Placenta praevia
| Placenta previa | |
|---|---|
 |
|
| Diagram showing a placenta previa (Grade IV ) | |
| Classification and external resources | |
| Specialty | Obstetrics |
| ICD-10 | O44, P02.0 |
| ICD-9-CM | 641.0, 641.1 |
| MedlinePlus | 000900 |
| MeSH | D010923 |
Placenta previa is an obstetric complication in which the placenta is inserted partially or wholly in the lower uterine segment. It is a leading cause of antepartum haemorrhage (vaginal bleeding). It affects approximately 0.4-0.5% of all labours.
In the last trimester of pregnancy the isthmus of the uterus unfolds and forms the lower segment. In a typical pregnancy the placenta does not overlie. If the placenta does overlie the lower segment, as is the case with placenta previa, it may shear off and a small section may bleed.
Women with placenta previa often present with painless, bright red vaginal bleeding. This commonly occurs around 32 weeks of gestation, but can be as early as late mid-trimester. This bleeding often starts mildly and may increase as the area of placental separation increases. Previa should be suspected if there is bleeding after 24 weeks of gestation.
Women may also present as a case of failure of engagement of fetal head.
Exact cause of placenta previa is unknown. It is hypothesized to be related to abnormal vascularisation of the endometrium caused by scarring or atrophy from previous trauma, surgery, or infection. These factors may reduce differential growth of lower segment, resulting in less upward shift in placental position as pregnancy advances.
Traditionally, four grades of placenta previa were used, however now it is more common to simply differentiate between 'major' and 'minor' cases.
The following have been identified as risk factors for placenta previa:
Placenta previa is itself a risk factor of placenta accreta.
History may reveal antepartum hemorrhage. Abdominal examination usually finds the uterus non-tender, soft 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. Malpresentation is found in about 35% cases. Vaginal examination is avoided in known cases of placenta previa.
Previa can be confirmed with an ultrasound. Transvaginal ultrasound has superior accuracy as compared to transabdominal one, thus allowing measurement of distance between placenta and cervical os. This has rendered traditional classification of placenta previa obsolete.
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