Causes of shoulder presentation could be maternal or fetal factors.

Maternal factors include:

Lax abdominal and uterine muscles: most often after several previous pregnancies
Uterine abnormality
Contracted (abnormally narrow) pelvis.

Fetal factors include:

Preterm labour
Multiple pregnancy
Polyhydramnios
Placenta previa.

What do 'placenta previa' and 'polyhydramnios' indicate?

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Placenta previa is when the placenta is partly or completely covering the cervical opening. Polyhydramnios is an excess of amniotic fluid. They are both potential causes of malpresentation.