A molar pregnancy occurs when the fertilized egg encounters issues in its development, causing it to grow abnormally and form a benign tumour. As such, both the embryo and the placenta fail to develop and the pregnancy fails to continue, often resulting in miscarriage. In the UK the occurrence of molar pregnancies is rather common - around 1 in 579 pregnancies develop molar growth whilst Asian women are at twice the risk of molar pregnancies than non-Asian.² High incidence, associated health complications and pregnancy loss it’s essential to understand its causes and factors that put women at risk of its development. This article will discuss what molar pregnancy is, the main causes and risk factors. 

What is molar pregnancy? types of molar pregnancies

Molar pregnancy (hydatidiform mole) is an example of a gestational trophoblastic disease (GTD). GTDs are characterized by the presence of abnormal cellular growth³ that originates from the tissues that form in the womb during pregnancy. These abnormal cells form tumours, which can be malignant (cancerous) or benign (non-cancerous) - the latter being the most common tumour subtype in molar pregnancy.² The tumorous mole appears as fluid-filled sacs that grow in the womb. 

Molar pregnancy develops when the fertilization process (sperm penetrating the egg) goes wrong. As a result, the fertilized egg can no longer develop normally and begins to grow in an unexpected manner in the womb. At this point, the pregnancy cannot continue and the mole often results in a miscarriage. 

Types of molar pregnancies

There are two types of molar pregnancies: complete molar pregnancy and partial molar pregnancy. Complete molar pregnancy occurs when the sperm (or sperm) fertilize an abnormal egg that lacks any maternal genetic material. Due to incomplete genetic material, the fertilized egg cannot develop properly. In a complete molar pregnancy, the mole does not contain any fetal material and forms from abnormal development of supporting tissue (such as the placenta). A partial molar pregnancy occurs when multiple sperm fertilize one egg at the same time. In normal fertilization, one sperm fertilizes an egg to ensure correct chromosomal numbers. If multiple sperm fertilize one egg, however, the chromosomal number is altered - the fertilized egg now contains twice as much paternal genetic material than maternal. As a result, the fertilized egg cannot develop normally. In partial molar pregnancy, the mole may contain some fetal tissue, yet cannot develop into a viable embryo. 

Causes of molar pregnancy

The most common causes of molar pregnancies are genetic abnormalities in the gametes (sperm/egg) or following the fertilization of the egg. As mentioned previously, incomplete genetic complement (complete molar pregnancy, where the female egg lacks genetic material) or incorrect (partial, where paternal chromosome number is doubled to maternal) cannot facilitate normal development of the egg and the subsequent embryo. For the mole to develop, the cells which make up the tissue undergo abnormal cellular division, which results in the typical mole appearance of fluid-filled tumorous sacs.⁴

Whilst all women can develop molar pregnancies, there are certain medical conditions that can predispose someone to developing a molar pregnancy. For instance, women carrying mutations in certain genes (ex. NLRP7 and KHDC3L) which are required for normal egg development are more likely to develop a complete molar pregnancy.⁵

Risk factors of molar pregnancy

Some groups of women have at higher chance of developing a molar pregnancy. These include women at a certain age, those with a history of molar pregnancy or miscarriages, women of specific ethnicities, malnutrition women and those who underwent fertility treatments.⁶ 

• Maternal age. Women under 18 and over 35 years old have at higher chance of developing a molar pregnancy. In young women, it’s often due to not finalized maturation of the reproductive systems. In older women, molar pregnancies can occur due to reduced quality of the eggs, which often takes place as a woman ages
• History of molar pregnancies and miscarriages: the risk of subsequent pregnancy being molar in a woman who already had a molar pregnancy in the past is 1 in 100
• Ethnicity: women of Asian descent are twice as likely to develop a molar pregnancy than non-Asian women
• Malnutrition: women who experience malnutrition/abnormal nutrition can develop molar pregnancy
• Fertility treatments: women who underwent in-vitro fertilization/other fertility treatments are at higher risk of developing a molar pregnancy

Symptoms, diagnosis and treatment

The danger of molar pregnancy is that often the symptoms are similar to a normal pregnancy (especially in the beginning stages), which makes it difficult to diagnose and therefore calls for a thorough understanding of causes and risk factors. It's typical for a patient with a molar pregnancy to experience: 

• Missed period
• Nausea, morning sickness 
• Fatigue 
• Sore breasts
• More frequent urination 
• Food cravings 

all of which are common symptoms of a normal pregnancy. 

Nevertheless, a warning sign of pregnancy abnormality, which is common in molar pregnancy is vaginal bleeding. However, whilst this symptom may be caused by a developing mole in the womb, it can also be a result of other pregnancy complications. Therefore medical examination is essential for the diagnosis of molar pregnancy.²

To diagnose a molar pregnancy, a doctor may perform an ultrasound and blood tests for hCG levels. Developing tumours associated with molar pregnancy can be seen on an ultrasound, making it a reliable tool for diagnosis. Molar pregnancy is also associated with very high levels of hCG. Therefore a combination of ultrasound and hCG hormone level results can aid doctors in diagnosing a molar pregnancy.¹

Currently, there is only one treatment for molar pregnancy and it is inducing/waiting for a miscarriage. This must happen as soon as possible as the mole can continue to grow for the duration of pregnancy and can develop into a malignant tumour. Due to the high incidence,and symptomatology that is similar to normal pregnancy/other pregnancy complications and associated complications, it is essential to understand the causes and risk factors of molar pregnancy. Moreover, it's key to inform women at risk of this potential pregnancy complication. 

Summary

Molar pregnancy (hydatidiform mole, a type of gestational trophoblastic disease) is a rather common pregnancy complication, which is characterized by abnormal growth of the tissue found in the womb during pregnancy. It is caused by genetic alterations in egg/or following fertilization which results in abnormal cell division and the appearance of fluid-filled tumours in the womb. Women of certain ages, ethnicities and those with specific medical histories are at higher risk of developing a molar pregnancy. It's essential to understand the underlying risk factors and causes of this pregnancy for better diagnosis and to inform these patients of this potential outcome to facilitate earlier detection.