What is triploidy?
Triploidy is a rare chromosomal abnormality in which fetuses are born with an extra set of chromosomes in their cells.
One set of chromosomes has 23 chromosomes. This is called a haploid set. Two sets, or 46 chromosomes, are called a diploid set. Three sets, or 69 chromosomes, are called a triploid set.
Typical cells have 46 chromosomes, with 23 inherited from the mother and 23 inherited from the father.
Triploidy occurs when a fetus gets an extra set of chromosomes from one of the parents. Triploidy is a lethal condition. Fetuses with the abnormality rarely survive to birth. Many are spontaneously miscarried during the first trimester. Others are stillborn before reaching full-term. The few infants that do survive to term have multiple severe birth defects. Some common defects include:
- growth retardation
- heart defects
- neural tube defects, such as spina bifida
Infants born with triploidy do not typically survive more than a few days after delivery.
Children born with mosaic triploidy usually survive for many years after birth. Mosaic triploidy occurs when the amount of chromosomes isn’t the same in every cell. Some cells have 46 chromosomes, while others have 69 chromosomes.
Trisomy is a condition similar to triploidy. It occurs when only certain pairs of chromosomes (the 13th, 18th, and 21st chromosomes being the most common) get an extra chromosome in every cell.
The most common types of trisomy are:
- trisomy 13, or Patau syndrome
- trisomy 18, or Edwards syndrome
- trisomy 21, or Down syndrome
These extra chromosomes also cause problems in physical and mental development. However, a higher number of children born with trisomy live to adulthood with long-term, full-time care.
Triploidy is the result of an extra set of chromosomes. This can occur when two sperm fertilizing one normal egg or a diploid sperm fertilizes a normal egg. It can also occur when a normal sperm fertilizes an egg that has an extra set of chromosomes.
Two different kinds of abnormal fertilization can cause triploidy:
- digynic fertilization, in which the mother provides the extra set of chromosomes. This usually occurs when the mother has a diploid ovum, or egg cell, that’s fertilized by a single sperm.
- diandric fertilization, in which the father provides the extra set of chromosomes. This usually occurs when one of the father’s diploid sperm (or two sperm at one time, in a process known as dispermy) fertilize the ovum.
Digynic pregnancies are more likely to end when the baby is closer to full-term. Diandric pregnancies are more likely to result in spontaneous miscarriages early on.
A partial molar pregnancy (or hydatidiform mole) can cause triploidy and is a nonviable pregnancy. It can also cause many complications, including cancer and an increased risk of another molar pregnancy.
Triploidy occurs in 1-3% percent of all conceptions, according to the National Organization for Rare Disorders. There aren’t any risk factors. It’s not more common in older mothers like other chromosome abnormalities, such as Down syndrome. Couples who experience one pregnancy with triploidy aren’t at higher risk for it in future pregnancies. Triploidy is also not hereditary.
Pregnant women carrying fetuses with triploid syndrome may have preeclampsia. Symptoms of this condition include:
- albuminuria, or high levels of the protein albumin in the urine
- edema, or swelling
Triploidy’s physical effects in a fetus depend on whether the extra chromosomes came from the father or the mother. Paternally inherited chromosomes can cause small heads and an enlarged, cyst-filled placenta. Maternally inherited chromosomes can cause severe growth problems, an enlarged head, and a small placenta without cysts.
Infants with triploidy who reach the full-term stage often have one or more birth defects after delivery, including:
- a cleft lip and cleft palate
- heart defects
- limb abnormalities, such as toe and finger webbing
- kidney defects, such as a cystic kidney
- neural tube defects, such as spina bifida
- wide-set eyes
A chromosome analysis (karyotype) test is the only test that can confirm a triploidy diagnosis. This test counts the number of chromosomes in a fetus’s genes to determine the existence of extra chromosomes.
To perform this test, a doctor needs a tissue sample from the fetus. Because the fetus and placenta come from the same fertilized egg, a doctor can use a placenta sample for testing.
An amniocentesis can also be used to diagnose triploidy. The doctor extracts amniotic fluid and then analyzes the sample for signs of abnormal chromosomes.
Triploidy may be suspected prenatally, or before the baby is born, if a maternal serum screening test is ordered and has certain abnormalities. This blood test, typically performed during the second trimester of pregnancy, is not designed to detect the condition. However, unusual test results such as too much or too little of certain proteins in the blood, can alert an obstetrician to potential problems. More tests may be needed if a serum screening test has abnormal results.
Triploidy can also be suspected during an ultrasound, which is commonly performed during pregnancy to examine the fetus. This test can help the doctor see if a fetus has certain anatomic abnormalities.
Triploidy cannot be treated or cured. Pregnancies that last until the baby is delivered are rare. If an infant does survive, the baby usually receives palliative care. Medicinal and surgical treatments are not used because of the ultimately lethal nature of the condition.
If doctors discover triploidy while a woman is pregnant, she can terminate the pregnancy or carry it to term or until a spontaneous miscarriage occurs. If she chooses to carry the baby to term, she should be monitored closely for complications caused by triploidy, including:
- preeclampsia, which can be life-threatening
- choriocarcinoma, a type of cancer rarely caused by tissue left over from a partial molar pregnancy
Losing your baby to triploidy can be difficult, but you don’t have to confront these feelings alone. Seek out support groups, online forums, or discussion groups to talk about your experiences. Thousands of other women have also had to face a situation similar to yours and have sought out help and support in these groups.
Carrying a baby with triploidy doesn’t increase your chances of another similar pregnancy, so it’s safe to conceive another child without having to worry about a higher risk of another baby with triploidy. If you do decide to have another child, continue to seek prenatal care and testing to make sure that your baby receives the best care possible.