A molar pregnancy is an abnormal form of pregnancy in which the uterus is filled with a mass of vesicles (small whitish cysts appearing like a huge cluster of pale grapes) rather than a fetus and placenta. This tissue is called a hydatidiform mole.
Each human cell normally contains 46 chromosomes in two sets of 23 each. The reproductive cells (the sperms in men, and the ova in females) contain only 23, however, because of the separation of the chromosomes in each pair during the formation of these specialized cells.
During conception, the two sets of chromosomes inside the ovum and the sperm fuse inside the ovum cell to form a new cell - the zygote - which now contains 46 chromosomes again. However, one set comes from the father through the sperm, and the other from the mother through the ovum - the paternal and maternal chromosomes, respectively.
Following the fusion of the sperm and the ovum, which is called fertilization, some of the cells form a nutritive structure called the trophoblast. This is destined to form the fetal membranes and the placenta, which nourish the developing baby and implant the fetus into the wall of the uterus.
A hydatidiform mole or vesicular mole results from an abnormality in the chromosomes that fuse to form the zygote, the cell that divides to form an embryo and later a fetus. When the fertilized ovum is empty (lacks any chromosomes) it normally dies, but sometimes develops to form a molar pregnancy. Because of the lack of maternal DNA, there is no sign of fetal development. This condition is called a complete mole.
Another mechanism is when two sperms fertilize the same ovum. This forms a cell with three copies of each chromosome, two paternal and one maternal. This leads to the presence of too much genetic material. The trophoblastic growth becomes excessive, and fetal growth is always restricted and abnormal. This is called a partial mole.
Both forms of molar pregnancy fail to produce a live viable baby because of the chromosomal anomaly. They are different forms of gestational trophoblastic disease (GTD), both benign. Rarely, they may give rise to the malignant form of GTD, called a choriocarcinoma.
The prevalence of molar pregnancy varies from place to place, being 1-3 per 1000 in developed countries. Its incidence is higher in the following conditions:
- Women younger than 16 or above 45 years have a higher risk of molar, but not partial molar, pregnancies
- Women who have already had one or more molar pregnancies: the incidence increases to 1-2 per 100 after one, and 15-20% after more than one molar pregnancy
- Women of Asian origin
The vesicular mole usually causes bleeding from the uterus during 4-12 weeks of pregnancy. Some women may not have any symptoms and it is diagnosed at the first scheduled scan, at 10-14 weeks.
Other symptoms include:
- Excessively severe nausea and vomiting during pregnancy (hyperemesis gravidarum)
- Abdomen feels bigger than expected for the stage of pregnancy
- Rare symptoms include hyperthyroidism and hypertension
Vesicles may be passed when the bleeding starts. More commonly, the molar pregnancy is diagnosed by an early pregnancy ultrasound, following the onset of bleeding, or scheduled. This is usually accompanied by hCG measurements.
A molar pregnancy must always be removed because it will never develop into a fetus. The most common technique is suction evacuation of the uterus. The removed tissue is always sent for histological examination.
Close follow up is mandatory to make sure the tumor is completely gone. A choriocarcinoma or other cancerous forms of gestational trophoblastic proliferation are rare complications of a molar pregnancy, and can be detected only when the follow-up shows rising hormone (hCG) levels even after evacuation. Such cancers can usually be completely treated with chemotherapy.
Reliable contraception is advised until the hCG levels remain normal for at least 6 months, following treatment of benign forms of GTD.