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Home > Reference Glossary P  >  Placenta Previa
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]
    LifeART  images © 2006 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.

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