Placenta previa

Placenta previa (PP) is an uncommon complication of pregnancy. Usually diagnosed on routine ultrasound done for other reasons, but may present with painless vaginal bleeding in the second or third trimester.

Classified as PP if the placenta is directly covering the cervical os, or as low-lying placenta if the placental edge is <2 cm from the cervical os.

Bleeding in PP may be (or become) torrential, and appropriate monitoring should be performed. Digital vaginal examination should not be performed on women with active vaginal bleeding until the position of the placenta is known with certainty.

Most seen on early ultrasound will resolve spontaneously.

Cesarean section will generally be necessary in all cases of PP persisting after 36 weeks' gestation. Cesarean sections may be complicated by excessive bleeding and the need for cesarean hysterectomy (or uterine artery embolization, or other interventional radiology procedure), and should only be performed by experienced physicians.

Placenta previa (PP) is defined as the placenta overlying the cervical os.[1]Jain V, Bos H, Bujold E; Society of Obstetricians and Gynaecologists of Canada. Guideline no. 402: diagnosis and management of placenta previa. J Obstet Gynaecol Can. 2020 Jul;42(7):906-17.e1. http://www.ncbi.nlm.nih.gov/pubmed/32591150?tool=bestpractice.com [2]Royal College of Obstetricians and Gynaecologists. Placenta praevia and placenta accreta: diagnosis and management. Green-top guideline no. 27a. Sep 2018 [internet publication]. https://www.rcog.org.uk/en/guidelines-research-services/guidelines/gtg27a [3]Reddy UM, Abuhamad AZ, Levine D, et al. Fetal imaging: executive summary of a joint Eunice Kennedy Shriver National Institute of Child Health and Human Development, Society for Maternal-Fetal Medicine, American Institute of Ultrasound in Medicine, American College of Obstetricians and Gynecologists, American College of Radiology, Society for Pediatric Radiology, and Society of Radiologists in Ultrasound Fetal Imaging Workshop. J Ultrasound Med. 2014 May;33(5):745-57. http://www.ncbi.nlm.nih.gov/pubmed/24764329?tool=bestpractice.com PP diagnosed in the second trimester is likely to resolve as pregnancy progresses.[1]Jain V, Bos H, Bujold E; Society of Obstetricians and Gynaecologists of Canada. Guideline no. 402: diagnosis and management of placenta previa. J Obstet Gynaecol Can. 2020 Jul;42(7):906-17.e1. http://www.ncbi.nlm.nih.gov/pubmed/32591150?tool=bestpractice.com [3]Reddy UM, Abuhamad AZ, Levine D, et al. Fetal imaging: executive summary of a joint Eunice Kennedy Shriver National Institute of Child Health and Human Development, Society for Maternal-Fetal Medicine, American Institute of Ultrasound in Medicine, American College of Obstetricians and Gynecologists, American College of Radiology, Society for Pediatric Radiology, and Society of Radiologists in Ultrasound Fetal Imaging Workshop. J Ultrasound Med. 2014 May;33(5):745-57. http://www.ncbi.nlm.nih.gov/pubmed/24764329?tool=bestpractice.com In women with a scarred uterus (most commonly from a prior cesarean section), PP may be associated with placenta accreta spectrum (previously known as abnormally adherent placenta and morbidly adherent placenta), where the placenta attaches to the myometrial layer of the uterus.[4]American College of Obstetricians and Gynecologists / Society for Maternal-Fetal Medicine. ACOG SMFM obstetric care consensus #7: placenta accreta spectrum. Dec 2018 [internet publication]. https://www.acog.org/clinical/clinical-guidance/obstetric-care-consensus/articles/2018/12/placenta-accreta-spectrum Vasa previa, where the fetal vessels lie over the internal cervical os, is an associated condition.[5]Society for Maternal-Fetal Medicine. SMFM consult series #44: management of bleeding in the late preterm period. Oct 2017 [internet publication]. https://www.smfm.org/publications/249-smfm-consult-series-44-management-of-bleeding-in-the-late-preterm-period

In this topic, we refer to all varieties of invasive placentation as placenta accreta spectrum.[4]American College of Obstetricians and Gynecologists / Society for Maternal-Fetal Medicine. ACOG SMFM obstetric care consensus #7: placenta accreta spectrum. Dec 2018 [internet publication]. https://www.acog.org/clinical/clinical-guidance/obstetric-care-consensus/articles/2018/12/placenta-accreta-spectrum There are three commonly defined variants: placenta accreta (where chorionic villi attach to the myometrium, rather than being restricted within the decidua basalis), placenta increta (where the chorionic villi invade into the myometrium), and placenta percreta (where the chorionic villi invade through the myometrium and sometimes into adjoining tissue).[4]American College of Obstetricians and Gynecologists / Society for Maternal-Fetal Medicine. ACOG SMFM obstetric care consensus #7: placenta accreta spectrum. Dec 2018 [internet publication]. https://www.acog.org/clinical/clinical-guidance/obstetric-care-consensus/articles/2018/12/placenta-accreta-spectrum Although these distinctions are important to attending obstetricians, they do not change management decisions for primary care providers.[Figure caption and citation for the preceding image starts]: Placenta previa (previously known as complete previa) at 22 weeksFrom the teaching collection of Janet R. Albers, MD [Citation ends].