interference with the ability of the uterus to contract

Note that while you cannot prevent an atonic uterus from occurring (nor can you always predict it), knowing the factors that make it more likely will help you to be alert to these possible signs of atonic PPH:

• Placenta previa or placental abruption: in both conditions the muscle fibres in the myometrium are damaged at the placental site.
• Retained placenta: this is when the whole or part of the placenta remains inside the uterus, interfering with the normal muscular contraction at the placental site.
• Incomplete separation of the placenta: this is when part of the placenta has separated from the uterine wall and part of it remains attached.
• Full bladder: the structural closeness of the uterus and the bladder means that a full bladder can interfere with the normal uterine contractions throughout labour and after delivery.
• High parity: this refers to a woman who has had more than five pregnancies; the muscles of the myometrium can lose their strength to contract firmly, due to repeated stretching.
• Multiple pregnancy: causes the uterus to increase its size to accommodate two or more babies (Study Session 10); following their delivery, the overstretched uterus may take a long time to contract firmly.
• Polyhydramnios: an excessive amount of amniotic fluid surrounding the baby (more than 3 litres) can overstretch the uterus in the same way as multiple pregnancy.
• Large baby: (over 4.0 kilograms) can also overstretch the uterus.
• Prolonged labour: when the labour extends more than 12 hours (Study Session 9), the muscles of the myometrium can become so exhausted by repeated contraction that they can no longer contract properly (uterine inertia).

Anaemia in the mother

Anaemia (lack of red blood cells due to low haemoglobin) puts the mother at greater risk of postpartum haemorrhage, because her blood does not clot as easily as in a non-anaemic person. Blood loss is also more serious in someone with anaemia. (You learned about the diagnosis and management of anaemia during pregnancy in Study Session 18 of the Antenatal Care Module.)

Mismanagement of the third stage of labour

Study Session 6 described how active management of delivery of the placenta should be conducted and what actions should be avoided.