- What is Placenta Accreta?
- Risk Factors
- Clinical Examination
- How is it Diagnosed
What is Placenta Accreta?
Placenta accreta is a potentially life threatening obstetric condition in which the placenta is abnormally attached to the uterus. This can lead to massive blood loss during or following delivery. This is thought to result from a defect in the layer of tissue that would normally separate the placenta from the uterus (deciduas basalis). As a result, projections of the placenta (chorionic villi) are able to invade the uterus to varying degrees.
While placenta accreta is often used as a general term to describe this condition, more specifically it refers to the presence of chorionic villi on the inner (myometrial) surface of the uterus. Where the chorionic villi invade the deeper muscular layer (the myometrium), this is termed placenta increta. Where infiltration occurs through the myometrium with or without infiltration of adjacent structures including the bladder this is termed placenta percreta.
Although the condition was quite rare 30 years ago, the incidence of placenta accreta has been steadily increasing, mirroring increased rates of caesarean section delivery, which is one of the principle risk factors for placenta accreta.
Currently, placenta accreta occurs in approximately 1 in 2,500 deliveries.
Placenta accreta most commonly occurs in the setting of placenta praevia with previous uterine surgery including caesarean section. In women with a previous caesarean section and current placenta praevia, the risk of placenta accreta is 25%. For women with two or more previous caesarean sections, this risk rises to 40%.
Other predisposing factors have been identified including:
- Advanced maternal age
- Increased parity (number of pregnancies beyond 20 weeks gestation)
- Unexplained elevated maternal serum alpha-fetoprotein
- Uterine myomas
- Asherman’s syndrome
During the pregnancy, the placental tissue can invade the different layers of the uterus to varying degrees. The main concern is that following delivery of the baby the placenta will fail to separate from the uterus with the potential for massive, life threatening blood loss.
Placenta accreta has the potential to cause massive, life threatening blood loss. It is for this reason that hysterectomy is the treatment of choice. Other treatment options, discussed in more detail below, include forced manual removal of the placenta but this is almost universally discouraged due to the potential for massive blood loss. The other treatment option involves taking a conservative approach whereby the placenta is left in position for variable lengths of time following delivery to allow it to resorb naturally. The latter has the benefit of preserving the woman’s fertility, but there still remains the possibility of massive blood loss and it also introduces the possibility of severe infection.
There is very little for a doctor to find on clinical examination of a woman with placenta accreta.
Instead the doctor will be interested in your previous obstetric history including details about previous caesarean sections/uterine surgery and the location of the placenta in the current pregnancy if known. This information is especially important given that placenta accreta most frequently occurs in women with a current placenta praevia and a history of uterine surgery.
How is it Diagnosed
Ultrasound is the most commonly used imaging method for the early diagnosis of placenta accreta. The most common feature on ultrasound that would suggest placenta accreta is a “moth-eaten” or “swiss cheese” appearance to the placenta. Ultrasound can be used from 15 weeks gestation, although it is more sensitive at gestations of 20 weeks or more.
Magnetic resonance imaging
MRI can be used to diagnose placenta accreta. However, in certain cases where the placenta accreta is located posteriorly (towards the back), it is often no better than ultrasound. Furthermore, MRI is costly and not readily accessible.
Maternal serum alpha fetoprotein
Placenta accreta should also be suspected in pregnant women with elevated maternal serum alpha-fetoprotein (MSAFP) levels, with no other obvious cause. This is a protein found in the blood, at highest concentrations in the foetus. The defect in the layer normally separating the placenta and uterus allows leakage of foetal alpha-fetoprotein into the mother’s circulation. Up to 45% of women with placenta accreta have elevated MSAFP levels in the absence of an obvious cause.
The probable outcomes of placenta accreta are largely dependant upon when it is diagnosed (during pregnancy vs. at delivery) and how it is treated. If diagnosed antenatally (during the pregnancy) this allows time for effective management planning and counselling. Currently, most women would undergo an elective caesarean section at approximately 38 weeks gestation to deliver the baby, followed by hysterectomy (surgical removal of the uterus). Indeed, placenta accreta is the most common reason for emergency hysterectomy during or following delivery.
Some of the complications that accompany hysterectomy include the need for blood transfusion, infection, injury to nearby tissues/organs and the formation of fistulas. As hysterectomy involves the surgical removal of the uterus, it means that the woman would no longer be able to bear children.
The rate of complications is minimised by performing the surgery as an elective procedure rather than in an emergency situation. Where the diagnosis is made during the pregnancy, planning the delivery at a suitable hospital with appropriate facilities such as obstetric anaesthesia, appropriate surgical expertise and an intensive care unit also helps to minimise adverse outcomes.
The first step in treatment of placenta accreta is recognition of risk factors. This allows further investigations such as ultrasound and/or MRI to be performed. Once an accurate diagnosis is made, this facilitates maternal counselling and effective management planning of the delivery to ensure safety.
Maternal counselling should include discussion pertaining to the choice and timing of an elective delivery, whether to make an attempt to separate the placenta at the time of delivery, the use of radiological interventions in an attempt to minimise blood loss and the use of particular drugs post-delivery.
Delivery of a woman with a diagnosis or suspicion of placenta accreta should take place in a hospital with adequate facilities for high volume blood transfusion, interventional radiology and specialist obstetric, anaesthetic and intensive care. Prior consultation with urology (specialist of the urinary system) and gynaecology oncology specialists (specialists of gynaecological cancer) may be appropriate and these specialists should be close at hand. Women with placenta accreta should reside close to the designated hospital throughout the third trimester of pregnancy in case of spontaneous labour, antepartum haemorrhage or other complications.
Effectively there are three treatment options including:
- Total abdominal hysterectomy – generally accepted as the treatment of choice;
- Forced manual removal – almost universally avoided due to the risk of massive haemorrhage and subsequent hysterectomy; and
- Conservative management – controversial as although it preserves future fertility, it exposes the woman to life threatening intra-abdominal infection and major bleeding.
Total abdominal hysterectomy is generally considered the treatment of choice for placenta accreta. This involves scheduled delivery of the foetus by caesarean section at 36-37 weeks provided there is foetal lung maturity. Once the foetus is delivered, no attempt is made to remove the placenta. The edges of the uterine incision are oversewn and hysterectomy performed. This will be performed by an experienced obstetric surgeon with a skilled obstetric anaesthetist. Urology and gynaecology oncology specialists should be close at hand.
Plans are made for emergency delivery should the women experience antepartum haemorrhage or go into spontaneous labour. However, emergency hysterectomy is associated with increased intraoperative blood losses and greater rates of blood transfusion.
Conservative management of placenta accreta involves leaving the placenta in place without a forcible attempt to remove it. Within the scope of conservative management are the use of drugs to contract the uterus and to prevent infection (antibiotics), as well as procedures including pelvic arterial embolisation, where selected arteries that feed blood into the uterus are blocked in an attempt to minimise bleeding.
In a study of 167 women from 25 University hospitals in France, conservative treatment was successful in 78.4% of cases. Spontaneous placental resorption occurred in 75% of women at a 13.5 week follow up. Surgical techniques were used to remove the retained placenta in 25% of women at a median follow up of 20 weeks. Just over 10% of women had hysterectomies performed within the first 24 hours following delivery and a further 10.8% had delayed hysterectomies.
Conservative management of placenta accreta remains highly controversial as although this technique preserves future fertility and avoids the complications of caesarean hysterectomy, it exposes the woman to potentially life threatening infection and massive haemorrhage.
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