Placenta previa refers to all or part of the placenta covering the internal cervical os (the inlet to the endocervical canal). This occurs when the placenta implants low. The placenta "precedes" the fetus if a vaginal delivery occurs.
The presence of placenta previa is easily determined with ultrasonography.
Situation
Term
the placenta's edge is close to but does not cover the internal os
placenta previa marginalis (marginal)
a part of the internal os is covered by the placenta
placenta previa partialis (partial)
entire internal os is covered by the placenta
placenta previa centralis (complete)
Complications of placenta previa:
(1) hemorrhage during the third trimester
(2) emergency cesarean section
(3) increased neonatal morbidity and mortality
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