In a normal pregnancy, the product of conception is derived from the original genetic material, the 23 chromosomes, inside the sperm and the ovum. When these cells fuse, a zygote is formed. This single cell with 46 chromosomes repeatedly splits before differentiating into all the many types of cells and tissues that make up a pregnancy - the fetus and its surrounding membranes.

Some of the cells formed from the zygote take part in forming the placenta, which is made up of trophoblastic or nutritive cells. These are the cells that are abnormal in a molar pregnancy.

The cause of the abnormality is chromosomal - the zygote has either too few (23) or too many (46) chromosomes. This results in a mass of vesicular or grape-like tiny white cysts clustered together, replacing the placenta and membranes. In complete molar pregnancy, the fetal part of the pregnancy, or the baby, does not develop at all. In a partial molar pregnancy, there is some abnormal non-viable fetal development.

Types

The two types of molar pregnancy, therefore, comprise:

• Complete moles – formed by the fertilization of an empty ovum, or an egg cell which failed to receive its share of chromosomes at cell division; only abnormal trophoblastic cells grow inside the uterus
• Partial moles – due to the fertilization of the ovum by two sperms, leading to the development of some vesicular placental tissue along with a non-viable fetus

A twin molar pregnancy is very rare, and includes the development of a normal fetus along with a molar pregnancy, because of the separate fertilization of two ova by two sperms

Symptoms and Diagnosis

A molar pregnancy is often completely asymptomatic, and the diagnosis is made only when the first pregnancy scan routinely scheduled at 10 to 14 weeks reveals the problem. The later the scan, the more unmistakable the picture becomes on ultrasound.
Other women may report the following signs and symptoms:

• A brownish-red discharge from the uterus, or frank bleeding, which may or may not be associated with the passage of vesicles. This is usually found between the fourth and twelfth week of pregnancy and is the most common symptom of a molar pregnancy.  
• Very severe and intractable nausea and vomiting, called hyperemesis gravidarum, which is due to abnormally high production of the pregnancy hormone called human chorionic gonadotropin (hCG) from the trophoblastic tissue
• Sickness and vomiting may be more severe than in a normal pregnancy, found in a tenth of patients
• Large ovarian cysts called theca-lutein cysts, due to ovarian stimulation by high hCG levels, which can cause pain, are found in a fifth of patients
• Grape-like cysts are passed out from the uterus in a tenth of patients

The size of the uterus does not correspond to the pregnancy date, being larger than normal in up to a quarter of cases due to the rapid proliferation of trophoblastic tissue inside the uterine cavity. Sometimes the uterus may be smaller than expected.
Other uncommon presentations include:

• Pain or pressure in the pelvis
• Post-delivery bleeding for longer than expected
• Anemia resulting in tiredness, breathlessness, giddiness, or palpitation
• Hypertension, with headaches and swelling of the legs, or reduced urine output, occurring before the middle of the second trimester of pregnancy, is almost always due to hydatidiform mole, and occurs in 1 of 20 patients
• Hyperthyroidism occurs in 1 of 20 cases, due to stimulation of the thyroid by hCG, which can manifest as:
  • Heat intolerance
  • Diarrhea
  • Palpitations and rapid heartbeats
  • Agitation, anxiety, or irritability
  • Warm and moist or sweaty skin
  • Hand tremors
  • Insomnia or sleep disturbances
  • Weight loss despite no obvious illness and despite eating normally

Diagnostic Procedure

A physical examination will reveal the patient’s general condition, including anemia or hyperthyroidism, and any masses. The past medical history and health status will also be recorded.

A pelvic examination will show the uterine size, shape, and feel, after checking the health of the vagina and cervix. In addition, the ovaries and nearby structures are assessed. The uterus is often larger than expected.

A pelvic ultrasound scan will show the presence of tissue inside the uterus, with the characteristic snowstorm appearance of a molar pregnancy. The scan may be performed via the abdominal scan or the vagina.

Blood tests are used to assess the level of hCG, which gives a clue as to the diagnosis. It is many times higher than normal in a complete molar pregnancy, going up to 100 000 IU/L. If hypertension or other illnesses are present, blood tests may be performed to evaluate the function of other organs such as the liver and kidney.

Urine analysis is used to check color and constitution of the urine, as well as the urine hCG level.

Diagnostic Features

When a patient presents with early pregnancy bleeding or other abnormal pregnancy features, an ultrasound scan is usually recommended. This may show the characteristic features of a hydatidiform mole.

When the ultrasound does not provide characteristic features of molar pregnancy, or looks like a miscarriage, the woman may be advised to have her hCG levels checked as well to differentiate between several possible diagnoses. If the levels are high, and no fetal parts are detected in the mass filling the uterus, the uterus will be emptied, and the tissue sent for microscopic examination, which will provide the true diagnosis.

Sometimes a woman may present with an abortion in progress, or with what looks like an incomplete abortion. The uterus is then emptied and the products are usually sent for histological examination. This occasionally reveals the presence of molar tissue.

When a woman presents with a pregnancy that is not progressing as expected, she may be advised to have a blood test to check hCG levels. These are much higher than normal in molar pregnancies and indicate the need for an ultrasound scan. The presence of the molar tissue is then detected.

Ultrasound scanning shows a honeycomb pattern produced by the numerous vesicles. As they enlarge the image is described to look like a snowstorm, which is due to swollen cysts with bleeding into the uterus. The ovaries are often seen to contain large cysts.

In a partial molar pregnancy, the fetus is seen along with hydropic villi forming vesicles. The fetus will have signs of abnormal chromosome number (three sets of chromosomes, or triploidy), which manifests as severe developmental aberrations or growth restriction. Molar change may be limited to a few scattered cysts. The ovaries may have only small cysts or appear normal. The hCG level may fall within the upper range of normal in such cases.

If the diagnosis is in doubt, a repeat scan is ordered after a week or two. In almost all partial moles the fetus is aborted spontaneously, and the diagnosis is established by microscopic examination and cytogenetic study of the aborted tissue.

References

• http://patient.info/in/health/hydatidiform-mole
• http://www.nhs.uk/conditions/Molar-pregnancy/Pages/Introduction.aspx
• https://medlineplus.gov/ency/article/000909.htm
• https://www.cancer.gov/types/gestational-trophoblastic/patient/gtd-treatment-pdq