Placenta previa pathophysiology
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
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Pathophysiology
No specific cause of placenta praevia has yet been found but it is hypothesized to be related to abnormal vascularisation of theendometrium caused by scarring or atrophy from previous trauma, surgery, or infection.
In the last trimester of pregnancy the isthmus of the uterus unfolds and forms the lower segment. In a normal pregnancy the placenta does not overlie it, so there is no bleeding. If the placenta does overlie the lower segment, it may shear off and a small section may bleed.
Women with placenta praevia often present with painless, bright red vaginal bleeding. This bleeding often starts mildly and may increase as the area of placental separation increases. Praevia should be suspected if there is bleeding after 24 weeks of gestation. Abdominal examination usually finds the uterus non-tender and relaxed. Leopold's Maneuvers may find the fetus in an oblique or breech position or lying transverse as a result of the abnormal position of the placenta. Praevia can be confirmed with an ultrasound. In parts of the world where an ultrasound not available, it is not uncommon to confirm the diagnosis with an examination in the surgical theatre.
The proper timing of an examination in theatre is important. If the woman is not bleeding severely she can be managed non-operatively until the 36th week. By this time the baby's chance of survival is as good as at full term.