Triploidy is a rare lethal chromosome abnormality caused by the presence of an entire extra set of chromosomes. A fetus with triploidy has 69 chromosomes, rather than 46. The majority of fetuses with triploidy are spontaneously miscarried during pregnancy. Those that survive until birth will have severe growth retardation and multiple birth defects. This condition is incompatible with life.
Triploidy is a devastating condition caused by having a full extra set of chromosomes. This extra set of chromosomes causes a variety of serious birth defects, placental problems, and severe growth problems in a fetus. In fact, most pregnancies in which the fetus has triploidy end in a spontaneous miscarriage. Very few infants with triploidy survive to term. Of those that do, most are stillborn and those that are born alive usually die shortly after birth. Infants with this lethal condition are generally small due to severe intrauterine growth retardation (IUGR) and they have multiple birth defects, including facial abnormalities, such as cleft lip, heart defects, neural tube defects (spina bifida), and other serious birth defects. The exact pattern of abnormalities depends on whether the extra set of chromosomes was inherited from the mother or from the father. Unfortunately, there is nothing that can be done to treat or cure triploidy.
Triploidy is a chromosomal disorder. Chromosomes are the structures that contain all of the body's genes (the basic unit of inheritance). Humans have 46 chromosomes in every cell of their body, with the exception of their sperm and eggs cells, which contain only 23 chromosomes. When a sperm and an egg unite at conception, the resulting fertilized egg will have 46 chromosomes: half from the mother and half from the father. This fertilized egg will continue to develop and grow into a fetus and, eventually, into a live-born infant with 46 chromosomes in every cell of their body.
Of these 46 chromosomes, 22 pairs (or 44 chromosomes) are called autosomes (or non-sex chromosomes) and the twenty-third pair is the sex chromosomes. Women have two X chromosome (46,XX) and men have an X and Y chromosome (46,XY). Fetuses with triploidy can be 69,XXX (female), 69,XXY (male), or 69,XYY (male). Twenty-three chromosomes (or one set) is referred to as a haploid set of chromosomes, 46 chromosomes (or two sets) is referred to as a diploid set of chromosomes, and 69 chromosomes (or three sets) is referred to a triploid set of chromosomes. A fetus with triploidy has three haploid sets of chromosomes.
Triploidy occurs in several different ways. The extra set of chromosomes can be inherited from the father (paternal inheritance) or they can be from the mother (maternal inheritance). The most common mechanism for triploidy is the fertilization of a single egg by two sperm. This results in a triploid egg with two sets of paternal chromosomes and one set of maternal chromosomes. This accounts for about 60% of cases of triploidy. The other mechanism is an error in cell division in which an egg cell ends up with 46 chromosomes instead of 23. This egg with 46 chromosomes is fertilized by a sperm with 23 chromosomes, resulting in a fertilized egg with 69 chromosomes, which then has two sets of maternal chromosomes and one set of paternal chromosomes. This mechanism is responsible for about 40% of cases of triploidy. The physical effects of triploidy differ depending on whether the extra set of chromosomes was inherited from the mother (maternally inherited) or from the father (paternally inherited).
In pregnancies in which the extra set of chromosomes is maternally inherited, the fetuses tend to be well-formed, with a small head (microcephaly). The placenta in these pregnancies is generally enlarged and cystic (filled with cysts). This type of placenta is often referred to as a hydati-form mole and the pregnancy as a whole may be referred to as a partial molar pregnancy. In pregnancies in which the extra set of chromosomes is paternally inherited, the fetuses have severe growth retardation, a large head, and a small, non-cystic placenta.
The physical birth defects seen in triploidy are variable. All fetuses will have some of these birth defects, but very few will have all of them. The birth defects most commonly seen are heart defects, cleft lip, neural tube defects, kidney malformation, abnormal genitalia (males), and defects in the abdominal wall. Regardless of the presence or absence of these birth defects, triploidy is incompatible with life.
Triploidy is a sporadic (or accidental) event. It is not caused by anything that a parent may or may not have done. Unlike some other chromosome abnormalities (trisomy 21 or Downs syndrome), triploidy is not associated with a mother's age. This means that there is not an increased risk for triploidy for an older mother to have a pregnancy. Because triploidy is an accidental event, there is no increased recurrence risk in future pregnancies. A woman who has had one triploid pregnancy is not at any increased risk to have a second one.
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