Molar Pregnancy | What Is It?

When a pregnancy doesn’t continue, it can be devastating to know there was a problem with the developing baby.

In some cases, gestational trophoblastic disease, or molar pregnancy, has happened.

Although it’s not common, it’s helpful to know what a molar pregnancy is, the symptoms and treatment, and how to prevent complications.

What is a molar pregnancy?

A molar pregnancy or hydatidiform mole is an abnormal growth of placental tissue in your uterus.

It’s a type of gestational trophoblastic disease (GTD). The layer of cells surrounding the embryo is called the trophoblast, aptly named because ‘tropho’ means nutrition and ‘blast’ means bud.

Normally in early pregnancy, the cells of the trophoblasts extend into the lining of the uterus and develop into the placenta.

In molar pregnancies, the trophoblast cells form an abnormal clump of cells and it isn’t viable due to the abnormal placenta.

What causes a molar pregnancy?

A hydatidiform mole is caused by abnormalities after a sperm has fertilized an egg. These are usually chromosomal abnormalities, causing the cells of what should be the placenta to be affected.

Molar pregnancies are rare, occurring in around 1 in every 1 000 pregnancies.

Types of molar pregnancy

There are two types of molar pregnancy or hydatidiform moles and both are benign, meaning they don’t cause cancer. Neither of them can survive to a term pregnancy.

Each types is caused by a different chromosomal abnormality. Normally, human cells have 23 pairs of chromosomes – one pair from each male and female parent.

With complete and partial molar pregnancies, this has somehow gone wrong.

Complete molar pregnancy

In complete molar pregnancies, the egg doesn’t contain any genetic material from the mother, so all the chromosomes come from the father and are duplicated. Sometimes more than one sperm has fertilized the egg.

The fertilized egg becomes a mass of abnormal cells that contain only placental tissue. There’s no signs of a developing baby in the uterus.

Partial molar pregnancy

In partial, or incomplete, molar pregnancies, the egg has the mother’s chromosomes but there are two sets of the father’s chromosomes, from either one or two sperm. This means the embryo ends up with 69 instead of 46 chromosomes.

The fertilized egg becomes a mass of abnormal cells that contain placental and fetal tissue, but the fetal tissue is abnormal and unable to develop into a baby.

hCG and molar pregnancy

In a molar pregnancy, the abnormal placenta produces very high levels of the pregnancy hormone human chorionic gonadotropin (hCG).

This is the hormone produced by the placenta and is the same hormone your care provider looks for to confirm your pregnancy.

You can read more about hCG Levels – What Should They Be Each Week? to get a better understanding.

Although high levels of hCG might indicate a molar pregnancy, this is not reliable on its own; a twin pregnancy can also cause elevated hCG levels.

Your doctor will organize other tests, check your symptoms, and assess your risk factors for diagnosis.

Molar pregnancy symptoms

At first, hydatidiform moles develop in the same way as a normal healthy pregnancy. Your body is producing large amounts of hCG, the pregnancy hormone that causes many of the signs and symptoms of pregnancy.

In a molar pregnancy, however, the symptoms are often more severe and you would usually experience one or more at the same time.

Your health care provider will look for these signs and symptoms:

Vaginal bleeding

Although spotting is common in the first trimester, molar pregnancy causes dark brown to bright red bleeding that might contain grape-like clusters or blood clots. This bleeding can be pretty frequent and should definitely be investigated.

Check out our article for more information on other causes of bleeding during pregnancy.

Severe pregnancy nausea and vomiting

Once again, this can be a normal symptom of early pregnancy caused by the hormone hCG.

In a molar pregnancy, the extra placental tissue causes very high hCG levels that make nausea and vomiting severely.

Pelvic pressure and pain

The abnormal tissue grows faster than it should in early pregnancy and can cause your uterus to look and feel larger than it should for this stage. It might also cause increased pressure and discomfort in the lower abdomen.

Preeclampsia and high blood pressure

Molar pregnancy can also cause early-onset preeclampsia. This condition causes high blood pressure and protein in your urine.

Read more about Preeclampsia – Symptoms, Signs, and Causes during pregnancy.

Molar pregnancy ultrasound

Ultrasound is part of the way your doctor will diagnose gestational trophoblastic disease.

A complete molar pregnancy is easier to detect than a partial molar pregnancy. It can usually be seen on ultrasound as early as eight or nine weeks.

In a healthy pregnancy, your doctor would point out the gestational sac, the yolk sac, and the fetal pole at 9 weeks.

In a complete molar pregnancy, these structures are absent and there’s only abnormal placental tissue that fills the uterine cavity.

In a partial molar pregnancy, the gestational sac and the fetal pole will be separate, and there might only be a thickening on the margin of the yolk sac, indicating further investigation is needed.

The ultrasound is only one tool your care provider will use to make a diagnosis for gestational trophoblastic disease. Most molar pregnancies are diagnosed in the first trimester.

Who is at risk for molar pregnancy?

Approximately 1 in every 1,000 pregnancies (0.1%) is diagnosed as a molar pregnancy. The risk factors for developing hydatidiform moles include maternal age and previous molar pregnancy.

Studies show women older than 35 years are more at risk for molar pregnancy.

There’s also evidence that women who have had a previous molar pregnancy are likely to have another. Repeat molar pregnancies occur in 1 out of every 100 women.

Molar pregnancies can happen even if you’ve previously had a normal pregnancy.

You can still have a normal pregnancy after a molar pregnancy, but your doctor might recommend waiting 6-12 months before trying to conceive again.

Treatment for molar pregnancy

Molar pregnancies need to be treated to prevent further complications. They can’t continue to full term.

In some cases, pregnancy loss will occur spontaneously with a miscarriage, possibly even before you realized that it was a molar pregnancy.

After you have been diagnosed with a molar pregnancy, if the pregnancy continues you will be given the option of having a dilatation and curettage (D&C).

This is a surgical procedure to remove the molar pregnancy.

You can read Dilatation and Curettage – What to Expect? to find out more.

Another option is to be given medication that will cause the uterus to contract and expel the tissue. Whether this treatment is available to you will depend on how far into the pregnancy you are, and how large the molar pregnancy is.

Complications of molar pregnancies

Once the molar pregnancy has been removed, you will be monitored for any complications.

It’s possible for molar tissue to be missed during the D&C, and continue to grow. This is called persistent gestational trophoblastic neoplasia (GTN). It happens in up to 20% of complete molar pregnancies and up to 5% of partial molar pregnancies.

Your doctor or health care provider will monitor your hCG levels after the molar tissue has been removed, as persistently high levels can be a sign of GTN.

In some cases, if the molar pregnancy penetrated deep into the middle layer of the uterine wall (myometrium), it can cause persistent vaginal bleeding. This is known as an invasive hydatidiform mole and can be treated with chemotherapy or hysterectomy.

Very rarely, a cancerous form of GTN called choriocarcinoma can develop and spread to other organs in the body.

This is more likely with complete molar pregnancies than with partial. Choriocarcinoma is often successfully treated with different cancer drugs.

It’s shocking and disappointing to hear that you have a molar pregnancy as it will result in pregnancy loss. Rest assured, though; it can be treated and you will be able to get pregnant after a molar pregnancy.

Your ob-gyn or health care provider might recommend waiting at least six months before trying to conceive again and might also discuss blood tests for prenatal genetic testing.

Although the risk of having a repeat molar pregnancy is low, it’s higher if you have had a previous molar pregnancy.

Your care provider will also do early ultrasound scans in your next pregnancy to offer you reassurance of normal development and a healthy pregnancy.