Molar pregnancy or hydatidiform mole — is a benign tumor that occurs in the uterus. It starts when an egg is fertilized, but instead developing into a normal, conceptus + placental tissue, the placenta tissue develops into a mass of small cysts. Here, the root system (trophoblast) of the embryo which under normal conditions develops into the placenta that connects the baby to the mother. With molar pregnancy, the roots of the trophoblast (chorionic villi) undergo cystic degeneration and when the woman bleeds, these cystic structures are passed in dark blood, giving rise to the common description of “white currants floating in red currant jelly”.

The condition usually presents with one or more of the following:

  • Vaginal bleeding in the first trimester
  • Very high beta hCG levels early on in pregnancy
  • Exaggerated pregnancy symptoms and pernicious intractable vomiting (hyperemesis gravidarum)
  • Rapid (often painful) enlargement of the early pregnant uterus
  • Ultrasound evidence of a typical “snow storm pattern)

A molar pregnancy can have serious complications. It can become invasive (an invasive mole or chorioadenoma destruens) and permeate the uterine wall or it can (albeit rarely) develop into a rare form of cancer known as choriocarcinoma.

Molar pregnancies are rare (about 1:2000 pregnancies) and having occurred.it infrequently (<1:1000) recurs in the same woman. It is at least twice as common among Asian women. On rare occasions (1%) a twin pregnancy will comprise of a normal baby and a mole. In about 20%-40% of cases the healthy baby will survive to delivery.

There are two types of molar pregnancies: a) the complete molar pregnancy: Here there’s no embryo or normal placental tissue is present. b) . A partial molar pregnancy, there is a developing embryo present but it is abnormal and non-viable, but there can be some “normal” placental tissue present as well.

“Complete Hydatidiform Molar Pregnancies” occur when an egg that has no chromosomal material (anuclear) is fertilized by a sperm and thereupon divides in two and propagates haphazard tissue growth. Like normal pregnancies, the complete mole has 46 chromosomes (two sets of 23), i.e., it is diploid.  However, unlike with normal fertilization, where one set of chromosomes comes from the mother and the other set from the father, both sets of chromosomes come from the father in the case of a complete molar pregnancy. This results from duplication of a sperm’s chromosomes after it has fertilized an “inactive” egg.  Since an embryo that has a YY karyotype is not viable, the chromosome gender of the complete molar pregnancy is invariably XX (female). Accordingly, with IVF, if one selectively only transfers only male (XY) embryos, the possibility of a complete molar pregnancy can be virtually eliminated.

A “Partial Molar Pregnancy” on the other hand, most often results from an egg being fertilized by 2 separate sperm, such that instead of the resulting embryo comprising 46 chromosomes (23 from the egg + 23 from the sperm), it instead has 69 chromosomes (23 from the egg + 46 from 2 separate sperm). However, it can also happen where one sperm fertilizes an egg, but one group of 23 chromosomes duplicates …again resulting in 3 groups of 23 chromosomes (triploidy)…..for a total of 69 chromosomes. Thus with partial moles, the sex chromosome configuration will be XXY or XYY. Partial moles can thus be avoided through selectively fertilizing an egg by intracytoplasmic sperm injection (ICSI), where a single sperm is injected, and thereupon performing PGD on the embryo(s) to exclude triploidy.

Persistent trophoblastic disease refers to the situation where following treatment to remove a molar pregnancy some molar tissue is retained and starts to grow again. It occurs in 8-10% of molar pregnancies. In such cases the woman will usually need to undergo chemotherapy

Treatment involves complete emptying of the uterus as soon as the diagnosis is made – even in cases where a spontaneous passage of the molar tissue appears to be complete. The reason is to avoid the development of an invasive mole (where the uterine wall is permeated by remaining tissue), and to limit the development of choriocarcinoma a very malignant tumor that invades the uterus and can spread rapidly via the blood system to bone, lungs, brain and other sites. Fortunately this cancer does respond well to hysterectomy, removal of ovaries plus aggressive chemotherapy.

In the vast majority of properly managed cases however, outcome after treatment is usually excellent. In cases where the beta hCG level fails to drop appropriately following evacuation of the uterus, chemotherapy will usually be curative. Close follow-up with serial quantitative blood hCG testing, ultrasound and/or Magnetic Resonance Imaging (MRI) is essential. After successful treatment, the woman must use very effective contraception for at least 6 to 12 months, so as to avoid pregnancy in order to allow for proper follow-up.