The major problem facing the sonographer when diagnosing placenta previa is that as many as 45% of women scanned at the beginning of the second trimester appear to have a placenta previa, which persists in only six to nine percent at term (1,2).

This phenomenon has been called "placental migration" and is thought to be due to differential growth of the placental site relative to the lower uterine segment (3). The placenta grows more slowly than the uterus and the development of the isthmus is more rapid than the growth in other parts of the uterus (4).

Another theory suggests that the blood supply to the lower uterine segment is weaker, resulting in degeneration of peripheral villi at a variable speed (5,6).

The likelihood of placenta previa at delivery is 5.1% (95% confidence interval) if the placenta extends at least 15mm over the internal cervical os at 12-16 weeks gestation (7).



  1. Wexier P, Gottefeld KR. Second trimester placenta previa: an apparently normal placentation. Obstet Gynecol 1973;50:706.
  2. Ballas S, Gitstein S, Jaffa AJ Midtrimester placenta previa: normal or pathologic findings. Obstet Gynecol 1979;54:12-14.
  3. King DL. Placental migration demonstrated by ultrasonography. Radiology 1973;100:167-170.
  4. Gruenwald P, Minh HN. Evaluation of body and organ weight in perinatal pathology. II. Weight of body and placenta of surviving and autopsied infants. Am J Obstet Gynecol 1961;82:312-319.
  5. Leerentveld RA, Gilberts ECAM, Arnold MJCW Accuracy and safety of transvaginal sonographic placental localization. Obstet Gynecol 1990;76:759-762.
  6. Norlander S, Sundberg B, Westin B Scintigraphic studies of uterine and placental growth and placental migration during pregnancy. Acta Obstet Gynecol Scand 1977;56:483-486.
  7. 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.