Understanding Molar Pregnancies: Types, Causes, and Diagnosis

Understanding Molar Pregnancies: Types, Causes, and Diagnosis

A Genetic Counselor's overview of Molar Pregnancies, and how Orchid's embryo screening is the best way to mitigate the chances for developing one

Written by Maria Katz, MS, CGC
Reviewed by Xiaoli Du, PhD, FACMG, DABMGG, CGC,CGMBS

Dr. Du is American Board of Medical Genetics and Genomics (ABMGG) certified geneticist. With a passion for precision medicine, she leverages cutting-edge molecular genetics and cytogenetics technology to improve outcomes for individuals affected by genetic conditions. Dr. Du has nine years of clinical genetic diagnosis experience in both the public and private sectors.

Understanding Molar Pregnancies: Types, Causes, and Diagnosis

Molar pregnancies, a result of a genetic anomaly in the embryo, affect approximately 1 in 1000 pregnancies in the United States. Instead of a typical embryo, these pregnancies give rise to abnormal tissue consisting mainly of excessively grown placental cells, occasionally accompanied by fetal cells. Molar pregnancies account for around 3% of miscarriages.

Molar pregnancies can be categorized into two main types: complete and partial.

In a complete molar pregnancy, there is an absence of a normal embryo or fetus. Instead, the pregnancy consists entirely of abnormal placental tissue. This type of molar pregnancy occurs when an empty egg is fertilized by a single sperm, resulting in the formation of a mass of cells without any fetal development.

In a partial molar pregnancy, also referred to as triploidy, both abnormal placental tissue and a fetus with severe abnormalities are present. This type of molar pregnancy arises when a normal sperm fertilizes an egg that has duplicated its chromosomes or when two sperms fertilize a single egg. Partial molar pregnancies have three sets of chromosomes instead of the usual two, resulting in a total of 69 chromosomes.

Causes and Risk Factors of Molar Pregnancies:

The exact causes of molar pregnancies are not yet fully understood. However, several risk factors have been identified:

  • Maternal Age: Women under the age of 20 or over the age of 35 have a higher risk of molar pregnancies.
  • Previous Molar Pregnancy: Women who have had one molar pregnancy are at an increased risk (1-2%) of having another.
  • History of Miscarriage: Women who have experienced multiple miscarriages may have a slightly higher risk of molar pregnancies.
  • Dietary Factors: Women with deficiency in animal fats and/or carotene/ vitamin A may increase risk for complete molar pregnancies. 

Diagnosis and Detection Methods for Molar Pregnancies:

Detecting molar pregnancies is of utmost importance due to the potential health risks they pose to the mother. If left undiagnosed or untreated, complications like persistent trophoblastic disease or the development of gestational trophoblastic neoplasia, a type of tumor, may arise. Fortunately, early detection and accurate diagnosis are possible through various methods:

  • Ultrasound: Transvaginal ultrasound is commonly employed to identify the distinct features of molar pregnancies, such as the absence of a fetus, abnormal growth patterns, and the presence of cystic structures
  • Blood Tests: Measuring the levels of human chorionic gonadotropin (hCG), a hormone produced during pregnancy, can provide valuable information. In molar pregnancies, hCG levels are often significantly higher than in normal pregnancies.

Once identified, appropriate medical intervention, including procedures like dilation and curettage, can be administered to remove the abnormal tissue. Furthermore, a sample of the collected tissue is typically sent to a laboratory for genetic analysis, confirming the diagnosis and type of molar pregnancy. Early detection not only facilitates timely medical care but also enables emotional support for those affected, helping them navigate the physical and psychological challenges associated with this type of pregnancy loss.

Can I test my embryos for this?

Yes. Orchid’s embryo screening assesses the embryos risk for both partial and complete molar pregnancies. 

Most other embryo screening methods miss the detection of both partial and complete molar pregnancies due to technical challenges associated with the technology used.

Molar embryos can result in failed implantation or abnormal pregnancies, potentially causing serious complications for the individual carrying the pregnancy. Early detection and precise diagnosis using methods like ultrasound, blood tests, and genetic testing are crucial for effective medical management and ensuring the mother's well-being. However, when it comes to reducing the risk of transferring a partial or complete molar embryo, the risk of transferring a molar embryo can be minimized through Orchid, offering a higher chance of a successful and healthy pregnancy.


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Brezina, P. R., Anchan, R., & Kearns, W. G. (2016). Preimplantation genetic testing for aneuploidy: what technology should you use and what are the differences?. Journal of assisted reproduction and genetics, 33(7), 823–832. https://doi.org/10.1007/s10815-016-0740-2

Ghassemzadeh, S., Farci, F., & Kang, M. (2023, May 22). Hydatidiform Mole. In StatPearls. Retrieved from https://www.ncbi.nlm.nih.gov/books/NBK459155/

Maisenbacher, M. K., Merrion, K., & Kutteh, W. H. (2019). Single-nucleotide polymorphism microarray detects molar pregnancies in 3% of miscarriages. Fertility and sterility, 112(4), 700–706. https://doi.org/10.1016/j.fertnstert.2019.06.015

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