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Placenta praevia (placenta previa AE) is an obstetric complication in which the placenta is attached to the uterine wall close to or covering the cervix.[1] It can sometimes occur in the later part of the first trimester, but usually during the second or third. It is a leading cause of antepartum haemorrhage (vaginal bleeding). It affects approximately 0.5% of all labours.

Placenta praevia is hypothesized[who?] to be related to abnormal vascularisation of the endometrium 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, as is the case with placenta praevia, it may shear off and a small section may bleed.

Diagnosis

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.[2] In parts of the world where ultrasound is unavailable, 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.

Placenta previa is classified according to the placement of the placenta:

• Type I or low lying: The placenta encroaches the lower segment of the uterus but does not infringe on the cervical os.
• Type II or marginal: The placenta touches, but does not cover, the top of the cervix.
• Type III or partial: The placenta partially covers the top of the cervix
• Type IV or complete: The placenta completely covers the top of the cervix

Placenta previa occurs approximately one of every 250 births. One third of all antepartum hemorrhage occurs due to placenta previa.[citation needed]