(Ultrasound Review) According to obstetricians from the Free University of Berlin, Germany, placenta previa can be predicted by assessing placental position with ultrasound at 20–23 weeks gestation when the anatomy is surveyed.
They studied the outcomes for 8,650 patients and determined that when the placenta overlaps the internal os by more than 25 mm at the specified time during the second trimester, a placenta previa would prevent vaginal delivery at term. These findings were published in the June issue of the Ultrasound in Obstetrics and Gynecology.
Placenta previa is an uncommon but dangerous complication of pregnancy and is the leading cause of antepartum hemorrhage in the third trimester. Early prediction of this problem can decrease the risk of abruptio placentae and postpartum hemorrhage.
Cesarean delivery can be planned in advance, thus decreasing the risks to mother and child because there is differential growth of the uterine wall with the portion where the placenta attaches, undergoing less growth compared with the rest when placental migration occurs.
"The consequence of this effect is a higher amount of false-positive cases of diagnosis of placenta previa in earlier phases of pregnancy," the authors stated.
Patients were first examined transabdominally with a moderately filled bladder. Those with suspicious uncertain placental site were further evaluated using transvaginal ultrasound. Placental coverage was classified into three groups: lower placental margin clear of the internal os, placental margin low but did not overlap os, and lower margin overlapped the os. There was no statistical difference for the first two groups.
"One patient with an overlap of 16 mm reported bleeding in the evening after vaginal examination. This incident was possibly due to contraction induction and abruption but resolved without further concern," the authors stated.
Determining the placental site at 20-23 weeks gestation enables a low false-positive rate compared with earlier stages of pregnancy.
"We conclude that overlapping placenta at 20–23 weeks has the consequence of a high probability of placenta previa at delivery. An overlap of 25 mm or more at 20–23 weeks seems to be incompatible with later vaginal delivery," the authors stated.
"The relevance of placental location at 20–23 gestational weeks for prediction of placenta previa at delivery: evaluation of 8650 cases"R H Becker et al
Free University of Berlin, University Klinikum Benjamin Franklin, Kurfürstendamm 199, 10719 Berlin, Germany
Ultrasound in Obstetrics & Gynecology 2001(June); 17:496–501
By Ultrasound Review
July 27, 2001
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