Molar pregnancy is part of the spectrum of diseases known as trophoblastic disease. A normal pregnancy consists of the fetus or developing baby and the placenta which is made of trophoblasts. A molar pregnancy (also called hydatidiform mole) is an abnormal overgrowth of the placenta caused by an imbalance in the number of chromosomes supplied from the mother and the father.
Molar pregnancies are uncommon, occurring in 1 out of every 600-1200 pregnancies. Risk factors for this condition include:
- Asian and Mexican ethnicity
- Teenage pregnancy
- Maternal age > 40 years
- History of two or more miscarriages
- Previous molar pregnancy.
Types of molar pregnancies
A molar pregnancy may be partial or complete depending on the genetic makeup of the fertilized egg:
- Complete hydatidiform moles (CHM) have only placental tissue that has the appearance of a cluster of grapes and no fetal parts are identified.
- Partial hydatidiform moles (PHM) have a fetus along with the abnormal placenta, often with sequelae of chromosomal problems and growth restriction.
What are the features suggestive of a molar pregnancy?
- Vaginal bleeding during pregnancy
- Nausea and vomiting, sometimes severe
- Development of high blood pressure early in the pregnancy
- Thyroid hormone abnormalities
- Ovarian cysts
In recent years, the occurrence of complications has reduced significantly owing to the early detection and treatment of this condition.
- On clinical examination, the uterine size may not correspond to the duration of the pregnancy. Fetal heart sounds will be absent in complete mole.
- Ultrasound examination reveals the presence of cystic spaces in the uterus.
- The blood levels of human chorionic gonadotropin (hCG) hormone are elevated.
- Sometimes, the diagnosis may be made in the pathology lab based on examination of a tissue sample after miscarriage.
How is a molar pregnancy treated?
The molar tissue has to be surgically removed by evacuation of the uterine contents under anaesthesia. The products of conception have to be sent to the pathology lab for examination in order to confirm the diagnosis. This has to be followed by surveillance of the blood levels of hCG (initially weekly and later monthly) for a period of 6 months or more.
In some (~10%) women, chemotherapy may be needed after surgical evacuation if the hCG levels plateau or rise during the follow-up, in order to destroy any remaining trophoblastic tissue. The number and type of drugs that are used depend on several factors such as age, type of pregnancy, blood levels of hCG before treatment, spread to other organs and how long it is since the pregnancy ended.
An effective method of contraception should be adopted and pregnancy avoided at all costs until the period of follow-up is completed. Intrauterine devices should not be inserted until the blood level of hCG is undetectable.
Persistent trophoblastic disease
In a small number of women (<20% with CHM and <5% with PHM), the levels of hCG may rise after surgical evacuation due to persisting molar tissue. This trophoblastic tissue can burrow deeply into the uterus (invasive mole) or rarely become cancerous and invade other parts of the body (choriocarcinoma).
Further investigations such as kidney function and liver function tests, and X-ray / CT scans in order to determine spread of the disease will be needed in these women. These conditions can be effectively treated with chemotherapy with an almost 100% cure rate.
The risk of a repeat molar pregnancy is around 1-2% which rises to almost 20% after two molar pregnancies. Repeat molar pregnancies may be due to a genetic problem. In future pregnancies, an early ultrasound to confirm or exclude molar pregnancy is advised. The blood level of hCG should also be monitored after subsequent pregnancies irrespective of the outcome.