PLACENTA PREVIA

Introduction

Placenta previa and placenta abruption (abruptio placentae), the two leading and major causes of antepartum hemorrhage, result in substantial maternal and perinatal morbidity and mortality. In their severe forms, both placenta previa and placenta abruption may have long-term maternal and neonatal sequelae.

Placenta previa is the implantation of placental tissue partially or entirely within the lower segment of the uterus after 20 weeks of gestation. The abnormally implanted placenta may partially or entirely cover the cervix. Pregnancies complicated with placenta previa often present with painless vaginal bleeding after 20 weeks of gestation and are thereafter confirmed and classified on obstetric ultrasonography.

Placental abruption refers to bleeding at the decidual–placental interface of normally implanted placenta, resulting in partial or complete placental detachment prior to delivery of the fetus. The diagnosis is typically reserved for pregnancies over 20 weeks of gestation. The major clinical findings are vaginal bleeding and abdominal pain, often accompanied by hypertonic uterine contractions, uterine tenderness, and a nonreassuring fetal heart rate (FHR) pattern.

In both placenta previa and abruptio, asymptomatic cases may be diagnosed during routine obstetric ultrasonography. In general, management and outcomes depend on the severity of clinical presentation and type of placenta previa or abruptio.

PLACENTA PREVIA
Definition

Placenta previa refers to placenta which is implanted partially or completely over the lower uterine segment (over and adjacent to the internal os) after 20 weeks of pregnancy. “Previa” comes from two words: “pre” (or “prae”) meaning before, and “via” meaning way. “Previa” usually refers to anything obstructing the presenting part. Placenta previa therefore means “placenta in the way, before the baby’s presenting part”. This definition is similar to that of vasa previa which means “vessels in the way, before the baby’s presenting part”.

Epidemiology

The overall prevalence of placenta previa is estimated as 5.2 per 1000 pregnancies, with marked regional variation.1,2 The prevalence is highest among Asian studies (12.2 per 1000 pregnancies) and lower in European (3.6 per 1000 pregnancies), North American (2.9 per 1000 pregnancies) and Sub-Saharan African (2.7 per 1000 pregnancies) studies.3 The risks of placenta previa increases 1.5–5-fold following cesarean delivery and with increasing numbers of cesarean deliveries, at 1% after one cesarean delivery, 2.8% after three cesarean deliveries, and 3.7% after five cesarean deliveries. Placenta previa following prior cesarean sections has been associated with high and increasing risk of placenta accreta syndromes.

Etiology and pathogenesis

Placenta previa arises from implantation by the embryo (embryonic plate) in the lower (caudad) uterine cavity (in close proximity to the cervical os). This implantation occurs as a result of defective decidual vascularization possibly from inflammation or atrophy. Due to continued placental growth, the placenta may remain at the lower segment or cover the cervical os partially or fully.

Placenta previa presents clinically as painless bleeding and is the leading single cause of major antepartum hemorrhage. Bleeding in placenta previa coincides with the development of the lower uterine segment in the third trimester. As the lower uterine segment thins in preparation for the onset of labor, placental attachment is disrupted leading to painless vaginal bleeding at the implantation site. The development of the lower segment also results in dilatation of the internal os, separation of some of the implanted placenta and subsequent bleeding. The myometrium of the lower uterine segment does not contract adequately to constrict and stop the flow of blood from the avulsed open vessels. Although thrombin released from the bleeding sites promotes uterine contractility, it also leads to a vicious cycle of bleeding–contractions–placental separation–bleeding.

Although the underlying cause of placenta previa is not known, a major risk is endometrial damage and uterine scarring.6 Other proposed hypotheses include the dropping down of the fertilized ovum and its implantation in the lower uterine segment, persistence of chorionic activity in the decidua capsularis and its contact with decidua vera of the lower uterine segment, defective decidualization and spread of the chorionic villi into the lower uterine segment, and large surface area of the placenta for example in multiple pregnancy. These pathogeneses may also explain placenta accreta syndromes and vasa previa.7 Due to lack of decidua basalis and incomplete development of the fibrinoid layer the implanting placenta may attach directly to the myometrium (accreta), invade the myometrium (increta), or penetrate the myometrium (percreta). Similarly, when sections of the placenta which undergo atrophic changes persist, they may form vasa previa.

Types of placenta previa

Based on proximity of the placental tissue to the internal cervical os, four types of placenta previa have been traditionally described.8 Type I or low lying placenta in which the placental edge is within 2 cm of but not reaching the internal cervical os. Type II or marginal placenta where the placental edge reaches the margin of but does not cover the internal cervical os, type III or partial or incomplete when the placental edge partially covers the internal cervical os (especially when closed but not entirely when fully dilated), and type IV or complete, when the placenta totally covers the internal cervical os including during full cervical dilatation. In addition, placenta previa has been described as anterior or posterior if lying in the anterior or posterior uterine wall, respectively. The majority of the placenta previa are on the posterior wall, and about one-third are placenta previa types III and IV. Clinically, placenta previa was also classified as minor (type I and II anterior) and major (type II posterior, III and IV). Other classifications systems characterized placenta previa as complete, partial, and marginal depending on how much of the internal endocervical os was covered by the placenta. Increased use of (transvaginal) ultrasonography for precise localization of the placental edge and the cervical os has led to a revision of the classification leading to elimination of the categories of “partial” and “marginal” placenta previa. The National Institutes of Health sponsored Fetal Imaging Workshop recommended two categories of placenta previa: placenta previa, when the internal os is covered partially or completely by placenta or low-lying placenta, when the placenta is implanted in the lower segment but the placental edge does not reach or cover the internal os and remains within 2 cm of the cervical os.

Placenta migration

The term “placental migration” has been used to describe placenta diagnosed as “low lying” in early pregnancy and which resolve by the third trimester. Occurring in more than 90% of cases, this is a misnomer as the placenta does not migrate but its proximal part grows toward better blood supply at the fundus (trophotropism), while the distal portions in the poorly vascularized lower segment regress and atrophy.10,11 Also, the differential growth of the lower segment relative to the upper segment due to the growing fetus may result in increased distance between distal edge of the placenta and the cervix. In general, owing to placental “migration”, any placenta previa diagnosed before 24 weeks should be confirmed by imaging studies between 28 and 32 weeks.

Multiple factors

Multiple factors that increase the risk of defective decidualization and, therefore, the placenta previa4,12,13,14,15,16 include:

Placenta previa increases with increasing parity. Grand multiparas have higher (5%) risk compared with nulliparous (0.2%).

Advanced maternal age: placenta previa occurs in an estimated 1% of deliveries among women aged 35 years and 2% of deliveries above 40 years. Compared to younger women, those more than 35 years and 40 years of age have more than a 4- and 9-fold greater risk for placenta previa, respectively.

Asian women have the highest rates of placenta previa compared to white and black women, 4.5, 3.3 and 3.0 per 1000 births, respectively.

Prior cesarean delivery: the risk of placenta previa increases with the number of cesarean sections from an estimated 0.9% after one, 1.7% after two, 3% after three and up to 10% after four or more cesarean deliveries due to endometrial scarring.

Other uterine surgeries like curettage, myomectomy have a slightly elevated risk of previa. Also, induced abortion and prior abortion due to endometrial scaring and inflammation increase the risk.

Factors that increase placental surface area as result of decreased uteroplacental oxygenation and increase the risk of placenta previa include:

Cigarette smoking (2–3-fold increase) because carbon monoxide hypoxemia causes compensatory placental hypertrophy and more surface decidual vasculopathy;

Maternal cocaine use (4-fold increase);

High altitude;

Multiple gestation is associated with 30–40% increase in placenta previa as result of larger placental surface area;

Male fetuses have large placental surface area and delayed implantation;

Prior placenta previa increases the risk of subsequent previa four to eightfold;

Other factors statistically associated with placenta previa include fetal malpresentation, preterm labor, preterm prelabor rupture of membranes, intrauterine fetal growth restriction, congenital anomalies, amniotic fluid embolism, prior infertility treatment, and abnormally elevated prenatal screening of maternal serum α-fetoprotein.

The diagnosis of placenta previa is based on history, clinical examination findings and supporting imaging studies. Increasingly, however, routine ultrasonography has resulted in earlier diagnosis of asymptomatic cases without or prior to clinical presentation.

History

Vaginal bleeding

The hallmark of placenta previa is painless per vaginal bleeding after 20 weeks of pregnancy, which occurs in more than 70% of the cases. Less than 10% of patients with placenta previa are asymptomatic. Although bleeding may be provoked by labor, pelvic examination, or sexual intercourse, often no predisposing factor is identified. The initial bleeding “herald or sentinel bleed” is seldom profuse and is typically followed by major bleeding. Some (10–20%) patients with placenta previa may have uterine contractions and such (painful) vaginal bleeding which may be confused with placenta abruptio.

Although the painless hemorrhage often occurs near the end of the second trimester or in the third trimester, placenta previa classically presents with painless third-trimester bleeding. About one-third of women bleed before 30 weeks, one-third between 30 and 36 weeks, and the rest after 36 weeks. Bleeding prior to 30 weeks is associated with increased maternal and perinatal mortality and morbidity including blood transfusion. The bleeding may stop spontaneously and recur in labor.

Other findings on history will depend on the severity of the bleeding. For example, patients may be hemodynamically stable, present with mild hypotension or present in shock from severe hemorrhage.

Clinical examination
General clinical examination

The findings on clinical examination depend on severity of the bleeding and may range from stable with no pallor and normal or minimally altered vital signs to clinical features of shock and raised shock index.

Abdominal examination

The size of the uterus may be disproportionate to the gestational age resulting in a higher symphysiofundal height compared to the gestational age. Placenta previa may prevent the fetus from establishing normal polarity resulting in abnormal fetal lie and malposition/malpresentations and unengaged presenting part. The uterus may be relaxed, soft and non-tender compared to findings of placenta abruptio. There may also be associated multiple pregnancy or uterine leiomyomata. Fetal heart sound is usually present, unless there is severe bleeding and placental separation when the fetal heart rate tracing may have repetitive late decelerations or other non-reassuring fetal heart rate patterns. Rarely performed, placenta previa can be suggested by Stallworthy’s sign, where the fetal heart rate slows down and soon recovers promptly on pressing the head down into the pelvis.17 This is more evident with posterior placenta previa.

Other clinical findings depend on the severity of the bleeding. For example, in extremely severe cases there may be absence of fetal heart tones due to intrauterine fetal exsanguination.

Pelvic examination

The only permitted routine pelvic examination is inspection of the vulva and clothing to ascertain continued bleeding, amount of blood loss, and the color of the blood. In placenta previa, the blood is bright red as the bleeding occurs from the separated uteroplacental sinuses close to the cervical opening and escapes out immediately. Digital or speculum examination can provoke further placental separation and massive fatal hemorrhage. Speculum examination and not digital vaginal examination should be performed in an operation theater when cesarean delivery can be performed immediately. However, experienced staff can safely conduct a careful speculum examination for mild cases, stable patients, if there is lack of immediate ultrasound for placental localization.

Diagnosis

The clinical presentation of painless and often recurrent vaginal bleeding after 20 weeks of pregnancy is often diagnostic of placenta previa unless proven otherwise. In such women placenta previa can only be excluded after imaging studies.

Now only for historical reasons and in very resource-limited settings, a double set-up technique can be diagnostic. In this procedure, the patient is set in the operating room with the surgical team ready for an immediate cesarean section. A digital vaginal examination is then performed. A finger is passed around the cervix through all the fornices to assess for bogginess suggestive of placental tissue. If the presenting part and not bogginess is felt clearly through all the fornices, the finger is gently introduced into the cervical canal to evaluate for the placenta (firm) and blood clots (soft and friable). Once placenta previa is confirmed a cesarean section is immediately performed. If the placenta previa is ruled out, then artificial rupture of membranes and membrane sweeping is performed, and labor augmented as the other common cause of bleeding in this case is likely to be placenta abruptio. With the growing availability of obstetric ultrasound, double set-up examination is rarely necessary.

Imaging studies

The imaging studies for placental location can be obstetric ultrasonography (transabdominal, transvaginal or transperineal) and magnetic resonance imaging. Ultrasonography is the initial imaging study for confirmation or ruling out placenta previa. Transabdominal sonography is confirmatory of placenta previa in 96% of cases. Precision is improved by emptying the maternal urinary bladder. False positive results may be due to a full bladder or myometrial contractions. Poor imaging could be due to maternal obesity and posterior placenta (poorly visualized as a result of acoustic shadow from the fetal presenting part), lack of anatomical landmark posteriorly (compared to anterior uterovesical angle) below which placenta is defined.

Transvaginal sonography (TVS) is safe and superior to the transabdominal ultrasound.18 Due to the proximity of the transducer to the uterus and higher frequencies, TVS has a superior resolution, does not require a full bladder and has nearly 100% accuracy .