The definition of placenta previa based on ultrasound findings is more practical, and the traditional definition (implantation of the placenta in the lower uterine segment) needs to be revised. The term ‘placenta previa’ should only be used when the placental edge overlaps or is within 2 cm of the internal cervical orifice in late pregnancy. If the placental edge is located further than 2 cm but within 3.5 cm from the internal cervical orifice, the placenta should be termed ‘low-lying’. Unless the placental edge at least reaches the internal orifice at mid-trimester, symptomatic placenta previa in the third trimester will not be encountered. Caesarean section is the recommended mode of delivery for placenta previa at term. Attempt at vaginal delivery is appropriate for low-lying placenta, but the possibility of post-partum haemorrhage should be kept in mind. The incidence of invasive placentation, such as placenta accrete, has progressively risen in the past 3 decades, possibly as a consequence of increasing caesarean section rates. Ultrasound has a sensitivity of 91% and a specificity of 97% for the identification of all forms of invasive placentation. Chorioangiomas are benign non-trophoblastic placental tumours with excessive vascular proliferation within the stroma of chronic villi. They are usually asymptomatic, although occasionally can be associated with adverse fetal outcomes. Chorioangiomas usually appear as well-circumscribed, rounded, hypo-echoic lesions next to the chorionic surface. Iatrogenic delivery or prenatal intervention are two options, if fetal compromise is present. Prenatal detection leads to a dramatic increase in survival compared with those cases unsuspected antenatally.
Introduction
Placenta previa remains a serious complication of pregnancy, and clinicians are increasingly facing invasive placentation. Prenatal diagnosis has been shown to decrease the rate of maternal and fetal morbidities that are usually higher if these conditions remain undiagnosed until delivery. Ultrasound is usually used as the primary tool in evaluating women at a risk for placental disorders. Prenatal magnetic resonance imaging (MRI) can be complementary to ultrasound, and may add information that helps clinicians in guiding the management. Increased awareness and improvements in ultrasound techniques have led to an increase in the prenatal diagnosis of placental chorioangiomas. Although usually asymptomatic, they can be associated with adverse fetal outcomes, and concerns have been raised about their association with poor neurodevelopmental outcome. Vasa previa is a rare, but potentially disastrous, condition, and can lead to severe fetal distress and death. Prenatal diagnosis of this condition has been shown to dramatically improve the outcome.
Therefore, knowledge of how to diagnose these conditions is mandatory for the obstetrician, especially if working in high-risk units. In this chapter, we provide an up-to-date review of prenatal diagnosis of placental disorders with ultrasound.
Placenta previa
Definition
Traditionally, implantation of the placenta, fully or partially, in the lower uterine segment is defined as placenta previa. This definition is not useful in practice, because the lower uterine segment is difficult to identify without opening the abdomen and inspecting the uterus, or with the use of ultrasound scan. Ultrasound is now the gold standard for identifying placenta previa. Therefore, a definition based on ultrasound findings is more practical, and a revision of the traditional definition is timely. Placenta previa is responsible for potentially life-threatening conditions for the mother, including severe ante- and postpartum bleeding, higher risk of invasive placentation, need for hysterectomy, blood transfusions, septicaemia, and thrombophlebitis. Furthermore, adverse fetal and neonatal outcome, such as perinatal death and preterm delivery, is increased in pregnancies complicated by placenta previa .
Incidence and prevalence
The prevalence of placenta previa is about 5.2 per 1000 pregnancies . The occurrence of placenta previa has increased in recent decades. A systematic review published in 2003 exploring the occurrence of this condition between 1966 and 2000 found an overall prevalence of placenta previa of 4.0 per 1000 pregnancies. This figure was different from that reported from a recent meta-analysis examining the occurrence of placenta previa in the past 30 years; in this study a prevalence of 5.2 per 1000 pregnancies was reported. The prevalence of placenta previa was reported to be highest among Asian studies (12.2 per 1000), and lower among studies from Europe (3.6 per 1000), North America (2.9 per 1000), and Sub-Saharan Africa (2.7 per 1000). In the review by Cresswell , the pooled prevalence of major placenta praevia was 4.3 per 1000 pregnancies. The increasing rate of caesarean deliveries in the past 3 decades may have contributed to this increase in the prevalence of placenta previa.
Risk factors
Previous caesarean delivery or uterine surgery represents a major risk factor for placenta previa . The occurrence of placenta previa seems to be correlated also with the number of caesarean sections. In a recent systematic review, Marshall et al. found that the incidence of placenta previa increased from 10 per 1000 deliveries with one previous caesarean section to 28 per 1000 with more than three caesarean deliveries. In addition, advanced maternal age, multiparity, smoking, cocaine abuse, history of induced abortions, and multiple pregnancy are other risk factors associated with the occurrence of placenta previa .
The pathophysiology of placenta previa is not yet completely understood. The relationship between the placental edge and the internal orifice changes with advancing gestation.
In most of the cases, the placenta implanted lower in the early second trimester of pregnancy seems to move away from the internal orifice in the third trimester. It has been hypothesised that this placental ‘migration’ from the lower uterine segment towards the fundus occurs because the fundus, being more vascularised than the rest of the uterus, would allow a better development of the trophoblastic tissue. This phenomenon would occur because of a process of a degeneration of the trophoblast close to the internal orifice secondary to decreased vascularisation rather than a true migration of the placental tissue . Distortion of the normal anatomy of the lower uterine segment induced by a previous caesarean scar would prevent this ‘migration’ . Alternatively, defective decidual vascularisation and subsequent endometrial hypoxemia may increase the surface area of the placental tissue so that the feto-maternal exchange of the oxygen may not be compromised. In this scenario, the placenta may have a larger diameter, and its lower edge would be more likely to implant in close proximity to the cervix .
Ultrasound definition and screening for placenta previa
Placenta previa is commonly diagnosed using ultrasound. Placental location is usually reported during the routine anomaly scan. A follow-up scan in the third trimester should be scheduled when the placental edge is found to be reaching or overlapping the internal cervical orifice, to confirm this finding and to plan the management of delivery. A conclusive diagnosis of placenta previa is possible only in the third trimester of pregnancies, because almost 90% of the placentas defined as low in the second trimester move away out of the lower uterine segment later on in gestation ( Fig. 1 a and b).
Although placenta previa can be easily diagnosed trans-abdominally, trans-vaginal ultrasound has been shown to better delineate the relationship between the lower placental edge and the internal cervical orifice .
Some investigators have attempted to correlate the position of the placenta in the second trimester, as detected at the scan, with the likelihood of migration in the third trimester of pregnancy. Oppenheimer et al. studied 36 women with placental edge within 3 cm of the internal cervical orifice at 26 weeks of pregnancy or later. The mean rate of migration was 5.4 mm per week in women in whom placental edge migration did occur. If the placental edge overlapped the internal cervical orifice by more than 2 cm, migration was not observed in any woman. When the placental edge was more than 2 cm from the internal cervical orifice, migration always occurred, and none of the women required a caesarean section for placenta previa. If the distance of the placental edge was less than 2 cm from the internal cervical orifice, placental migration occurred in most (88.5%) cases.
Ghourab reported on the significance of the shape of the placental edge detected at 28–32 weeks in predicting placental migration. The placental edge was defined as ‘thick’ if the thickness was 1 cm or less, within 1 cm from the edge, the angle between the basal and the chorionic plate exceeded 45 0, or both. All other cases were defined as ‘thin’ placental edge. Placental migration was seen in 29.6% where the placental edge was thin, but only in 5.8% where it was thick. A significantly higher rate of ante-partum haemorrhage, abdominal delivery, adherent placenta, and low birth weight was found in cases where the placental edge was thick.
The above two studies show that, in some cases of mid-trimester low-lying placenta, the placental edge is more likely to ‘migrate’ than others are. Ultrasound may be useful in predicting both the likelihood and extent of placental migration as a function of time.
Mode of delivery
Caesarean section is the recommended mode of delivery for major placenta previa, whereas for minor previa an attempt at vaginal delivery is deemed appropriate. The introduction of ultrasound in clinical practice has raised the issue of which sonographic threshold of distance between the lower placental edge and the cervix should be used to achieve a safe vaginal delivery. Oppenheimer et al. found that the mean distance of the placental edge from the internal cervical orifice in women requiring a caesarean section for placenta previa was 1.1 cm (range, 0.0–2.0 cm).
In a study of 121 women with placenta previa, all women required a caesarean section when the placental edge was within 1 cm of the internal cervical orifice within 2 weeks of delivery. In contrast, if the placental edge to internal cervical orifice distance was 2 cm or more, the likelihood of achieving a vaginal delivery was at least 63% . According to these findings, the term placenta previa should only be used when the placental edge overlapped or was within 2 cm of the internal cervical orifice in late pregnancy. If the placental edge is located further than 2 cm, but within 3.5 cm from the internal cervical orifice, the placenta should be termed low-lying. In the latter case, although there is a good chance of a vaginal delivery, the incidence of post-partum haemorrhage remains high . Therefore, a low-lying placenta deserves an attempt at vaginal delivery, but should warn the clinician of the possibility of haemorrhagic complications, so that appropriate precautions can be taken.
Placenta previa
Definition
Traditionally, implantation of the placenta, fully or partially, in the lower uterine segment is defined as placenta previa. This definition is not useful in practice, because the lower uterine segment is difficult to identify without opening the abdomen and inspecting the uterus, or with the use of ultrasound scan. Ultrasound is now the gold standard for identifying placenta previa. Therefore, a definition based on ultrasound findings is more practical, and a revision of the traditional definition is timely. Placenta previa is responsible for potentially life-threatening conditions for the mother, including severe ante- and postpartum bleeding, higher risk of invasive placentation, need for hysterectomy, blood transfusions, septicaemia, and thrombophlebitis. Furthermore, adverse fetal and neonatal outcome, such as perinatal death and preterm delivery, is increased in pregnancies complicated by placenta previa .
Incidence and prevalence
The prevalence of placenta previa is about 5.2 per 1000 pregnancies . The occurrence of placenta previa has increased in recent decades. A systematic review published in 2003 exploring the occurrence of this condition between 1966 and 2000 found an overall prevalence of placenta previa of 4.0 per 1000 pregnancies. This figure was different from that reported from a recent meta-analysis examining the occurrence of placenta previa in the past 30 years; in this study a prevalence of 5.2 per 1000 pregnancies was reported. The prevalence of placenta previa was reported to be highest among Asian studies (12.2 per 1000), and lower among studies from Europe (3.6 per 1000), North America (2.9 per 1000), and Sub-Saharan Africa (2.7 per 1000). In the review by Cresswell , the pooled prevalence of major placenta praevia was 4.3 per 1000 pregnancies. The increasing rate of caesarean deliveries in the past 3 decades may have contributed to this increase in the prevalence of placenta previa.
Risk factors
Previous caesarean delivery or uterine surgery represents a major risk factor for placenta previa . The occurrence of placenta previa seems to be correlated also with the number of caesarean sections. In a recent systematic review, Marshall et al. found that the incidence of placenta previa increased from 10 per 1000 deliveries with one previous caesarean section to 28 per 1000 with more than three caesarean deliveries. In addition, advanced maternal age, multiparity, smoking, cocaine abuse, history of induced abortions, and multiple pregnancy are other risk factors associated with the occurrence of placenta previa .
The pathophysiology of placenta previa is not yet completely understood. The relationship between the placental edge and the internal orifice changes with advancing gestation.
In most of the cases, the placenta implanted lower in the early second trimester of pregnancy seems to move away from the internal orifice in the third trimester. It has been hypothesised that this placental ‘migration’ from the lower uterine segment towards the fundus occurs because the fundus, being more vascularised than the rest of the uterus, would allow a better development of the trophoblastic tissue. This phenomenon would occur because of a process of a degeneration of the trophoblast close to the internal orifice secondary to decreased vascularisation rather than a true migration of the placental tissue . Distortion of the normal anatomy of the lower uterine segment induced by a previous caesarean scar would prevent this ‘migration’ . Alternatively, defective decidual vascularisation and subsequent endometrial hypoxemia may increase the surface area of the placental tissue so that the feto-maternal exchange of the oxygen may not be compromised. In this scenario, the placenta may have a larger diameter, and its lower edge would be more likely to implant in close proximity to the cervix .
Ultrasound definition and screening for placenta previa
Placenta previa is commonly diagnosed using ultrasound. Placental location is usually reported during the routine anomaly scan. A follow-up scan in the third trimester should be scheduled when the placental edge is found to be reaching or overlapping the internal cervical orifice, to confirm this finding and to plan the management of delivery. A conclusive diagnosis of placenta previa is possible only in the third trimester of pregnancies, because almost 90% of the placentas defined as low in the second trimester move away out of the lower uterine segment later on in gestation ( Fig. 1 a and b).
Although placenta previa can be easily diagnosed trans-abdominally, trans-vaginal ultrasound has been shown to better delineate the relationship between the lower placental edge and the internal cervical orifice .
Some investigators have attempted to correlate the position of the placenta in the second trimester, as detected at the scan, with the likelihood of migration in the third trimester of pregnancy. Oppenheimer et al. studied 36 women with placental edge within 3 cm of the internal cervical orifice at 26 weeks of pregnancy or later. The mean rate of migration was 5.4 mm per week in women in whom placental edge migration did occur. If the placental edge overlapped the internal cervical orifice by more than 2 cm, migration was not observed in any woman. When the placental edge was more than 2 cm from the internal cervical orifice, migration always occurred, and none of the women required a caesarean section for placenta previa. If the distance of the placental edge was less than 2 cm from the internal cervical orifice, placental migration occurred in most (88.5%) cases.
Ghourab reported on the significance of the shape of the placental edge detected at 28–32 weeks in predicting placental migration. The placental edge was defined as ‘thick’ if the thickness was 1 cm or less, within 1 cm from the edge, the angle between the basal and the chorionic plate exceeded 45 0, or both. All other cases were defined as ‘thin’ placental edge. Placental migration was seen in 29.6% where the placental edge was thin, but only in 5.8% where it was thick. A significantly higher rate of ante-partum haemorrhage, abdominal delivery, adherent placenta, and low birth weight was found in cases where the placental edge was thick.
The above two studies show that, in some cases of mid-trimester low-lying placenta, the placental edge is more likely to ‘migrate’ than others are. Ultrasound may be useful in predicting both the likelihood and extent of placental migration as a function of time.
Mode of delivery
Caesarean section is the recommended mode of delivery for major placenta previa, whereas for minor previa an attempt at vaginal delivery is deemed appropriate. The introduction of ultrasound in clinical practice has raised the issue of which sonographic threshold of distance between the lower placental edge and the cervix should be used to achieve a safe vaginal delivery. Oppenheimer et al. found that the mean distance of the placental edge from the internal cervical orifice in women requiring a caesarean section for placenta previa was 1.1 cm (range, 0.0–2.0 cm).
In a study of 121 women with placenta previa, all women required a caesarean section when the placental edge was within 1 cm of the internal cervical orifice within 2 weeks of delivery. In contrast, if the placental edge to internal cervical orifice distance was 2 cm or more, the likelihood of achieving a vaginal delivery was at least 63% . According to these findings, the term placenta previa should only be used when the placental edge overlapped or was within 2 cm of the internal cervical orifice in late pregnancy. If the placental edge is located further than 2 cm, but within 3.5 cm from the internal cervical orifice, the placenta should be termed low-lying. In the latter case, although there is a good chance of a vaginal delivery, the incidence of post-partum haemorrhage remains high . Therefore, a low-lying placenta deserves an attempt at vaginal delivery, but should warn the clinician of the possibility of haemorrhagic complications, so that appropriate precautions can be taken.
Invasive placentation
Definition
Invasive placentation encompasses a spectrum of conditions characterised by an abnormal adherence of the placenta to the implantation site. Three major variants of adhesive placentation can be recognised according to the degrees of trophoblastic invasion through the myometrium and the uterine serosa: placenta accreta, increta, and percreta. Placenta accreta is defined as a placenta that strongly attaches to the myometrium, without penetrating it; the progressive penetration of the trophoblastic tissue through the myometrium defines the placenta increta, whereas the invasion of the uterine serosa and adjacent organs, such as the bladder, is known as placenta percreta .
Incidence
The incidence of invasive placentation has progressively risen in the past 3 decades, especially as the consequence of the increasing rates of caesarean delivery, which represents the major risk factor for this condition. Among the different types, placenta accreta probably represents the most common variant of abnormally adherent placenta. The incidence of invasive placentation is highly variable, and mostly dependent upon the population analysed and type of invasive placentation considered. A recent systematic review assessing the predictive accuracy of ultrasound in identifying invasive placentation in a population at risk for this condition, reported an incidence of 9%; however, when limiting the analysis to a sub-population of women with an anterior placenta previa diagnosed in the third trimester of pregnancy and a previous uterine surgery, the incidence increases to 19%.
Risk factors
A defective development of the decidua basalis allowing the invasion of the myometrium by the trophoblastic tissue constitutes the pathophysiological explanation for the occurrence of invasive placentation. This can occur as a consequence of a primary defect of the trophoblast function, leading to excessive invasion of the uterine myometrium or as an inability of the decidua to complete is development due to the presence of a uterine scar . Focal hypoxemia induced by an abnormal vascularisation in the area of the uterine scar, leading to both excessive trophoblastic invasion and defective decidualisation, may represent an alternative hypothesis for this phenomenon .
Placenta previa and previous uterine surgery, such as caesarean section, represent the most commonly reported risk factors associated with invasive placentation. In particular, the finding that most of the invasive placentas are implanted on the area of the previous uterine scar poses particular relevance on the role of a previous uterine surgery in determining this condition in women with placenta previa. The risk of invasive placentation seems to be related also to the number of caesarean sections; in a meta-analysis assessing the role of multiple caesarean deliveries on maternal morbidity, Marshall et al. found that the likelihood of invasive placentation in women with placenta previa progressively increases as the number of previous caesarean section increases, with an odd ratio of 29.8 in women who have undergone caesarean delivery . Advanced maternal age, multiparity, uterine fibroids, second-trimester serum levels of alpha-fetoprotein, and free beta-hCG greater than 2.5 multiples of the median, hypertensive disorders, and smoking represent other risk factors associated with invasive placentation .
Ultrasound signs
Prenatal diagnosis of invasive placentation is associated with a reduced risk of maternal complications, such as peripartum blood loss, need for transfusions, and rate of hysterectomies, as it allows a pre-planned treatment of the condition . Ultrasound is usually used as the primary modality for antenatal diagnosis of invasive placentation, and is carried out especially in the second, third trimester of pregnancy, or both; however, early diagnosis of invasive placenta during the first trimester has been reported . All women presenting with a history of previous caesarean delivery or uterine surgery and an anteriorly situated low-lying or placenta previa at the anomaly scan require a follow-up scan later on in the third trimester for the identification of invasive placentation.
Different sonographic signs have been described to be associated with an increased risk of invasive placentation; the most commonly reported are discussed below.
Placental lacunae
Placental lacunae are hypoechoic irregular vascular spaces within the placental parenchyma, usually showing turbulent flow within them and giving a ‘moth-eaten’ appearance to the placenta ( Fig. 2 a). Placenta lacunae are usually classified according to their number, dimension and shape . The pathophysiology of placenta lacunae is not clear, although their presence has been associated with an increased likelihood of invasive placentation.
