Molar Pregnancy: What Is It?

Molar Pregnancy: What Is It?


The young adult woman gazes out of the hospital room window as she worries about her lab results.

While the hope is always for a healthy and normal pregnancy and baby, unfortunately, sometimes things go wrong at conception that complicate pregnancy and prevent a healthy baby from forming. One of these complications is a molar pregnancy, also called a hydatidiform mole.

Knowing the potential early pregnancy complications can help some women cope with the anxiety of the unknown in early pregnancy. So here we will discover what a molar pregnancy is, its different types, causes, symptoms, and more.

What is Molar Pregnancy?

When conception occurs, typically, one sperm fertilizes one egg, and they each contribute half of the new embryo’s genetic material. A molar pregnancy can happen when something goes wrong with the amount or combination of genetic material at the beginning of a pregnancy.1

There are two types: complete and partial. Both cause a growth or tumor, or mole, to develop inside the uterus.1

Complete Molar Pregnancy

A complete molar pregnancy consists of only a tumor, no fetus. It occurs when a woman’s empty egg is fertilized either by two sperm or by one sperm with a double batch of genetic material. Either way, the embryo has the correct number of chromosomes, but they all come from the father, not the mother.2

This causes the placenta to form abnormally and grow into a tumor, but no baby ever forms in a complete one. These are more common than partial ones.3,4

Partial Molar Pregnancy

In the case of a partial molar pregnancy, both a fetus and a growth exist in the uterus.

A partial occurs when a woman’s normal egg (as opposed to an empty egg) is fertilized by two sperm or one sperm with double the amount of DNA. This creates an embryo with a whole extra set of chromosomes—two from the father and one from the mother.3,5

As with a complete molar pregnancy, the placental formation goes wrong and creates a tumor, but a fetus may also form in a partial one. However, the baby is rarely genetically normal.6

A partial molar can also be found in a twin pregnancy. In fraternal or dizygotic twins, one normal fetus can exist alongside a hydatidiform mole.2

How Common is Molar Pregnancy?

Molar pregnancy is rare. While miscarriage is common — some reports say up to one in three pregnancies ends in miscarriage — only one in 41 miscarriages is a molar pregnancy. The frequency in North America and Europe is between 60 and 120 per 100,000 pregnancies but may be higher in other parts of the world.2,4,7

Symptoms of Molar Pregnancy

Symptoms include the following:

High hCG Levels

In a normal pregnancy, the placenta begins to form and secrete human chorionic gonadotropin (hCG) as soon as implantation occurs. The hormone hCG helps pregnancy progress and is responsible for a positive home pregnancy test.8

Since a molar pregnancy is an uncontrollable growth of an abnormal placenta, blood and urine hCG levels are abnormally high.2

Vaginal Bleeding

In a molar pregnancy, vaginal bleeding will likely begin during the first trimester and may be accompanied by the passing of molar grape-like cysts out of the vagina.1

Uterine Enlargement

Because the hydatidiform mole proliferates, causing the uterus to fill and expand, a woman’s womb may measure much larger than expected for pregnancy dating.2

Diagnosis of Molar Pregnancy

If a woman presents with the above symptoms, her provider may suspect a molar pregnancy. It can be diagnosed via ultrasound through the vagina or on the abdomen.

On ultrasound, a complete mole has a “snowstorm” appearance in the uterus. The cysts formed from the abnormal placenta appear like multiple translucent areas of different shapes and sizes, filling the uterine cavity.2

A partial mole will have an enlarged placenta and look like Swiss cheese on an ultrasound.

Ultrasounds may also reveal “soap bubble ovaries” because ovaries grow multiple cysts due to extremely high levels of hCG.2

An ultrasound can usually diagnose a molar pregnancy between nine and 12 weeks after your last menstrual period. In a twin pregnancy with a partial mole, the mole may not be diagnosed until a bit later — around 15-20 weeks of pregnancy.2

Risk Factors for Molar Pregnancy

Molar pregnancies are more common in certain circumstances.

  • Maternal age. Very young mothers under 15-20 or older than 43 are more likely to have one.1,4
  • Previous molar pregnancy. People who have experienced one are likelier to experience another, but it is still rare.1
  • Ovulation induction. It is thought that induction may increase the risk for hydatidiform mole.2

Treatment Options

Most molar pregnancies are removed via a procedure called dilation and curettage. This is when the cervix is dilated, and a tool or suction is used to remove the contents and lining of the uterus gently.9

If it is a partial molar with a viable fetus, a woman can choose to continue the pregnancy in hopes of it resulting in a healthy newborn. This is rare but possible and would be considered an extremely high-risk pregnancy.9


Treatment is usually successful. However, if left untreated, complications like the following can arise.9


High blood pressure and pre-eclampsia can occur on their own later in pregnancy, but when they occur before 20 weeks, a molar pregnancy is often the cause.4


Extremely high levels of circulating hCG can cause hyperthyroidism with symptoms such as jitteriness and rapid heart rate.4


The hormone hCG is thought to cause morning sickness, so when its levels are much higher than normal, a woman may experience extreme nausea and vomiting.4


A woman may experience a low red blood cell count due to vaginal bleeding associated with molar pregnancy.4

Ovarian Cyst and Torsion

High hCG levels secreted by the hydatidiform mole can cause cysts on the ovaries. The ovaries can twist, cutting off blood flow. This is called torsion.2

Lung Complications

A piece of the mole can break off and enter a woman’s bloodstream. If this happens, that tissue can travel to the lungs, causing breathing problems.2,4

Premature Delivery

In rare cases where a molar pregnancy has a viable baby and the pregnancy is allowed to continue, it will likely be born early before it fully develops.9

Invasive Mole

The hydatidiform mole can grow deep into the uterine wall and cause internal bleeding.9


It is rare but possible for a complete molar pregnancy to turn into a fast-growing form of cancer called choriocarcinoma. This usually responds well to chemotherapy but can be life-threatening.9

Healing and the Future

Most women who have had a molar pregnancy go on to have a normal pregnancy, and they are not thought to affect future fertility. However, your doctor may recommend waiting to conceive again for up to a year. This allows for hCG levels to return to baseline, ensures the abnormal placental tissue does not grow back, and can prevent complications. Even though waiting may be best for your health, a year can feel like a long wait when you are ready to grow your family.1,3

After a molar pregnancy, some women feel eager to conceive again, while others may need to take time to process the loss of what they thought their pregnancy would become. A whole range of feelings after this situation is normal. If you have experienced a hydatidiform mole and are having trouble processing it, your doctor, a therapist, or a support group may be able to help.

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