A molar pregnancy is a complication of pregnancy, also known as a hydatidiform mole. It is a gestational trophoblastic disease characterized by the abnormal growth of trophoblast cells – the cells that normally develop into the placenta. It is a relatively rare pregnancy complication and occurs in 1 in 1.000 pregnancies. The incidence is more likely in women younger than 20 or older than 35.

What causes a molar pregnancy?

Male and female gametes normally contain 23 pairs of chromosomes. When the gametes unite to form an embryo, a complete set of 46 chromosomes develops – one chromosome in each pair comes from the father and the other from the mother. 

A molar pregnancy is caused by an abnormally fertilized egg. We distinguish perfect and imperfect or partial molar pregnancy. 

Complete molar pregnancy

A complete molar pregnancy results in the fertilization of an abnormal or “empty” oocytes . This is what we call an egg cell that has lost its nucleus or the nucleus. Such an egg can be fertilized by one (90% of cases) or two sperm (10% of cases), and all the genetic material comes from the father. Chromosomes from the mother’s egg are lost or inactivated, and the father’s chromosomes are duplicated. The genotype is thus usually diploid (46, XX or 46, XY).

In a complete molar pregnancy, the trophoblastic tissue (placental tissue) is abnormal, swollen, and appears to form fluid-filled cysts. Fetal tissue is absent. 

Complete molar pregnancy has a 2–4% risk of developing choriocarcinoma and accounts for 50% of all choriocarcinoma cases.

Partial molar pregnancy

In a partial or incomplete molar pregnancy, a normal ovum is fertilized by one or two sperm, which then divides. The mother’s chromosomes remain and the father provides two sets of chromosomes. As a result, the embryo has 69 ( triploid genotype) or even 92 (tetraploid genotype) chromosomes instead of 46. It happens when two or more sperm fertilize an egg, resulting in an extra copies of the father’s genetic material.

A partial molar pregnancy involves a combination of normal and abnormal trophoblastic tissue (placental tissue). The fetus may develop, but the fetus cannot survive in such a pregnancy.

Symptoms and diagnosis

In the past, most cases of molar pregnancy were only discovered after miscarriage. However, with the development of high-resolution ultrasound devices, most molar pregnancies are now diagnosed in the first trimester of pregnancy.

Women with a molar pregnancy will initially have a positive pregnancy test and normal, typical symptoms of early pregnancy. 

Over time, however, signs and symptoms of molar pregnancy appear, which may include:

• faster, too fast or abnormal growth of the uterus 
• vaginal bleeding or dark vaginal discharge in early pregnancy
• pressure, pelvic pain
• more severe nausea and vomiting
• development of anemia due to vaginal bleeding
• discharge of tissue from the vagina (when bleeding)
• the absence of a fetus or absence of fetal movements and/or heartbeat
• preeclampsia – a pregnancy complication that causes high blood pressure and swelling of the feet, ankles and legs after 20 weeks of pregnancy
• hyperthyroidism – overproduction of thyroid hormones, which can lead to a fast heart rate, high blood pressure, weight loss and increased appetite

A molar pregnancy is usually diagnosed with a combination of ultrasound and blood tests. A complete molar pregnancy is easier to diagnose because the bulk of the abnormal trophoblastic tissue makes it easier to detect with ultrasound. Also, β-hCG levels in the blood are often abnormally high (even over 100,000 IU/L), which is not the case for partial molar pregnancy, where the values ​​are often within the normal range.

Treatment

Molar pregnancy can lead to serious complications (including a rare form of cancer) and requires early treatment.

When a molar pregnancy is diagnosed (before a miscarriage), due to the high risk of developing complications in the case of continued pregnancy, a surgical procedure called dilation and curettage (D&C) is most often recommended. A D&C can also be performed using vacuum aspiration (suction) to remove molar tissue. It is a less demanding, shorter procedure that is performed under general anesthesia.

In some cases, a woman may be given a medication that causes the uterus to contract to eliminate the pregnancy (misoprostol, mifepristone), but still require a D&C afterward to remove the remaining abnormal tissue.

Women who no longer want children, can consider a hysterectomy, as this completely eliminates the possibility of developing cancer.

Follow-up activities

Treatment is usually successful. However, because there is a small risk that the abnormal molar cells that remain in the uterus continue to grow (gestational trophoblastic neoplasia – GTN) or develop into cancerous tissue, follow-up is usually required for a period of 2 to 12 months . 

During this time, the drop in β-hCG levels in the blood is mainly monitored. The absence of a drop or even an increase in the value may indicate that not all of the molar tissue has been removed from the uterus. It is not recommended to become pregnant again during the follow-up period.