INTRODUCTION TO PLACENTA PREVIA

The problems involved in accurately imaging the lower uterine segment transabdominally, together with the fact that the type of placenta previa is dependent on the gestational age at the time of the ultrasound, result in a false positive diagnosis of placenta previa in two to six percent of patients scanned in the third trimester (1).

A false positive diagnosis may lead to hospitalization and patient anxiety, whereas a false negative result may result in catastrophic bleeding from vaginal examination (2).

The prevalence of second trimester placenta previa on transabdominal scans during the second trimester = 1.9-6.2% (3,4).

This figure increases to 9.1% -45% if low lying placentas are included (3,5). The prevalence of placenta that extend over the internal os at transvaginal scanning between 12-16 weeks gestation is 4.5% (6).

REFERENCES

1. Crenshaw C, Jones DED, Parker RT. Placenta previa: A survey of 20 years experience with improved perinatal survival by expectant therapy and caesarean section. Obstet Gynecol Surv 1973;28:461.
2. Pauzner D, Barrett J, Farine D. Transvaginal scanning in the management of placenta previa. J SOGC 1995;17:231-235.
3. Anderson ES, Steinke NMS. The clinical significance of asymptomatic mid-trimester low placentation diagnosed by ultrasound. Obstet Gynecol Scand 1988;57:339-341.
4. Townsend RR, Laing FC, Nyberg DA et.al. Technical factors responsible for "placental migration": Sonographic assessment. Radiology 1986;160:105-108.
5. Wexler P, Gottesfeld KR. Second trimester placenta previa. An apparently normal placentation. Obstet Gynecol 1977;50:706-709.
6. Taipale P, Hiilesmaa V, Ylostalo P. Diagnosis of placenta previa by transvaginal sonographic screening at 12-16 weeks in a nonselected population. Obstet Gynecol 1997;89:364-367.