Placenta Praevia and the Placenta Accreta Spectrum

Placenta Praevia

The term placenta praevia refers to implantation of the placenta in the lower uterine segment that overlies or abuts the internal os ( Fig. 28.1 ). The incidence of placenta praevia is 4 per 1000 deliveries, with evidence of geographical variation. The aetiology of placenta praevia is poorly understood but risk factors include advancing maternal age, smoking, multiple pregnancies, assisted reproduction technologies and any form of uterine trauma such as previous caesarean section, intrauterine infections, abortions or curettage ( Table 28.1 ).

As the lower uterine segment develops and grows in the third trimester, painless antepartum haemorrhage (APH) may develop, which is most commonly caused by disruption of small uteroplacental veins as the anchored placenta is gradually stretched. Recurrent episodes of bleeding may generate thrombin and cause extravasation of blood into the myometrium resulting in uterine irritability, which may stimulate uterine contractions and ultimately result in a placental abruption in a small number of cases. The use of tocolytics to treat ongoing APH in placenta praevia is no longer advised. Several studies have failed to support the use of tocolysis in this setting and a recent placebo-controlled trial demonstrated an increased risk of emergency caesarean section for haemorrhage in women who received maintenance nifedipine, particularly if the placental edge was less than 2 cm from the internal os.

Diagnosis

The most common presentation of placenta praevia is a painless, self-limiting APH in the third trimester, with no evidence of fetal distress. Hence placental localization should be confirmed prior to performing a vaginal examination. However, up to 20% of major placenta praevias will not bleed and one should retain a high index of suspicion, particularly in a multiparous woman who presents with a malpresentation or transverse lie after 34 weeks’ gestation. Transvaginal ultrasound is the most accurate approach for diagnosis of placenta praevia; however, routine assessment of cervical length to predict timing of delivery is no longer recommended.

Antepartum Management

Caesarean Section for Uncomplicated Placenta Praevia

This can usually be performed via a transverse skin incision under neuraxial analgesia. Preparation is key, as up to 22% of women will have a postpartum haemorrhage (PPH). Adequate vascular access is vital. The key surgical manoeuvre is to adequately mobilize the bladder before delivery of the fetus to allow access to the entire lower segment and to perform haemostatic measures as outlined below. Mobilization of the bladder flap may identify significant aberrant vessels which can be ligated using individual sutures or a bipolar energy device such as a Ligasure. Generally, monopolar diathermy is not sufficient to control these larger vessels.

Although most obstetricians can quickly cut through an underlying placenta, use of modified hysterotomy (Ward procedure) reduces blood loss from placenta praevia. The myometrium is incised down to the placenta, the operator’s hand is inserted between the myometrium and the placenta, and the membranes are ruptured to deliver the baby through the uterine incision. After delivery, most of the placenta may still be attached to the lower segment, and best practice is to allow spontaneous detachment of the placenta. Manual removal of the placenta should not be attempted if there is a suspicion of placenta accreta syndrome, as this may lead to catastrophic bleeding. The most common cause of significant PPH after elective delivery is atony of the lower uterine segment. Some important considerations include:

• Rapid closure of the hysterotomy wound to allow the uterus to contract – both angles should be secured individually.

• Exteriorizing the uterus to enable adequate bimanual compression.

• Liberal use of uterotonics including ergometrine (if not contraindicated), oxytocin, misoprostol and prostaglandin F 2 alpha (PGF _2α ).

• Administration of tranexamic acid.

Additional haemostatic measures, which will often be based on available skills and equipment, may be necessary as significant haemorrhage can occur.

Simple and inexpensive techniques include bimanual compression of the lower uterine segment or placing a tourniquet around the lower segment, which can only be placed if the bladder was mobilized prior to delivery. In an unstable patient, manual aortic compression at the level of the sacral promontory will aid to arrest bleeding in an emergency. Furthermore, application of an Eschmarch’s compression bandage around the uterus can help arrest bleeding in the hypovolaemic coagulopathic patient and aids reinfusion of blood accumulated inside the uterus (intrauterine lakes). Uterine wrapping with two rounds of elastic bandage from the uterine fundus towards the cervix reduces the uterine axial volume by 50% as a result of the high band pressure. When haemodynamic and haemostatic parameters are stable, the elastic uterine wrap can be removed and a Bakri balloon in addition to a B-Lynch technique (sandwich technique) may compress the lower uterus to achieve haemostasis.

Peripartum hysterectomy may be necessary, as outlined in Chapter 27 . Treatment of hypovolaemia and coagulopathy is outlined in Chapter 29 . It is important to allow time for these measures to take effect prior to proceeding to hysterectomy. Primary haemostasis can be achieved via aortic compression if necessary, which can allow replacement of fluids, blood and clotting factors, administration of uterotonics and allow time for additional surgical help to arrive if necessary.

Vaginal Delivery in Placenta Praevia

Although caesarean delivery is universally accepted as the optimal approach for major placenta praevia, there are occasional clinical scenarios where vaginal delivery may be warranted, particularly in the setting of major fetal abnormality or intrauterine death, usually after 24 weeks. In this situation, uterine artery embolization may be considered a safe option to reduce the risk of massive maternal haemorrhage, as outlined in Chapter 38 . In this situation, use of endovascular hemostasis by branches of iliac internal arteries may be a safe option to reduce the risk of massive maternal haemorrhage.

If the placenta is 2 cm or greater from the internal os, a trial of labour is appropriate and the risk of haemorrhage is acceptable. If there is less than 1 cm between the edge of the placenta and the internal os, the risk of significant haemorrhage is extremely high and an elective caesarean delivery is the safest option. The optimal management of patients with a placenta between 1 and 2 cm from the os is uncertain. However, small series suggest that it may be safe to consider vaginal delivery, with success ranging between 76% and 93%. The rate of PPH is increased in this population and these women should only be managed in a unit with adequate expertise, access to blood products and ability to perform an emergency caesarean section. Active management of the third stage is advised and prophylactic measures as outlined in Chapter 34 should be considered.

It is reasonable to individualize the management of these women. Induction of labour in this group has not been studied in any detail. Awaiting spontaneous labour may be the safest approach as the lower segment continues to mature in the third trimester and may allow further placental migration. In a woman with a singleton cephalic term pregnancy who requires induction of labour, an amniotomy may be performed once adequate vascular access, blood products and maternal and fetal monitoring are in place.

A sterile finger is inserted vaginally to palpate the vaginal fornices and determine if thick placental tissue is present between the lower uterine segment and the fetal head. If the fetal head is easily palpated through a thin lower uterine segment it is then deemed safe to advance the examining one or two fingers through the cervix, to explore for any intervening placental tissue. Blood clot and placental tissue may be difficult to distinguish, though placental tissue is firm and may have a gritty feel. If no placenta is found upon digital exploration, then labour can be safely induced with amniotomy and an oxytocin infusion.