Objectives: To determine the extent of overlapping of the internal cervical os by the lower placental edge at 11-14 weeks' gestation which best predicts placenta previa at term.
Patients and methods: This was a prospective study initially involving 381 singleton pregnancies with a live fetus at 11-14 weeks attending for routine antenatal care. The distance between the lower placental edge and the internal cervical os was longitudinally evaluated by transvaginal ultrasound examination at 11-14 weeks', 20-24 weeks', and 30-34 weeks' gestation. The first 203 cases were selected at random (first phase) and after this period only cases with the lower placental edge reaching and/or overlapping the internal cervical os were followed up (n = 170, second phase). Thus a total of 373 cases were analyzed, 351 of whom were examined in all three trimesters. Multiple regression analysis was used to estimate the probability of predicting placenta previa at term using the overlap of the lower placental edge over the internal cervical os in the first trimester of pregnancy.
Results: A change in the relative position of the placenta (placental migration) was observed in all 351 cases examined in the three trimesters of pregnancy. In the general population, represented by the 203 cases (first phase), the incidence of placenta previa at 11-14 weeks' gestation was 42.3% (86/203), at 20-24 weeks' 3.9% (8/203) and at term 1.9% (4/203). A total of 18 cases of placenta previa and 17 cases of marginal placenta were observed at term. It was estimated that when the lower placental edge overlaps the internal cervical os by 23 mm at 11-14 weeks the probability of placenta previa at term is 8% with a sensitivity of 83.3% and specificity of 86.1%.
Conclusion: The present study establishes the probability of placenta previa at term depending on the relationship of the lower placental edge to the internal cervical os at 11-14 weeks.