ABNORMALLY INVASIVE PLACENTA

What is the 'abnormally invasive placenta' (AIP)?

The placenta develops along with the baby in the uterus (womb) during pregnancy. It connects the baby with the mother’s blood system and provides the baby with its source of oxygen and nourishment. The placenta is delivered after the baby, and is also called the afterbirth.

Rarely, placenta development may be complicated by a problem known as AIP or placenta accreta. This is when the placenta grows into the muscle of the uterus, making separation at the time of birth difficult. AIP is more commonly found in women with placenta praevia (placenta cover a part or all of the cervix) who have previously had a caesarean section.

Incidence

The incidence of placenta accreta has increased and seems to parallel the increasing cesarean delivery rate. Researchers have reported the incidence of placenta accreta as 1 in 533 pregnancies for the period of 1982–2002. This contrasts sharply with previous reports, which ranged from 1 in 4,027 pregnancies in the 1970s, increasing to 1 in 2,510 pregnancies in the 1980s.

Repeat Cesarean Delivery and Other Risk Factors

Women at greatest risk of placenta accreta are those who have myometrial damage caused by a previous cesarean delivery with either anterior or posterior placenta previa overlying the uterine scar. The authors of one study found that in the presence of a placenta previa, the risk of placenta accreta was 3% for the first, 11% for the second, 40% for the third, 61% for the fourth, and 67% for the fifth or greater repeat cesarean deliveries, respectively. Placenta previa without previous uterine surgery is associated with a 1–5% risk of placenta accreta. Risk factors include: advanced maternal age, multiparity, any condition resulting in myometrial tissue damage followed by a secondary collagen repair, such as previous myomectomy, endometrial defects due to vigorous curettage resulting in Asherman syndrome, submucous leiomyomas, thermal ablation, and uterine artery embolization.

What are the risks to me and my baby?

When there is an AIP, especially if the placenta is in the lower part of the womb, there is a risk that you may bleed in the second half of pregnancy. Bleeding can be heavy, therefore it is extremely important to let us know as soon as you see some blood and/ or experience abdominal pain. If the placenta is in the way of your baby being born, you will need a caesarean section.

AIP may be suspected in the antenatal period by an ultrasound scan, but while additional tests such as magnetic resonance imaging (MRI) scans may help with the diagnosis, we will only be able to tell for sure if you have this condition at the time of your caesarean section.

If AIP is suspected before your baby is born, we will discuss your options and the extra care that you will need at delivery. Delivery may be planned earlier, for example between 35 and 36 weeks, depending on individual circumstances. You may need to have your baby in a hospital, which has additional facilities such as interventional radiology available. We will discuss all the details with you.

AIP can also cause bleeding when an attempt is made to remove your placenta. The bleeding may be severe and you may require a hysterectomy (removal of the womb) to stop the bleeding. It may be possible to leave the placenta in place after birth, to allow it to absorb over a few weeks and months. Unfortunately this latter type of treatment is not always successful and some women will still need a hysterectomy. 

The Doctor is a member of the European Working Group on the Abnormally Invasive Placenta, a multidisciplinary task force of experts around Europe that strive toward a better understanding and management of AIP. As a member of this group, we can have direct access to super experts of the field to improve detection rate and ensure proper management of the condition.