Written by Ada’s Medical Knowledge Team
What is a molar pregnancy?
A molar pregnancy, sometimes called a hydatidiform mole, is a rare complication of pregnancy. It forms part of a group of disorders known as gestation trophoblastic disease (GTD), in which tumors grow inside a woman’s uterus. A molar pregnancy results from a problem, which is related to the genetic makeup of the fused egg and sperm, and which occurs during conception. This causes a fluid-filled mass of tissue to grow inside the womb, instead of a healthy pregnancy. A molar pregnancy will not develop into a baby.
A molar pregnancy will often have the same symptoms as a normal pregnancy in its early stages and may go undetected until the first routine ultrasound scan, if it is not miscarried before this time. Most molar pregnancies are diagnosed in the first trimester, before any classic signs and symptoms are present. However, symptoms which could indicate a molar pregnancy include vaginal bleeding, severe nausea and vomiting, tummy growth which can be faster or slower than normal, and signs and symptoms of hyperthyroidism. 1 2
A molar pregnancy can be removed, and most people will make a full recovery. Very rarely, it may become cancerous. However, cancers that develop from molar pregnancies tend to have a very good prognosis, with a high cure rate of nearly 100 percent. 3
Molar pregnancy affects about one in every 1.000 or 2.000 pregnant women. 4 The reported incidence varies widely in different regions of the world.
Many people experience emotional distress after having a molar pregnancy, particularly if it is felt as a loss if it occurs during a planned pregnancy, or if chemotherapy is necessary. A range of therapeutic treatment options are available.
If you are worried that you may have a molar pregnancy, try the Ada app for a free symptom assessment.
Molar pregnancy symptoms
Molar pregnancy often has no symptoms differing from a normal pregnancy and is frequently diagnosed during a routine ultrasound. However, the most common signs and symptoms that could indicate molar pregnancy are: 5 6
- Vaginal bleeding early in the pregnancy, which may contain little fluid-filled sacs
- High level of human chorionic gonadotropin (hCG) (pregnancy hormone)
- Bigger abdomen (tummy) than expected for the number of weeks pregnant
- Pelvic pressure or pain.
Less common signs and symptoms include: 5
- Hyperemesis gravidarum: severe nausea and vomiting
- Early onset preeclampsia: high blood pressure in pregnancy
- Signs of an overactive thyroid, known as hyperthyroidism, such as nervousness, insomnia and heart palpitations.
Worried you may be experiencing a molar pregnancy? Check your symptoms with the Ada app.
Molar pregnancy causes
A molar pregnancy occurs when there is a problem with the genetic information of a fused ovum (egg) and sperm. While a molar pregnancy cannot be predicted, because little is known about how and why it occurs, factors that are thought to increase the chances of having one include: 6 7
- Being younger than age 15 or older than age 35
- A history of molar pregnancy
- A history of miscarriage
- A diet low in carotene (a form of vitamin A)
Types of molar pregnancy
A molar pregnancy may be complete or incomplete, depending on the number of chromosomes in the fertilized egg.
In normal conception, a single sperm with 23 chromosomes fertilizes an egg with 23 chromosomes, making 46 in all.
Complete molar pregnancy
Most molar pregnancies are complete. 8 Once ovulation occurs, one sperm, or sometimes two, fertilize an egg that has no genetic material. No embryo will grow, because there are no chromosomes from the mother, but two sets from the father.
Normally, a fertilized empty egg dies, but in rare cases it implants in the uterine wall. This causes trophoblast cells, the cells that would become a placenta, to grow rapidly into clusters of fluid-filled sacs called cysts or a complete mole. 9 10
Partial molar pregnancy
A partial mole occurs when two sperm fertilize the egg instead of one. This means that there are 69 instead of 46 chromosomes, creating too much genetic material.
Molar pregnancy complications
In some cases, after removal of a molar pregnancy, a number of cells may remain behind and develop into gestational trophoblastic neoplasia (GTN), which is cancerous and can be very serious if left untreated. However, GTN has a very good prognosis, with a near 100 percent cure rate. There are several types of GTN that can result from a molar pregnancy, including:
- Invasive mole: Also known as an invasive hydatidiform mole or chorioadenoma destruens, this is where part of the mole remains in the body and can grow quickly into a locally invasive tumor. This is more likely to occur after a complete molar pregnancy.
- Choriocarcinoma: This is the most aggressive form of GTN, a cancer which grows from the placenta and can metastasize (spread) throughout the body if left untreated. It can also occur postpartum after a healthy pregnancy or a miscarriage, but those with a molar pregnancy are at a higher risk. 11
- Placental site trophoblastic tumor (PSTT): This tumor is a very rare form of GTN that develops where the placenta attaches to the uterus lining. It most often develops months to years after a normal pregnancy, but may also develop after a spontaneous abortion or after a molar pregnancy is removed. 12 Most PSTTs do not spread to other sites in the body, but they do have a tendency to grow into the muscle layer of the uterus. Unlike most other forms of GTN, PSTTs are, in many cases, not sensitive to chemotherapy. Instead, they are most often treated with surgery, usually removal of the womb to remove disease completely and, occasionally, with chemotherapy. ref15
Molar pregnancy ultrasound diagnosis
Molar pregnancies are commonly discovered during the first routine ultrasound scan of a pregnancy. If a person develops bleeding in early pregnancy, the doctor will normally arrange a scan, in which case the condition may be found sooner.
The image from the ultrasound scan may be characteristic of a molar pregnancy, showing cysts that resemble a cluster of grapes, in which case the condition will be treated. It may also reveal enlarged ovaries caused by non-cancerous ovarian cysts stemming from abnormally high amounts of the pregnancy hormone human chorionic gonadotropin (hCG).
Often, however, the image may resemble that of a miscarriage. The further along the molar pregnancy is, the more characteristic the ultrasound picture becomes. If the doctor suspects a molar pregnancy, a blood test will be ordered to check if the hCG levels are higher than normal, far higher in comparison to possible hCG levels that may occur in normal pregnancies. See these resources for more information on miscarriage and signs of miscarriage.
If there is both an ultrasound image that looks like a miscarriage and higher-than-normal hCG levels, the person will be treated to remove the abnormal tissue in the uterus.
Tissue miscarried or removed from the uterus will be sent to a laboratory and tests will be done to see if molar tissue is present. A molar pregnancy diagnosis can then be confirmed.
Malignant trophoblastic tumors primarily metastasize to the lungs, so once diagnosis of a molar pregnancy is confirmed, this should be ruled out by taking a baseline chest radiograph. 13
There may be a delay of a few days or weeks between treatment and the receipt of confirmation that the person had a molar pregnancy. 10
Molar pregnancy treatment
Treatment for a molar pregnancy usually takes the form of dilation and curettage (D&C), sometimes known as surgical uterine evacuation, dilation and evacuation, surgical management of miscarriage (SMM), evacuation of retained products of conception (ERPC) or a scrape. This is a minor operation done under general anesthetic, in which the cervix, the passage leading from the uterus to the vagina, is dilated so that abnormal tissue in the uterus can be removed with suction and a spoon-like instrument (curette). There is likely to be vaginal bleeding for up to six weeks after surgery, which may be heavy at first.
Women who have completed childbearing may have the option of hysterectomy, a surgical procedure to remove the womb.
Molar pregnancy follow-up appointments
There is a small risk of developing gestational trophoblastic neoplasia (GTN) after a molar pregnancy has been treated. GTN is a serious condition that requires rapid treatment, so all women who have had molar pregnancies should be monitored afterwards.
During a molar pregnancy, hCG (pregnancy hormone) levels rise, but should return to normal once the person has been treated. For at least six months after the end of treatment for a molar pregnancy, hCG levels will typically be monitored through urine tests. If they do not return to normal, this could indicate GTN.
It is important to take steps to avoid becoming pregnant for about six months after treatment, as this would interfere with the monitoring of hCG levels for signs of GTN.
After a complete mole, the likelihood of developing GTN is significantly increased. After a partial mole, this risk only increases slightly. The reported incidence of gestational trophoblastic neoplasia (GTN) after each type of molar pregnancy is:
- Complete mole: 15-20 percent
- Partial mole: 1-5 percent
Molar pregnancy and emotions
It is normal to feel stress after a molar pregnancy and, in some people, these emotions can develop into depression and anxiety if left untreated.
The Miscarriage Association suggests that a person with molar pregnancy might experience a variety of associated feelings, including:
- Shock and confusion. These emotions are especially common if the pregnancy was planned and thought to be healthy prior to an ultrasound scan or miscarriage. It can be shocking for a person to find out they have a condition which they probably never heard of before.
- Fear and anxiety. A molar pregnancy may lead to fear of the possibility of having cancer, and fear that problems will happen again with any future pregnancies.
- Loss and grief. It is normal to feel grief if a baby was expected.
- Feeling stuck. It is common to feel stuck if wanting to try to conceive again and having to wait some time while having follow-up and repeated blood and urine tests.
Counseling is available for people affected by a molar pregnancy. Support groups such as MyMolarPregnancy.com are also available and may be able to put a person in touch with others in a similar situation.
Health practitioners will be able to provide advice to those affected by a molar pregnancy on choosing the most appropriate kind of support and direct them towards available services.
Other names for molar pregnancy
- Hydatidiform mole
- Complete mole
- Partial mole
Molar pregnancy FAQs
Q: Can a molar pregnancy survive?
A: No, a molar pregnancy cannot develop into a baby, as it does not have a correct set of chromosomes. Molar pregnancies may at first seem to have the same symptoms as a normal pregnancy and generate a positive result on a pregnancy test. In the case of complete molar pregnancy, no fetus will develop. In a partial molar pregnancy, fetal tissue may initially develop, it will, however, be genetically abnormal and cannot survive and will be overtaken by rapidly-growing cysts.
Q: Will a person who has had a molar pregnancy be able to have a normal pregnancy in future?
A: Yes, a molar pregnancy will not normally cause infertility or lead to complications with future pregnancies. 9 The chances of another molar pregnancy is increased, but is still only around 1-2 percent. The recurrence rate after two molar pregnancies has been reported to range from 11 to 25 percent. However, a person should take measures to avoid becoming pregnant for at least six months after the end of molar pregnancy treatment, to allow hCG monitoring for remaining molar tissue. If chemotherapy follow-up treatment has been used, a person should take measures to avoid becoming pregnant for at least one year after the end of treatment, to allow for hCG monitoring and also to allow the chemotherapy, which can be very damaging to an unborn baby, to clear the body.
Q: Blighted ovum vs molar pregnancy: what is the difference?
A: Blighted ovum, also known as anembryonic pregnancy, is a condition in which a fertilized egg attaches itself to the uterine wall, but there is no embryo development, usually due to chromosomal abnormalities or abnormal cell division. Molar pregnancy is caused by chromosomal abnormalities due to abnormal conception, and causes a mass of fluid-filled cysts to grow, instead of a healthy pregnancy. Blighted ovum and partial molar pregnancy can look similar on an ultrasound. However, a partial mole usually shows the remains of embryonic tissue, which never appears in a blighted ovum. 14
Q: What is the outcome if there is a twin pregnancy with a molar pregnancy and a live baby?
A: In extremely rare cases, there may be a viable fetus co-existing with a molar pregnancy in the uterus. It may be possible for the pregnancy to continue; in as many as 40 percent of these cases, the woman is able to give birth to a baby that survives. However, the risk of the mother developing persistent trophoblastic disease is higher, making continuing the pregnancy a risk. 15