Placenta previa is diagnosed when the placenta overlies or is close to the internal cervical os, obstructing delivery. The incidence is of 5 in 1000 deliveries and it has recently increased due to the rise in cesarean deliveries. PP is associated with a higher risk of adverse maternal outcomes.

Abstract: Placenta previa (PP) is diagnosed when the placenta overlies or is close to the internal cervical os, presenting an obstruction to delivery. The incidence is of 5 in 1000 deliveries and it has recently increased due to the rise in cesarean deliveries. PP is associated with a higher risk of many adverse maternal outcomes, mostly related to hemorrhage. Ultrasound diagnosis in the second and third trimester allows tailored management and a planned cesarean section in order to optimize maternal and neonatal outcomes.

Keywords: placenta previa, placenta praevia, low-lying placenta, abnormal placentation.

Authors: Maddalena Morlando1, Johanna Bjurstrom2, Lene Gronbeck 2.

1. Prenatal Diagnosis and High Risk Pregnancy Unit, Department of Woman, Child and General and Specialized Surgery, University “Luigi Vanvitelli”, Naples, Italy.

2. Department of Obstetrics, Copenhagen University Hospital, Rigshospitalet, Denmark.

Reviewer: Dr. J. J. Duvekot

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Definition

The term placenta previa refers to a placenta that overlies or is close to the internal cervical os, presenting an obstruction to the cervix and thus to delivery.  Traditionally, placenta previa has been categorized into 4 types:

  1. Complete placenta previa, where the placenta completely covers the internal os.
  2. Partial placenta previa, where the placenta partially covers the internal os. 
  3. Marginal placenta previa, which just reaches the internal os, but does not cover it.
  4. Low-lying placenta, which extends into the lower uterine segment but does not reach the internal os.1 Types 1 and 2 are also often defined as ’major’ placenta previa, whereas 3 and 4 are also referred to as ’minor’ placenta previa.

The widespread use of transvaginal (TV) scanning in clinical practice has allowed for a more precise evaluation of the distance between the placental edge and the internal os. According to a definition based on the TV ultrasound aspect of the placenta, the term ‘placenta previa’ should be used when the placenta lies directly over the internal os. The placenta should be reported as ‘low lying’ when the placental edge is less than 2 cm from the internal os and as normal when the placental edge is 2 cm or more from the internal os.2
 

ICD code

ICD-10: O44 - Placenta previa.

Incidence

The incidence of placenta previa is estimated to be 5 in 1000 deliveries and varies throughout the world.3 It has increased in the last decades relative to the increasing rate of cesarean deliveries, the main risk factor for this condition.4 Other risk factors include previous spontaneous and elective pregnancy terminations and previous uterine surgery, increasing maternal parity, increasing maternal age, smoking, cocaine use, multiple gestations, and prior previa.5

Pathogenesis

The underlying cause of placenta previa is not yet completely understood. There is a clear association between prior endometrial damage and uterine scarring from uterine surgery and placental implantation in the lower uterine segment.6 Many placenta previas identified at midpregnancy will no longer be present by the time of delivery. Only 10–20% of previas identified at 20 weeks of gestation will remain previas in the late third trimester.7. The relationship between the cervix and the placenta changes over time with the placenta typically “migrating” to the uterine fundus. Actually, the placenta does not truly “migrate.” It is thought that the placenta undergoes a process termed trophotropism, growing toward the area of the uterus with the best blood supply (typically the fundus), while the portion of the placenta closest to the cervix regresses and atrophies.8 The presence of a previous caesarean scar would induce distortion of the normal anatomy of the lower uterine segment and prevent this ‘migration’

Pathology

Abnormal placentation in the poorly vascularized lower uterine segment is associated with inadequate uteroplacental perfusion reflected by histopathological changes of placenta. In fact, PP is significantly associated with maternal underperfusion lesion, including villous infarction and increased intervillous fibrin. Hemorrhage in the myometrium and infiltration with acute inflammatory cells in myometrium and blood vessels wall are also seen in PP.9 In addition to this, compensatory placental growth with increased surface area in response to reduced placental perfusion is often present. These morphological changes may play important roles in maintaining adequate uteroplacental and fetal perfusion, which may prevent adverse neonatal outcomes.10

Associated anomalies

Placenta previa is an important risk factor for abnormal invasive placenta (AIP), a spectrum of conditions characterized by an abnormal adherence of the placenta to the myometrium with a high risk of severe hemorrhage. In women with both a previous caesarean section and a placenta previa the incidence of AIP rises from 1.7 per 10.000 to 577 per 10.000.11 Women with a placenta previa and a previous uterine surgery, such as cesarean section, should be specifically screened for AIP by a by a skilled operator with experience in the ultrasound diagnosis of AIP.

Recurrence risk

The recurrence rate of placenta previa in a subsequent birth has been estimated to be around 4.8%.12

Diagnosis

The classic clinical history for PP is that of painless bleeding. Bleeding may be provoked by labor, vaginal examination, or sexual intercourse but it usually has no identifiable cause. PP is typically confirmed by ultrasound. Many studies have compared different sonographic techniques and TV ultrasound has been shown to better delineate the relationship between the lower placental edge and the internal cervical os.13-15 Concerns regarding the potential for the TV approach to provoke bleeding are unfounded.16 The diagnosis of a PP is usually made during the second trimester scan. When this occurs a subsequent scan in the third trimester is usually scheduled, to confirm the finding of PP. In fact, a conclusive diagnosis of PP is possible only in the third trimester of pregnancies, because most low lying placentas will migrate to the uterine fundus with increasing gestational age.7, 17, 18 The likelihood of persistence to term of a PP found in the second trimester is related to the degree to which the placenta overlies the cervix. When the placental edge overlaps the internal os by more than 2 cm, migration is not likely to occur. When the placental edge is more than 2 cm from the internal os, migration is the most likely scenario. If the distance of the placental edge is less than 2 cm from the internal os, placental migration occurs in most cases (88.5%.)19 The thickness of the placental edge, the advanced gestational age at diagnosis, the presence of a major previa, and prior cesarean sections can also increase the likelihood of persistence to term.7, 17, 20

Differential diagnosis

PP might be differentiated from other conditions causing antepartum hemorrhage (APH). Abruption of the placenta can lead to APH, but this condition is usually painful, associated with uterine tenderness and with signs of fetal compromise. Other causes of bleeding (for example bleeding from the vulva, vagina or cervix) might be responsible for APH and must be ruled out. All these conditions can coexist with PP, but the ultrasound examination will usually show a placenta implanted away from the internal os.

Implications for sonographic diagnosis

The second trimester routine fetal anomaly scan should include placental localization in order to identify women at risk of persisting PP. If the placenta is thought to be low-lying or previa at the second trimester scan, a follow-up transvaginal ultrasound examination at 32 weeks of gestation should be arranged to diagnose persistent low-lying placenta and/or placenta previa. In women with persistent low-lying placenta at 32 weeks an additional TV scan is recommended at around 36 weeks of gestation to safely plan delivery and evaluate the need for caesarean delivery.21 Women with a PP and a previous uterine surgery, such as cesarean section, should be specifically screened for AIP by a by a skilled operator with experience in the ultrasound diagnosis of AIP. 

Prognosis

PP is associated with an increased risk of many adverse maternal and fetal–neonatal complications, mostly depending on maternal hemorrhage.  Women diagnosed with PP have a 10-fold increased risk of antenatal bleeding22, and the pregnancy outcome is mainly dependant on whether the woman is symptomatic or not.23 Bleeding is associated with an increased risk of preterm delivery, blood transfusion, hysterectomy, maternal intensive care unit admission, and even maternal death.22, 24, 25 The risk of hysterectomy is even higher in the presence of previous caesarean sections.  The type of PP can also affect the outcome, with complete previas associated with a greater risk of admission due to APH, severe peripartum hemorrhage and hysterectomy compared to partial previas.26 Fetal complications are also more frequent with PP and are primarily associated with prematurity. Increased risk of lower Apgar scores and admission to neonatal intensive care unit have been demonstrated in women with PP.25 Perinatal mortality rates are increased by threefold to fourfold.27

Management

The guiding principle in the management of PP is expectant management. Antenatal care must be tailored to the characteristics and on the medical history of each woman. Decision about hospitalization must take into account several variables including distance between home and hospital and availability of transportation, previous bleeding, and the presence of anemia.28, 29 The timing of delivery must consider the benefits of a planned delivery before the onset of labor or bleeding and the risks of prematurity. As the risk of hemorrhage increases rapidly after 36 weeks of gestation, women with uncomplicated placenta previa should undergo scheduled delivery at 36-38 weeks.30, 31 Women with placenta previa should be delivered by caesarean section. In fact, if the placenta overlies the internal os or there is less than 1 cm between the placenta and the internal os, the risk of hemorrhage is so high as to indicate a cesarean delivery. However, if the placenta is 2 cm or more from the internal os, a trial of labor is a reasonable choice.32, 33 The optimal mode of delivery in women with a placental edge between 1.0 and 2.0 cm between the os and the placenta is uncertain and must be individualized. In women with a previous history of caesarean section the risk of massive hemorrhage and hysterectomy is higher and delivery should be arranged in a maternity unit with on-site blood transfusion services and access to high-dependency care. The cesarean section should be carried out by an appropriately experienced operator with a consultant obstetrician and consultant anesthetist present within the delivery or theatre suite.21 

References

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This article should be cited as: Maddalena Morlando, Johanna Bjurstrom, Lene Gronbeck: Placenta previa, Visual Encyclopedia of Ultrasound in Obstetrics and Gynecology, www.isuog.org, February 2019.
 


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