Abnormal placentation refers to situations in which the placenta is either abnormally situated or abnormally invasive, both of which are conditions associated with an increased risk of haemorrhage and subsequent maternal and neonatal morbidity and mortality.
The placenta usually implants in the fundus of the uterus. Placenta praevia is the term used to describe an abnormally low placenta, implanted in the lower uterine segment. The previously widely used classification of grade 1 (placenta low-lying), grade 2 (reaches the os), grade 3 (asymmetrically covers the os) or grade 4 (symmetrically covers the os) has been replaced with a simpler division into minor (leading edge of the placenta is in the lower uterine segment, but not covering the cervical os) or major (placenta lies over the os). This classification may be further subdivided into anterior or posterior.
A low-lying placenta is noted in about 5% of early ultrasound scans, but most of these have moved into the fundus by the third trimester, and this finding is thus only regarded as significant after 27 weeks’ gestation. The incidence at term is ~1 in 200-250. It occurs more frequently in mothers who have previously delivered by caesarean section (incidence is ~10% after four caesarean sections), and is also associated with increased parity, increasing maternal age and multiple gestation.
In addition to the above, the placenta may be morbidly adherent. The incidence is ~1 in 2500 overall, increasing to ~5-9% in placenta praevia, 25-30% in placenta praevia with previous caesarean section, and 40-50% in placenta praevia with two previous caesarean sections. The terms placenta accreta, increta and percreta are used to describe placental penetration through the uterine decidua, into the myometrium, and through the myometrium’s serosa, respectively.
Vasa praevia refers to the presence of fetal vessels lying within the membranes over the os, unprotected by placental tissue or the umbilical cord, associated either with a velamentous cord insertion or a multi-lobed placenta. It occurs in 1 in 2000-6000 pregnancies.
Problems and special considerations
There are two main problems caused by abnormal placentation:
1. The physical presence of placental tissue and vessels low down in the uterus, and in major placenta praevia or vasa praevia between the presenting part of the baby and the os, means that compression or rupture of vessels may occur during pregnancy as well as during labour and/or vaginal delivery (in caesarean section, placental tissue may be encountered before the baby can be reached). Thus there is a risk of fetal compromise and severe haemorrhage during pregnancy, labour and/or delivery.
2. The relative lack of muscle in the uterine lower segment compared with the upper segment results in less effective haemostasis when the uterus contracts after delivery; this is compounded by the presence of the placental bed itself and any penetration of the myometrium by the placenta (i.e. accreta/increta/percreta). The incidence of postpartum haemorrhage is therefore also increased.
Diagnosis
Antenatally, transabdominal ultrasound may suggest a low-lying placenta, and its position may be more accurately identified by transvaginal ultrasound. High-resolution ultrasound, magnetic resonance imaging and colour flow Doppler imaging may define the degree of invasion of the placenta, although none is totally accurate. The mother who presents with late bleeding should undergo urgent ultrasonography to determine the position of the placenta. The differential diagnosis is of placental abruption, in which bleeding is normally accompanied by abdominal pain and tenderness (see Chapter 72, Placental abruption). If there is uncertainty as to whether vaginal delivery is possible, then an examination in theatre may be performed with a view to proceeding to immediate caesarean section if necessary.
Presentation
Placenta praevia usually presents as painless bleeding, with the first bleed commonly occurring at 27-32 weeks’ gestation. Occasionally, bleeding may not be apparent until the mother goes into labour, which is more likely to be preterm. If there has been recurrent bleeding, the mother is usually kept in hospital, with cross-matched blood continuously available. Vasa praevia may present as abrupt onset of bleeding with rupture of the membranes and, since blood loss is entirely fetal, it is associated with a high perinatal mortality.
Mode of delivery
Although lesser degrees of placenta praevia, where the placenta does not encroach on the os, may be managed conservatively, caesarean section is the normal method of delivery. Timing of delivery will depend on the individual circumstances, including antenatal symptoms, maternal and fetal condition, risk of preterm delivery and imaging results. When the mother is actively bleeding, emergency caesarean section and delivery of the placenta may be essential to preserve the life of the mother and the baby. Placenta praevia may interfere with the development of the usually thin lower uterine segment and thus increase blood loss. Occasionally it may be necessary for the obstetrician to divide an anterior placenta praevia in order to gain access to the fetus, and this is usually accompanied by very heavy blood loss. If the placenta is morbidly adherent, placental separation may be difficult or even impossible to achieve, and torrential haemorrhage may occur, which can only be controlled by removing the uterus.
Management options
Immediate resuscitation
Management of the bleeding mother should follow basic principles of resuscitation. Two large-bore peripheral cannulae should be inserted and blood taken for haemoglobin estimation and emergency cross-match. The possibility of disseminated intravascular coagulation should be borne in mind if blood loss is very heavy, and coagulation factors should be replaced (fresh frozen plasma, cryoprecipitate, platelets) according to local haematological guidelines for massive transfusion.
Caesarean section
The presence of one or more consultant anaesthetist(s) and obstetrician(s) is mandatory for caesarean section with placenta praevia; other specialists (e.g. vascular surgeon, urologist) may be required for extreme cases of placenta percreta. Preparations should be made for major blood loss; this might include cell salvage, especially in elective cases.
Placenta praevia has commonly been regarded as an indication for general anaesthesia, because of the risk of heavy, uncontrolled bleeding. Regional anaesthesia has traditionally been contraindicated because of the perceived risk of vasodilating the patient who is, or is about to become, hypovolaemic. However, in recent years, the use of regional anaesthesia in these circumstances has become more acceptable, and many senior anaesthetists would consider a regional technique for caesarean delivery, depending on each individual situation. In addition to the well-known advantages offered by the use of regional anaesthesia, including reduced blood loss, the avoidance of general anaesthesia in this situation enables clinical monitoring of the mother’s cerebral function, which is an indication of the adequacy of perfusion. Points that would tend to favour a regional anaesthetic would be a posterior placenta that will not interfere with delivery (although bleeding from a posterior placental bed may be more difficult to control), no or little active bleeding, prior cardiovascular stability, and a low risk of placenta accreta (no previous sections). However, the mother and her partner should be warned about the potential for major haemorrhage and the increased risk of conversion to general anaesthesia. The patient who is bleeding heavily, or who has an anterior placenta or a history of previous caesarean section, may be best managed with general anaesthesia.
Interventional radiology is recommended by the Royal College of Obstetricians and Gynaecologists as an adjuvant in the prophylactic management of placenta praevia and accreta. In the elective situation, balloon catheters may be inserted prophylactically in the internal iliac/uterine arteries (or even aorta) via the femoral arteries before caesarean section, with a view to inflating the balloons should uncontrolled haemorrhage occur. Risks of catheter placement include lower limb ischaemia and fetal bradycardia necessitating emergency delivery.
An alternative is to place femoral artery sheaths through which balloon or embolisation catheters can be passed should they be required subsequently. In the emergency situation, embolisation procedures may be performed after angiography to identify the bleeding vessels, and this may be life-saving. The procedure is not without risks - not least those associated with transferring a potentially unstable patient to the radiology suite. In some facilities, elective caesarean sections may be performed in adequately equipped radiology suites.
Occasionally, when there are signs of acute placental insufficiency, the risks to the fetus of waiting for cross-matched blood must be balanced against the risk to the mother of proceeding without it; these are decisions that must be taken coolly and rationally, with full consultation between the parties.
Key points
• The chances of placenta accreta increase with the number of previous caesarean sections.
• The risk of massive haemorrhage should be assessed when choosing an anaesthetic technique.
• Successful management of the parturient with an abnormally implanted placenta relies on advance planning and interdisciplinary communication, with the involvement of senior clinicians.
Further reading
Fitzpatrick KE, Sellers S, Spark P, et al. The management and outcomes of placenta accreta, increta, and percreta in the UK: a population-based descriptive study. BJOG 2014; 121: 62-70.
Jauniaux E, Alfirevic Z, Bhide AG, et al.; Royal College of Obstetricians and Gynaecologists. Placenta praevia and placenta accreta: diagnosis and management. Green-top Guideline 27a. BJOG 2019; 126: e12-48.
Jauniaux E, Alfirevic Z, Bhide AG, et al.; Royal College of Obstetricians and Gynaecologists. Vasa praevia: diagnosis and management. Green-top Guideline 27b. BJOG 2019; 126: e49-61.
Lilker SJ, Meyer RA, Downey KN, Macarthur AJ. Anesthetic considerations for placenta accreta.
Int J Obstet Anesth 2011; 20: 288-92.
Snegovskikh D, Clebone A, Norwitz E. Anesthetic management of patients with placenta accreta and resuscitation strategies for associated massive hemorrhage. Curr Opin Anaesthesiol 2011; 24: 274-81.
Taylor NJ, Russell R. Anaesthesia for abnormally invasive placenta: a single-institution case series. Int J Obstet Anesth 2017; 30: 10-15.
Vinas MT, Chandraharan E, Moneta MV, Belli AM. The role of interventional radiology in reducing haemorrhage and hysterectomy following caesarean section for morbidly adherent placenta. Clin Radiol 2014: 69: e345-51.