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
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
Another theory suggests that the blood supply to the lower uterine segment
is weaker, resulting in degeneration of peripheral villi at a variable speed
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).
- Wexier P, Gottefeld KR.
Second trimester placenta previa: an apparently normal placentation.
Obstet Gynecol 1973;50:706.
- Ballas S, Gitstein S, Jaffa
AJ et.al. Midtrimester placenta previa: normal or pathologic findings.
Obstet Gynecol 1979;54:12-14.
- King DL. Placental migration
demonstrated by ultrasonography. Radiology 1973;100:167-170.
- 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
- Leerentveld RA, Gilberts
ECAM, Arnold MJCW et.al. Accuracy and safety of transvaginal sonographic
placental localization. Obstet Gynecol 1990;76:759-762.
- Norlander S, Sundberg B,
Westin B et.al. Scintigraphic studies of uterine and placental growth and
placental migration during pregnancy. Acta Obstet Gynecol Scand
- Taipale P, Hiilesmaa V,
Ylostalo P. Diagnosis of placenta previa by transvaginal sonographic
screening at 12-16 weeks in a nonselected population. Obstet Gynecol