The placenta is implanted in the lower uterine segment near or over the internal cervical os. The degree to which the internal cervical os is covered by the placenta has been used to classify four types of placenta previa; total, partial, marginal and low–lying. In total previa the internal os is entirely covered by the placenta. Partial placenta previa implies incomplete coverage of the internal os. Marginal placenta previa indicates that only an edge of the placenta extends to the margin of the internal os. And the last is the low – lying placenta has been used when the placenta is implanted in the lower uterine segment but not reach the os. The more descriptive classification that includes placenta previa is in the third trimester.
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