Placenta accreta

Overview
Placenta accreta is a severe obstetric complication involving an abnormal superficial attachment of the placenta to the myometrium (the middle layer of the uterine wall). There are three forms of placenta accreta, distinguishable by the depth of penetration.

The placenta usually detaches from the uterine wall relatively easily, but women that encounter placenta accreta during childbirth are at great risk of haemorrhage during its removal. This commonly requires surgery to stem the bleeding and fully remove the placenta, and in severe forms can often lead to a hysterectomy or be fatal.

Placenta accreta affects approximately 1 in 2,500 pregnancies.

Variants
The most common form of placenta accreta is an invasion of the myometrium which does not penetrate the entire thickness of the muscle. This form of the condition accounts for around 75-78% of all cases, and has no name other than placenta accreta.

There are two further variants of the condition that are known by specific names and are defined by the depth of their attachment to uterine wall. Placenta increta occurs when the placenta further extends into the myometrium and happens in around 17% of all cases. Placenta percreta, the worst form of the condition and occurring in 5-7% of cases, is when the placenta penetrates the entire myometrium to the uterine serosa (invades through entire uterine wall). This variant can lead to the placenta attaching to other organs such as the rectum or bladder.

Diagnosis
Placenta accreta is very rarely recognised before birth, and is very difficult to diagnose. While it can lead to some vaginal bleeding during the third trimester, this is more commonly associated with the factors leading to the condition. In some cases the second trimester can see elevated maternal serum alpha-fetoprotein levels, though this is also an indicator of many other conditions.

Risk factors
The condition affects around 10% of cases of placenta praevia, and is increased in incidence by the presence of scar tissue from a past caesarean section. A thin decidua can also be a contributing factor to such trophoblastic invasion. Some studies suggest that the rate of incidence is higher when the fetus is female.