Placenta Previa

Author: Saju Joy, MD, MS; Chief Editor: Carl V Smith, MD more...

Updated: Feb 25, 2010

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Background
Pathophysiology
Epidemiology
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Background

Placenta previa involves implantation of the placenta over the internal cervical os. Variants include complete implantation over the os (complete placenta previa), a placental edge partially covering the os (partial placenta previa) or the placenta approaching the border of the os (marginal placenta previa). A low-lying placenta implants in the caudad one half to one third of the uterus or within 2-3 cm from the os.

Placenta previa.

Placenta previa covering the entire cervical os.

Placenta previa partially separated from the lower uterine segment.

Placenta previa invading the lower uterine segment and covering the cervical os.

Pathophysiology

Placental implantation is initiated by the embryo (embryonic plate) adhering in the lower (caudad) uterus. With placental attachment and growth, the developing placenta may cover the cervical os. However, it is thought that a defective decidual vascularization occurs over the cervix, possibly secondary to inflammatory or atrophic changes. As such, sections of the placenta having undergone atrophic changes could persist as a vasa previa.

A leading cause of third trimester hemorrhage, placenta previa presents classically as painless bleeding. Bleeding is thought to occur in association with the development of the lower uterine segment in the third trimester. Placental attachment is disrupted as this area gradually thins in preparation for the onset of labor. When this occurs, bleeding occurs at the implantation site as the uterus is unable to contract adequately and stop the flow of blood from the open vessels. Thrombin release from the bleeding sites promotes uterine contractions and a vicious cycle of bleeding-contractions-placental separation-bleeding.

Frequency

United States

Placenta previa occurs in 0.3-0.5% of all pregnancies. The risks increase 1.5- to 5-fold with a history of cesarean delivery. With an increased number of cesarean deliveries, this risk can be as great as 10%. Recent studies show that a previous cesarean delivery did not increase the odds of detecting a placenta previa on second-trimester ultrasonography. However, the rate of placental migration observed at 28-36 weeks' gestation may identify patients who are more likely to deliver vaginally with resolution of the previa.

Of all placenta previas, the frequency of complete placenta previa ranges from 20-45%, partial placenta previa accounts for approximately 30%, and marginal placenta previa accounts for the remaining 25-50%.

Mortality/Morbidity

Table. Morbidities and Their Relative Risk (Open Table in a new window)

Morbidities	Relative Risk
Antepartum bleeding	10
Need for hysterectomy	33
Blood transfusion	10
Septicemia	5.5
Thrombophlebitis	5
Endometritis	6.6^[1]

The perinatal mortality rate associated with placenta previa ranges from 2-3%.

Maternal mortality is 0.03% in the United States.

Race

Placenta previa has no predilection for any race.

Sex

Placenta previa only occurs in pregnant women.

Age

Age is associated with a varying prevalence of placenta previa. The risk of placenta previa in relation to age is as follows:

Aged 12-19 years - 1%
Aged 20-29 years - 0.33%
Aged 30-39 years - 1%
Older than 40 years - 2%

Proceed to Clinical Presentation

Contributor Information and Disclosures

Author

Saju Joy, MD, MS Associate Director, Division Chief of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, Carolinas Medical Center

Saju Joy, MD, MS is a member of the following medical societies: American College of Obstetricians and Gynecologists, American Institute of Ultrasound in Medicine, American Medical Association, and Society for Maternal-Fetal Medicine

Disclosure: Nothing to disclose.

Coauthor(s)

Deborah Lyon, MD Director, Division of Gynecology, Associate Professor, Department of Obstetrics and Gynecology, University of Florida Health Science Center at Jacksonville

Deborah Lyon, MD is a member of the following medical societies: American College of Obstetricians and Gynecologists, Association of American Medical Colleges, Association of Professors of Gynecology and Obstetrics, and Florida Medical Association

Disclosure: Nothing to disclose.

Ryan A Stone, MD Fellow, Department of Obstetrics and Gynecology, Maternal-Fetal Medicine Section, Wake Forest University Health Sciences

Ryan A Stone, MD is a member of the following medical societies: Academic Pediatric Association, American College of Obstetricians and Gynecologists, American Medical Association, and Society for Maternal-Fetal Medicine

Disclosure: Nothing to disclose.

Specialty Editor Board

Francisco Talavera, PharmD, PhD Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Medscape Salary Employment

Richard S Legro, MD Professor, Department of Obstetrics and Gynecology, Division of Reproductive Endocrinology, Pennsylvania State University College of Medicine; Consulting Staff, Milton S Hershey Medical Center

Richard S Legro, MD is a member of the following medical societies: American College of Obstetricians and Gynecologists, American Society for Reproductive Medicine, Endocrine Society, Phi Beta Kappa, and Society of Reproductive Surgeons

Disclosure: Korea National Institute of Health and National Institute of Health (Bethesda, MD) Honoraria Speaking and teaching; Greater Toronto Area Reproductive Medicine Society (Toronto, ON, CA) Honoraria Speaking and teaching; American College of Obstetrics and Gynecologists (Washington, DC) Honoraria Speaking and teaching; National Institute of Child Health and Human Development Pediatric and Adolescent Gynecology Research Think Tank Panel (Bethesda, MD) Honoraria Speaking and teaching; University of Illinois (Chicago, IL) Honoraria Speaking and teaching; Georgetown University Hospital (Washington, DC) Honoraria Speaking and teaching; Heilongjiang University (Harbin, China) Speaking and teaching; New England Fertility Society (Nashua, NJ) Honoraria Speaking and teaching; William Beaumont Hospital Division of Reproductive Endocrinology and Infertility (Detroit, MI) Honoraria Speaking and teaching; Wayne State University School of Medicine (Detroit MI) Honoraria Speaking and teaching

Frederick B Gaupp, MD Consulting Staff, Department of Family Practice, Hancock Medical Center

Frederick B Gaupp, MD is a member of the following medical societies: American Academy of Family Physicians

Disclosure: Nothing to disclose.

Chief Editor

Carl V Smith, MD The Distinguished Chris J and Marie A Olson Chair of Obstetrics and Gynecology, Professor, Department of Obstetrics and Gynecology, Senior Associate Dean for Clinical Affairs, University of Nebraska Medical Center

Carl V Smith, MD is a member of the following medical societies: American College of Obstetricians and Gynecologists, American Institute of Ultrasound in Medicine, Association of Professors of Gynecology and Obstetrics, Central Association of Obstetricians and Gynecologists, Council of University Chairs of Obstetrics and Gynecology, Nebraska Medical Association, and Society for Maternal-Fetal Medicine

Disclosure: Nothing to disclose.

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