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Ultrasound was used as a diagnostic tool in 859 patients for the localization of the placenta in the second trimester prior to genetic amniocentesis. A greater incidence of total placenta previa was found in the study group (5.6%) than is reported for the general population (0.25–0.5%). Maternal age > 35 years was the indication or co-factor in 59% of the patients studied, which might explain this difference. It was possible to predict total placenta previa at term early in the second trimester. It was demonstrated that if a placenta was located centrally over the cervical os and did not change position with variations in the degree of bladder-filling or changes in uterine position, the risk of having a total placenta previa at term increased from 1/143 to 1/8 pregnancies.