Diagnosis of Placental Position by Early First-Trimester Ultrasound: A Pilot Study

Conventional wisdom is that placental location cannot be identified before 8 weeks' gestation when the placenta first becomes hyperechogenic on ultrasound. We sought to evaluate whether placental location could be reliably diagnosed between 5 and 6 weeks' gestation.

This was a retrospective analysis of prospectively acquired data. Early placental location was diagnosed by evaluation of the embryonal and yolk sac position inside the gestational sac on transvaginal ultrasound. Placental position was described as anterior, posterior, fundal, or lateral. Early and mid-pregnancy placental locations were compared and coded as being the same, having migrated to an adjacent surface, or being on an opposite surface.

A total of 111 patients met study criteria, providing 141 placental locations, comprising 85 singleton and reduced pregnancies and 28 dichorionic twin pregnancies. The most common placental location was anterior in both singleton and twin/triplet pregnancies. Placental location at the mid-pregnancy ultrasound was consistent with early pregnancy location in 100% of cases, with 79.5% (112/141) being on the same surface and 20.5% (29/141) having expanded onto an adjacent surface. Placental location was not associated with pregnancy outcome, although our study may have been underpowered to detect a significant difference.

Placental location diagnosed at 5 to 6 weeks' gestation is consistent with the location on mid-pregnancy ultrasound. Excluding the presence of an ectopic, cornual, or cesarean section scar and uterine subseptation pregnancy in early first trimester would allow a more effective tailoring of pregnancy follow-up.