Placenta previa is a condition in which the
placenta (including
the marginal veins of the palcenta) partially or completely covers the opening of the cervix (cervical os).
Degrees of placenta previa described are:
Complete or total placenta previa |
 |
The cervix is completely covered by the
placenta
|
Partial placenta
previa |
 |
The
placenta partially covers the opening of the cervix. If the placenta
overlaps by 25 mm or more at 20 to 23 weeks' vaginal delivery appears
to be less likely at term [1]. |
Marginal
placenta previa |
 |
The
edge of the placenta or marginal veins are located 0.5 mm or less from the
internal os [2] |
Incomplete previa |
|
|
|
The inferior placental edge partially covers or reaches the
margin of the cevical opening. This definition Includes marginal and partial previa.[3] |
|
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Lippincott Williams & Wilkins. All rights reserved. |
Some investigators have
proposed that the term placenta previa be used for all placentas with the
lower edge within 2 cm from the internal cervical opening since such
patients have been found to have a low chance of successful vaginal delivery
in some studies. This definition would include placentas that have been
described as being low-lying in the past [5]. The term low lying placenta would be used if the placental edge is located
farther than 2 cm but within 3.5 cm from the internal cervical opening.[4].
Placenta previa occurs in one in 200 to 250 births overall,
but is much more common if a woman has given birth before, has had a cesarean
section, has had placenta previa with a previous pregnancy, or is over the age
of 35. It is uncommon in nulliparous women (women who have never given birth)
[5].
The main symptom of placenta previa is vaginal
bleeding. The bleeding is typically painless unless there is coexisting
abruption or labor. About 20% of third trimester bleeding may be attributed to
placenta previa[ 5].
Nearly all cases of placenta previa are delivered by cesarean section.
Infrequently patients with marginal previa and minimal bleeding are allowed to
deliver vaginally as are patients with intrauterine fetal demise (stillbirth) or
a previable pregnanc y[6].
1.Becker RH et al., ,The relevance of placental location at 20-23 gestational weeks
for prediction of placenta previa at delivery: evaluation of 8650 cases.
Ultrasound Obstet Gynecol. 2001 ;17(6):496-501.
PMID: 11422970
2.Mustafá SA, et al,Transvaginal ultrasonography in predicting placenta previa at
delivery: a longitudinal study.Ultrasound Obstet Gynecol. 2002 ;20(4):356-9.
PMID: 12383317
3.Dashe JS, et al. Persistence of placenta previa according to gestational age at
ultrasound detection. Obstet Gynecol. 2002;99:692-7.
PMID: 11978274
4.Bhide A and Thilaganathan B. Recent advances in the management of placenta
previa.Curr Opin Obstet Gynecol. 2004;16(6):447-51.
PMID: 15534438
5.Green JR , Placenta previa and Abruptio Placenta In Resnik R, ed., Maternal-Fetal Medicine, 5th ed., pp.
Philadelphia: Saunders.
6. Benedetti TJ Obstetric hemorrhage in
Gabbe ed: Obstetrics - Normal and Problem Pregnancies, 4th ed New York, NY,
Churchill Livingstone; 2002
Created: 7/12/2007
Reviewed: Mark Curran, M.D.
