If you know of any twins in your life, you may have heard about the diferencias between identical and fraternal twins. Identical twins, who share all of their DNA, come from a single fertilized egg that splits into two early on in its development. These two identical fertilized eggs become two babies that are both unique human beings but share the same DNA.
Fraternal twins, which are much more common and can also be more likely if they run in your family, are formed from two separate eggs and two separate sperm, developing two babies that gestate together in the womb but are not genetically identical in any way.
Though twins (and other multiples) are rarer than singleton pregnancies, there is another type of pregnancy which also involves multiple sets of DNA from the father, but they do not undergo typical fertilization and unfortunately cannot result in a viable pregnancy. These pregnancies are known as trophoblastic molar pregnancies.
What are molar pregnancies?
Just like in a fraternal twin pregnancy, many molar pregnancies begin with two sperm cells. However, instead of fertilizing one egg each, both sperm cells fertilize one egg.
Complete molar pregnancies
En un completa molar pregnancy, there can either be two sperm cells, or just one sperm cell that is copied, which then fertilizes an egg where the mother’s chromosomes are either not present or do not work the way they are supposed to. This makes it so the newly created cell is no a zygote (an egg fertilized with a sperm that typically contains 23 chromosomes from the mother and 23 from the father), but instead a double of the father’s DNA only — 46 chromosomes from the father and none from the mother. Complete molar pregnancies do not create an embryo, as the 23 chromosomes from a female sex cell is necessary to do so, but (fascinatingly) they do lead to the creation of a placenta. (This provides further credence to the somewhat recent discovery that the father’s DNA drives the gene expression in the baby’s side of the placenta.)
En un completa molar pregnancy, there can either be two sperm cells, or just one sperm cell that is copied, which then fertilizes an egg where the mother’s chromosomes are either not present or do not work the way they are supposed to. This makes it so the newly created cell is no a zygote (an egg fertilized with a sperm that typically contains 23 chromosomes from the mother and 23 from the father), but instead a double of the father’s DNA only — 46 chromosomes from the father and none from the mother. Complete molar pregnancies do not create an embryo.
Incomplete molar pregnancies
Incomplete (or partial) molar pregnancies, on the other hand, do have the mother’s chromosomes present in the mix. This means a zygote is created, but instead of having 46 chromosomes, the embryo has 69—23 from the mother, and 46 from the father’s two copied sperm cells. While an incomplete molar pregnancy does result in an embryo—sometimes even an embryo with a detectable heartbeat (possible, but rare)—the extra copy of the father’s genes makes this embryo nonviable, and an incomplete molar pregnancy typically results in an early (first trimester) miscarriage or later stillbirth.
Incomplete (or partial) molar pregnancies, on the other hand, do have the mother’s chromosomes present in the mix. This means a zygote is created, but instead of having 46 chromosomes, the embryo has 69—23 from the mother, and 46 from the father’s two copied sperm cells. While an incomplete molar pregnancy does result in an embryo—sometimes even an embryo with a detectable heartbeat (possible, but rare)—the extra copy of the father’s genes makes this embryo nonviable, and an incomplete molar pregnancy typically results in an early (first trimester) miscarriage or later stillbirth.
(A note about twin pregnancies with an incomplete mole: According to this 2023 case report, “Twin pregnancies with one fetus and a coexisting mole, which were formerly uncommon, are becoming increasingly prevalent, possibly due to the rise in ovulation induction… A partial mole with a concomitant fetus is reported to occur in 0.005% to 0.01% of pregnancies” [1]. In fact, in a 2022 case report from China, a healthy baby girl was eventually delivered at-term, despite her “twin” being a partial mole [2].)
Atypical placental tissue
In both complete and incomplete molar pregnancies, there is an atypical growth of cells called trophoblasts. While in a typical pregnancy the father’s DNA works to build the placenta during early pregnancy, the abnormality of having two sperm copies where there should only be one causes the newly created placental tissue to swell and form cysts filled with fluid (often described as “grapelike” in their appearance), making it unable to sustain life.
How common are molar pregnancies?
Fortunately, molar pregnancies are raro—only affecting about 1 in 1,000 pregnancies. According to the Mayo Clinic, molar pregnancies are more common in women who are either under 15 or over 43, have a history of molar pregnancies, have had frequent miscarriages, or are of Asian descent.
Many women do not experience symptoms of molar pregnancies and are unaware they’re experiencing one until they have an ultrasound (or follow-up genetic testing on the tissue passed after a miscarriage). However, molar pregnancies can cause síntomas like:
- Vaginal bleeding during the first trimester
- Severe nausea and vomiting
- Cysts the size of grapes passing vaginally
- Pelvic pain or swelling
- Abnormally high HCG levels
- Hipertensión arterial
- Anemia
Molar pregnancies can be diagnosed during routine prenatal tests such as an ultrasound. A uterine ultrasound of a woman with an incomplete molar pregnancy will show an embryo, but also many fluid-filled sacs (trophoblasts) instead of the placenta that should have formed. A complete molar pregnancy will show the same trophoblasts, but with no embryo developing.
HCG levels can also be an indicator, as oftentimes molar pregnancies produce abnormally high levels of hCG. However, this cannot be judged by an at-home pregnancy test, and will need the specificity of a blood test in order to determine whether or not the hCG levels are too high or not.
Treatment for molar pregnancies
Unfortunately, there is no way to ‘fix’ or ‘save’ a molar pregnancy. With a complete molar pregnancy, there is no embryo to save, and with an incomplete molar pregnancy, the extreme chromosomal abnormalities mean the developing embryo is nonviable, and will eventually die in utero.
For the health of the mother, it is vital to remove the trophoblastic tissue (those fluid-filled cysts which were meant to be the placenta) with either medication or surgery. Leaving the tissue in the uterus could potentially cause those cells to become cancerous (more on this later), so todos of the cells must be removed.
Medicación
There are two options for removing the trophoblastic tissue: either medication or surgery. The medicación used for removal of a molar pregnancy allows the trophoblastic tissue to pass through the vagina.
Surgery: D&C or D&E
It’s more common, however, to undergo cirugía to remove the tissue from a molar pregnancy. Your healthcare practitioner may recommend that you receive dilation and curettage (D and C) or dilation and suction evacuation (D and E). In either of these cases, you would receive an ultrasound after the procedure to make sure all of the abnormal cells were removed.
Histerectomía
In some cases, a doctor may recommend a hysterectomy in the case of a molar pregnancy. This is typically recommended if you are having a lot of excess bleeding that cannot be controlled, or if you currently have a uterine issue such as fibromas.
Potential complications of molar pregnancies
While the prognosis after treatment for molar pregnancies is generally good, there is a chance for complications if all of the trophoblastic cells are not removed,
Gestational trophoblastic neoplasia (GTN)
If not all of the trophoblastic cells are removed, there is a chance of gestational trophoblastic neoplasia (GTN), which is when the remaining cells continue to grow and develop. This is a more common complication of a complete molar pregnancy than of an incomplete molar pregnancy, and it is typically diagnosed when there is still a high level of the hCG hormone after the molar pregnancy has been removed.
Choriocarcinoma
GTN growths can sometimes be cancerous, which is called choriocarcinoma. This type of cancer is fast-growing and can spread to other organs. It usually causes vaginal bleeding and pelvic pain, and will also be indicated by a high hCG test after the removal of a molar pregnancy. Chemotherapy is the main method of treatment and is usually effective.
Mole
GTN can also be noncancerous, and simply continue to multiply the trophoblastic cells as they form a mass (called a mole) that grows deeper into the muscle layer of the uterus. This also causes vaginal bleeding, and if it is persistent, may also be treated by chemotherapy. In some cases, a doctor may recommend a hysterectomy in the case of GTN (whether or not it is choriocarcinoma).
How likely are complications?
While a molar pregnancy and its complications can seem scary, only about 13% of complete molar pregnancies and 1% of incomplete molar pregnancies will lead to GTN. The chance of a molar pregnancy leading to choriocarcinoma is even more rare — only 5% of GTN cases lead to choriocarcinoma, which amounts to about 7 in every 100,000 pregnancies. If you’ve experienced a molar pregnancy, it is essential to make sure your doctor checks your HCG levels and performs an ultrasound after treatment, to ensure that all of the tissue has been removed.
The bottom line on molar pregnancies
It is somewhat rare to experience a molar pregnancy, and while you can notice symptoms, it is more likely to discover the existence of a molar pregnancy during an ultrasound. A molar pregnancy can not be “fixed” and the tissue must be removed, either by medication or surgery. Without proper diagnosis and treatment, molar pregnancies can turn into cancer (which is another example of why it’s so important that all women, no matter whether they plan to continue their pregnancies or not, should receive ultrasound confirmation during early pregnancy of a normal, intrauterine pregnancy). One piece of good news is that having a molar pregnancy does not affect your chances of getting pregnant and having future children.
