What is a molar pregnancy?

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A molar pregnancy is a nonviable pregnancy characterized by an abnormal growth of placental tissues.1 The resulting growth is not an embryo, but it triggers the symptoms of early pregnancy. It is a rare gynecological condition that ends in pregnancy loss and can affect women at any reproductive age. However, the risk is often higher in women who are younger than 15 years old or older than 45 years old and in those who had prior molar pregnancies.1

Molar pregnancy, also known as hydatidiform mole, occurs in 1 out of every 1000 pregnancies.1 A mole represents a lump of growing tissue that is genetically abnormal. Molar pregnancy can take the form of a partial or complete mole. Each form has a unique genetic profile, but both carry the risk of progressing into a locally invasive or metastatic malignancy.2

Complete moles lack identifiable fetal tissues. Partial moles have identifiable fetal tissues but exhibit congenital anomalies such as syndactyly (webbed or fused fingers) or cleft lip.3

Molar pregnancy is usually diagnosed with transvaginal ultrasound. The ultrasound can detect atypical features such as the presence of cysts and the absence of a clear embryonic structure or gestational sac.4 The characteristic ultrasound appearance shows a snowstorm pattern, described as a uterus full of dots, that usually follows a benign course.

Causes:

There are two kinds of molar pregnancy. Usually, both are benign.

  • Complete hydatidiform mole (CHM) occurs when a single sperm duplicates after fertilizing an empty oocyte.5 The resulting mole is diploid (46 chromosomes), and the chromosomes are entirely from the paternal genome. A complete moral pregnancy has only placental tissue that will produce the human chorionic gonadotropin (HCG) hormone. An ultrasound will detect the presence of a placenta without a fetus.6
  • Partial hydatidiform mole (PHM) occurs when an oocyte is fertilized by two sperm cells (di-spermic fertilization).5 The resulting mole is triploid (69 chromosomes) with two sets of paternal haploid chromosomes and one set of maternal haploid chromosomes. A partial moral pregnancy contains both abnormal cells and a fetus with severe malformation and is never viable.6

Symptoms:

Apart from the typical pregnancy symptoms, molar pregnancy6 is clinically manifested with uterine bleeding, anemia, an enlarged uterus, and an abnormal level of the hCG hormone. It may also lead to other medical complications such as:

  • Hyperthyroidism: signs of an overactive thyroid include feeling tired, excessive sweating, and an irregular heartbeat
  • Pre-eclampsia: high blood pressure
  • Ovarian theca lutein cysts: are benign cysts that usually develop due to the high levels of hCG
  • Abdominal distension
  • Acute respiratory failure

Detection:

Advancement in transvaginal ultrasound allow for early detection of molar pregnancies in the first trimester, often before clinical signs and symptoms appear.7 Testing for elevated hCG levels helps the physician to distinguish an early complete mole from a missed abortion. A definitive diagnosis is confirmed by a pathologist.

Side Effects:

Post-molar pregnancy side effects vary widely and may include the risk of:6

  • Repeated abortions
  • Stillbirths
  • Preterm births
  • Recurring molar pregnancies

Treatment:

A molar pregnancy cannot grow into a normal pregnancy. Suction dilation and curettage (known as D&C) are typically the first-choice options without the need for further treatment.4 Unlike induction procedures, suction and curettage carry a significantly low risk of bleeding or retaining molar tissue in the uterus.3

If the mole was not completely evacuated, the remainder tissue might metastasize and would require treatment with chemotherapy. hCG levels would fall rapidly after uterine evacuation and would remain within the normal range.

Occasionally, hCG levels may rise again which may indicate an invasive disease that develops into a tumor.4 Hysterectomy or bilateral salpingectomy (the removal of both fallopian tubes) might be performed. Hysterectomy prevents the development of local invasion at the uterine level, but it does not eliminate the possibility of developing a metastatic disease beyond the uterus.3

Post-procedural monitoring of HCG levels continues every two weeks for a period of 6 months for evidence of a progressing disease and to ensure that the patient has a sustained and complete remission.2

Despite the slightly increased risk of a recurrent molar pregnancy, subsequent pregnancy is generally considered safe for patients and their partners as soon as hCG testing has been completed.3

Resources

  1. Tantengco OAG, De Jesus FCC, Gampoy EFS, Ornos EDB, Vidal MS, Cagayan MSFS. Molar pregnancy in the last 50 years: A bibliometric analysis of global research output. Placenta (Eastbourne). 2021;112:54-61. doi:10.1016/j.placenta.2021.07.003
  2. Albright BB, Shorter JM, Mastroyannis SA, Ko EM, Schreiber CA, Sonalkar S. Gestational Trophoblastic Neoplasia After Human Chorionic Gonadotropin Normalization Following Molar Pregnancy: A Systematic Review and Meta-analysis. Obstetrics and gynecology (New York 1953). 2020;135(1):12-23. doi:10.1097/AOG.0000000000003566
  3. Berkowitz RS, Goldstein DP. Molar Pregnancy. The New England journal of medicine. 2009;360(16):1639-1645. doi:10.1056/NEJMcp0900696
  4. Taylor F, Short D, Harvey R, et al. Late spontaneous resolution of persistent molar pregnancy. BJOG : an international journal of obstetrics and gynaecology. 2016;123(7):1175-1181. doi:10.1111/1471-0528.13867
  5. Mohamed SA, Al-Hendy A, Ghamande S, Chaffin J, Browne P. Atypical Presentations of Molar Pregnancy. Journal of ultrasound in medicine. 2016;35(3):643-649. doi:10.7863/ultra.15.05027
  6. Capozzi VA, Butera D, Armano G, et al. Obstetrics outcomes after complete and partial molar pregnancy: Review of the literature and meta-analysis. European journal of obstetrics & gynecology and reproductive biology. 2021;259:18-25. doi:10.1016/j.ejogrb.2021.01.051
  7. Stamatopoulos N, Espada Vaquero M, Leonardi M, Nadim B, Bailey A, Condous G. Pre‐operative classification of molar pregnancy: How good is ultrasound? Australian & New Zealand journal of obstetrics & gynaecology. 2020;60(5):698-703. doi:10.1111/ajo.13130
  8. Image by Sharon McCutcheon from Pixabay 





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