During pregnancy, an exchange between mother and baby needs to occur in order to deliver essential oxygen and nutrients to the growing fetus.
The placenta is the organ that is the interface between maternal and fetal blood.
It acts as the baby’s lungs, digestive system and kidneys, as well as supplying vital pregnancy hormones, until birth occurs.
The placenta must act as this interface without actually invading the mother’s uterine muscle.
When pregnancy occurs, the lining of a woman’s uterus becomes the decidua. One of the functions of the decidua is to prevent the placenta from invading the uterus.
If the decidua is too thin (deficient) it can allow the placenta to embed too deeply.
If this occurs, the placenta can’t detach from the uterine wall after the birth of the baby. This is called placenta accreta, and it's reported to occur in about 1 in 7000 pregnancies.
What Is Placenta Accreta?
The placenta normally attaches to the uterine wall by projections called chorionic villi. How severe placenta accreta is depends on how deeply the chorionic villi have penetrated.
- Placenta accreta: the chorionic villi attaches to the muscle of the uterine wall and occurs in about 75% of all cases of placenta accreta
- Placenta increta: the chorionic villi extends into the muscles of the uterus and occurs in approximately 15% of cases
- Placenta percreta: the chorionic villi extends through the entire wall of the uterus and often into nearby organs. This is the deepest form of attachment, occurring in approximately 7% of cases.
Most medical professionals use the term placenta accreta when referring to any of the three variations.
After the birth of your baby, high levels of oxytocin normally trigger the uterus to contract down and the placenta to shear off the uterine wall. If the placenta is too deeply embedded, it can’t detach or may only partially detach. The problem occurs when the maternal blood vessels that are open can’t be shut off. Massive bleeding will occur until the placenta is removed.
What Are The Risk Factors For Placenta Accreta?
Placenta accreta is more likely to occur in a woman who also has placenta previa. Placenta previa is when the placenta attaches low in the uterus and grows over the cervix.
Placenta previa is linked to previous uterine surgery, most notably c-sections.
If you've had a previous c-section and you have placenta previa in your current pregnancy, the risk of placenta accreta is 25%.
If you've had two or more c-sections and currently have placenta previa, the risk increases to 40%.
Placenta accreta without placenta previa is rare, but other risk factors that increase your risk for placenta accreta are:
- Your placenta has attached over a uterine scar
- You've had a dilation and curettage (including after a termination) involving your uterus lining being scraped
- You are over the age of 35
- Increased number of pregnancies beyond 20 weeks
- Maternal smoking
- Unexplained elevated maternal serum alpha-fetoprotein (protein found in the blood)
- Uterine fibroids
- Asherman’s syndrome (causes uterine scar tissue or adhesions to form).
Placenta Accreta Symptoms
If you have risk factors for placenta accreta, it's likely your doctor will make sure you have a thorough ultrasound to check for the placenta's position – especially if you've had a previous c-section.
Often the most consistent signs of a placental problem during pregnancy is vaginal bleeding that occurs into the second trimester and beyond. However, bleeding doesn't always occur, and the first sign of placenta accreta can happen during birth.
If you do have risk factors for placenta accreta, you may choose to have screening done:
- Ultrasound from 15 weeks of pregnancy can identify placenta accreta, although the best results are around 20 weeks, as the ultrasound is more sensitive at this time. The placenta will have a ‘swiss cheese’ appearance if placenta accreta has occurred.
- Blood testing for maternal serum alpha-fetoprotein (MSAFP) levels. This is a protein found in the blood, at highest concentrations in the baby. Due to the decidua being thin, this allows leakage of fetal alpha-fetoprotein into the mother’s blood. If these levels are raised with no other obvious cause, placenta accreta is the likely diagnosis.
Treatment For Placenta Accreta
The major risk factor for placenta accreta is massive, life-threatening blood loss, because the uterus can’t contract down and shut off the blood vessels. More than half of all women will need blood transfusions if placenta accreta occurs during birth.
If it does occur during birth, there are three treatment options available:
- An emergency hysterectomy (the most likely outcome)
- Very rarely would the placenta be forcibly removed, as this increases the risk of massive bleeding and subsequent hysterectomy
- Conservative management, which leaves the placenta in place. Drugs are used to contract the uterus and prevent infection, as well as blocking certain arteries to minimise bleeding. The placenta is reabsorbed by the body — studies have shown it's successful in about 80% of cases. It's considered a very controversial option due to the risk of bleeding and infection.
If placenta accreta is diagnosed before the birth, then management can be carefully planned. The usual outcome is an elective c-section at around 38 weeks of pregnancy. The risk of complications from placenta accreta are lessened if the surgery is planned rather than performed in an emergency.
At the time of the c-section, the uterus will be surgically removed (hysterectomy). Counselling and planning is an important part of the diagnosis of placenta accreta during pregnancy.
While placenta accreta can occur in the absence of uterine scarring, it's most likely to happen after a previous c-section. Research has shown between 60-80% of placental accretes occur in women who had a previous c-section.
While women are also experiencing higher numbers of other risk factors – such as increased maternal age, more likelihood of having uterine procedures for fibroids – the ever increasing rate of c-sections are having an impact. Avoiding c-section in the first instance would increase your chances of avoiding this serious, life-threatening complication in future pregnancies.