BACKGROUND

Placenta previa is defined as a placenta covering the internal cervical os. The incidence is estimated to range from 1 in 200 to 1 in 250 pregnancies at term, but it varies worldwide.^1,2 Risk factors include advanced maternal age, multiparity, multifetal gestation, male fetal gender, prior cesarean delivery, previous spontaneous and elective pregnancy terminations, previous uterine surgery, maternal tobacco or cocaine use, and use of assisted reproductive technology.^1–4 There is also a direct correlation between the number of previous cesarean deliveries and risk for placenta previa.

CLINICAL IMPLICATIONS

Placenta previa is associated with numerous adverse maternal outcomes, owing to a ten-fold increased risk of antepartum vaginal bleeding, and subsequently the increased need for blood transfusion, peripartum hysterectomy, intensive care unit (ICU) admission, sepsis, thrombosis, and even death.¹ The presence of one or more prior cesarean deliveries further increases these risks.⁵

Fetal complications are primarily attributable to preterm delivery. The average gestational age at delivery among women with previa is 35 weeks, with approximately half of all women delivering at ≤37 weeks gestation.^1,5 The risk of neonatal mortality appears to be threefold to fourfold higher compared to neonates born to women without placenta previa,⁶ while long-term outcomes do not differ.⁷

DIAGNOSIS

The classic presentation of a patient with placenta previa is one with painless vaginal bleeding. However, the presence of pain should not reduce the clinician’s suspicion for previa. In developed countries, the majority of placenta previas are diagnosed during midtrimester ultrasound (Figure 43-1). In cases of suspected placenta previa on transabdominal ultrasound, the use of transvaginal ultrasound can and should be employed, as it has greater diagnostic accuracy. Theoretical concerns for increased risk of bleeding with the use of transvaginal ultrasonography largely have been disproven.⁸ Translabial ultrasonography is another, more accurate alternative to transabdominal imaging.¹

Ultrasonography Tips^9,10

Sagittal, parasagittal, and transverse images should be obtained with the bladder partially full. Transvaginal or translabial ultrasonography can aid in visualization of the placenta if the fetal vertex is low within the maternal pelvis. Using a cutoff of 4.2 cm between the placental edge and internal cervical os on transabdominal ultrasound has a 99.8% negative predictive value for previa Color Doppler can aid in differentiating between vasculature arising from the placenta or the cervix.

RECOMMENDATIONS FOR MANAGEMENT

Antenatal Management

Asymptomatic women with previae are managed on an outpatient basis. Women with symptomatic placenta previae and who are hemodynamically stable also may be managed on an outpatient basis. Studies demonstrate no differences in maternal or neonatal outcomes with inpatient vs. outpatient management, except that the former costs more. Nonetheless, most experts recommend hospitalization for repetitive vaginal bleeding, threatened preterm labor, limited access to care, or shortened cervical length.^1,11–13

Antenatal Corticosteroids

Given the high rate of preterm delivery, it is reasonable to consider a course of corticosteroids in all patients with placenta previa. A single course of corticosteroids is recommended between 24 0/7 weeks and 33 6/7 weeks gestation who are at risk for preterm delivery within the next 7 days, and it can be considered as early as 23 0/7 weeks following a detailed discussion surrounding periviability and the need for neonatal resuscitation.

A single repeat dose of corticosteroids should be considered in women who are <34 0/7 weeks gestation and are at risk for preterm delivery within the next 7 days, and whose prior course of corticosteroids was administered >14 days earlier. A single course of corticosteroids is recommended between 34 0/7 and 36 6/7 weeks gestation for women who are at risk for preterm delivery within the next 7 days, are nondiabetic, and have not previously received a course of corticosteroids.^1,14

Timing of Delivery

The optimal timing of delivery for asymptomatic placenta previae is controversial. The benefits of a scheduled delivery with optimal staffing, presence of neonatal intensivists, anesthesiologists, and available blood products must be weighed against the risk of neonatal prematurity and the risk of antenatal hemorrhage and emergent delivery with advancing gestational age. Current recommendations are for delivery between 36 0/7 and 37 0/7 weeks gestation. Fetal lung maturity does not need to be confirmed prior to delivery. In cases of maternal instability due to ongoing bleeding, delivery should occur immediately, and it should not be delayed waiting for the administration of antenatal corticosteroids.^1,15

Mode of Delivery

For women whose placental edge is ≤1 cm from the internal cervical os, the only safe mode of delivery is cesarean. If transvaginal ultrasonography shows the placenta to be ≥2 cm from the internal cervical os, vaginal delivery is safe. Cases in which the placental edge lies between 1 cm and 2 cm from the internal cervical os should be individualized, with consideration given to preparing for both vaginal and cesarean delivery.^1,16

In the absence of severe prematurity, transverse fetal lie or placenta accreta, hysterotomy is generally low transverse. Care should be taken to avoid cutting through the placenta, as this can increase blood loss. Preoperative mapping of the placenta with ultrasonography may aid in delineating the proper location for hysterotomy. Delayed cord clamping should be avoided.¹⁷ Women with placenta previa are at increased risk for postpartum hemorrhage owing to bleeding from the placental implantation site.

In cases of placenta previa with one or more prior cesarean deliveries, the possibility of placenta accreta should always be considered and preoperative surgical planning undertaken. See the section entitled “Placenta Accreta,” later in this chapter, for further details.

BACKGROUND

Vasa previa occurs when fetal vessels traverse unprotected through amniotic membranes over or near the internal cervical os.¹ Its incidence is estimated to be approximately 1 in 2500 to 1 in 5000 pregnancies,¹ although the condition may be as common as 1 in 208 pregnancies achieved by assisted reproductive technology.¹⁸ Risk factors for this condition include succenturiate or bilobed placentae, velamentous cord insertion, placenta previa or low-lying placentae, fertilization achieved by assisted reproductive technology, and multiple gestation.^1,19 There are two types of vasa previa: (1) those associated with a velamentous umbilical cord insertion (type I vasa previa), and (2) those involving vessels that connect either bilobed or succenturiate lobes of the placenta (type II vasa previa).²⁰ This differentiation is important not only to ensure completion of the third stage of labor but also to allow fetoscopic laser ablation in certain cases of type II vasa previa.^21,22

CLINICAL IMPLICATIONS

The major concern with vasa previa is sudden and catastrophic bleeding, which can lead to rapid fetal exsanguination. For this reason, perinatal mortality approaches 60% when undiagnosed, underscoring the importance for early diagnosis and planned preterm cesarean delivery. Conversely, survival approaches 97% when diagnosed antenatally.^1,23

DIAGNOSIS

The majority of vasa previas today are diagnosed via midtrimester ultrasound, which carries a median detection rate of 93% and a specificity of 99%.²⁴ Findings that raise suspicion for vasa previa include placenta previa or low-lying placenta, a succenturiate or bilobed placenta, or velamentous cord insertion.¹ Grayscale ultrasonography will demonstrate a tubular, echolucent structure overlying the interval cervical os (Figure 43-2). Color Doppler can confirm vasa previa¹ by demonstrating a rate consistent with the fetal heart rate (FHR) (Figure 43-3). The course of the vessel should be carefully evaluated to exclude other vessels in close proximity to the cervix, such as funic presentation, marginal vein, or a venous sinus.²⁴

RECOMMENDATIONS FOR MANAGEMENT

Antenatal Management

The distinction between type I and type II vasa previa is important, as type II vasa previa is occasionally amenable to fetoscopic laser photocoagulation.^21,22 The proper management of stable, nonbleeding vasa previa is unclear. Many experts advise routine hospitalization at 30 to 34 weeks gestation; however, the benefit of this management remains unproven. Factors that may influence a provider’s decision to consider inpatient vs. outpatient management include a history of antenatal bleeding, prior spontaneous preterm birth, access to care, patient reliability, and cervical length.^1,13,24

Antenatal Corticosteroids

Given the high rate of iatrogenic preterm delivery among women with vasa previa, it is reasonable to consider a course of corticosteroids in all patients with this placental abnormality. Steroids can be considered as early as 23 0/7 weeks gestation following a detailed discussion surrounding periviability and the need for neonatal resuscitation.

A single repeat dose of corticosteroids should be considered in women who are <34 0/7 weeks gestation and are at risk for preterm delivery within the next 7 days, and whose prior course of corticosteroids was administered >14 days previously. In the setting of overt fetal compromise with an abnormal FHR pattern and a known vasa previa, delivery should be effected immediately, regardless of the status of antenatal corticosteroids.

Timing of Delivery

As with placenta previa, the risk of iatrogenic preterm delivery must be weighed against the increasing risk of hemorrhage with advancing gestational age and the onset of labor or spontaneous rupture of membranes. Current recommendations suggest that delivery between 34 and 36 weeks gestation effectively balances maternal and fetal risks. However, delivery should occur immediately in any clinical scenario characterized by vaginal bleeding, suspected/confirmed vasa previa, and FHR abnormalities.^1,24

Mode of Delivery

Given the risk for prematurity and neonatal anemia due to sudden and massive hemorrhage, women with vasa previa should deliver via cesarean in a facility capable of immediate neonatal resuscitation and blood transfusion.²⁴ Delayed umbilical cord clamping should be avoided.^17,24 While case reports of vaginal delivery following laser photocoagulation of type II vasa previa exist, delivery is generally via cesarean.^21,22

BACKGROUND

Placenta accreta is defined as an abnormal trophoblastic attachment to the myometrium. The term encompasses any abnormal placental attachment to the myometrium, including placenta increta (invasion into the myometrium), and placenta percreta (invasion through the myometrium into the uterine serosa).¹ The incidence of placenta accreta has risen dramatically in developed countries in recent decades, ranging from 1 in 30,000 pregnancies in the 1960s to as high as 1 in 300 pregnancies in 2010.^1,25 This rise is thought to be related to the similar increase in the rate of cesarean delivery.

Several studies, both prospective and retrospective, have demonstrated that placenta previa and prior uterine scarring are the strongest risk factors for development of placenta accreta. Among those with placenta previa, the risk of placenta accreta is 3%, 11%, 40%, 61%, and 67% with the first, second, third, fourth, and fifth cesarean deliveries, respectively.²⁶ Additional risk factors include advanced maternal age, multiparity, a history of endometrial curettage, thermal ablation or pelvic radiation, uterine leiomyomata, hypertensive disorders, in vitro fertilization (IVF), and tobacco use.^1,25

CLINICAL IMPLICATIONS

The major concern with placenta accreta is massive obstetric hemorrhage, which can lead to disseminated intravascular coagulopathy (DIC), hemorrhagic shock, transfusion of multiple blood products, hysterectomy, any number of surgical complications, the need for mechanical ventilation, and death. Median blood loss is reported to range from 2500 to 7800 cc, and many women often require multiple units of blood products. As many as half of all women with accreta require admission to the ICU postoperatively. The maternal death rate is estimated to be as high as 7%. Delivery frequently is undertaken between 34 and 36 weeks gestation, with neonatal complications primarily resulting from iatrogenic preterm delivery.

DIAGNOSIS

A high index of suspicion for placenta accreta cannot be overstated. Any woman with placenta previa and a history of cesarean delivery or other uterine surgery should be screened for placenta accreta. This can be performed easily using grayscale ultrasonography, which has a reported sensitivity and specificity of 80% to 90% in the second and third trimesters. Box 43-1 shows various image findings that are highly specific for placenta accreta.¹