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1.1 TRIPLOIDY

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EPIDEMIOLOGY/GENETICS

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Definition Rare, lethal chromosomal abnormality. An entire extra haploid set of chromosomes results in 69 chromosomes instead of the usual 46.

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Epidemiology Of human conceptions, 1% to 2% are triploid, but most end in spontaneous abortion. Accounts for 20% of chromosomally abnormal spontaneous abortions. Very rare at birth, occurring in 1 in 10,000 live births.

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Embryology A complete extra set of chromosomes results in 69 XXX (digynic) or XXY (diandric). Sixty percent result from fertilization with two sperm; 40% result from fertilization of a diploid egg. However, more recent prospective information, at the time of first-trimester screening, suggests that digynic triploidy may be more common than diandric. Central nervous system malformations include hydrocephalus, holoprosencephaly, and neural tube defects. Hypertelorism, cleft lip/cleft palate, syndactyly of the third and fourth fingers, and congenital heart defects are typical features.

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Inheritance Patterns Sporadic

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Teratogens None

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Screening Serum levels of human chorionic gonadotropin and inhibin A may be extremely high in triploidy with dispermy as the cause. Very low levels of α-fetoprotein, estriol, inhibin A, and human chorionic gonadotropin (hCG) are seen in pregnancies in which fertilization of a diploid egg occurs. In the first trimester, screening, using nuchal translucency and serum markers will detect approximately 85% of fetuses with triploidy. Diandric triploids have an increased nuchal translucency, elevated hCG, and normal, or low, pregnancy-associated plasma protein A (PAPP-A). Digynic triploidy has a normal nuchal translucency, very low hCG, and very low PAPP-A. Diandric triploidy will generally look like an increased risk for trisomy 21, and digynic will generally look like a high risk for trisomy 18.

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Prognosis Lethal antenatally or in the newborn period. Rare mosaic cases survive with moderate-to-severe mental retardation.

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SONOGRAPHY

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Findings: Sonographic findings vary with the source of the extra chromosome.

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FREQUENT FINDINGS
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  • First-trimester thickened nuchal translucency

  • Intrauterine growth restriction

    • Asymmetric growth restriction with large fetal head is associated with digynic triploidy

    • There is early-onset symmetrical growth restriction.

    • Some fetuses with diandric triploidy have grown normally.

  • Ventriculomegaly

  • Holoprosencephaly

  • Micrognathia

  • Low-set ears

  • Single umbilical artery

  • Syndactyly of third and fourth fingers

  • Clubfeet

  • Oligohydramnios

  • Enlarged placenta with multiple cystic spaces; similar to a partial mole (diandric, XXY)

  • Small placenta (digynic, XXX)

  • Bilateral theca lutein cysts

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LESS FREQUENT
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  • Neural tube defects

  • Dandy-Walker malformation

  • Agenesis of the corpus callosum

  • Microphthalmia

  • Hypertelorism

  • Cystic hygroma

  • Coarctation of aorta

  • Hypoplastic left ventricle

  • Omphalocele

  • Pyelocaliectasis

  • Ambiguous genitalia

  • Nonimmune hydrops

    • Pleural effusion

    • Pericardial effusion

    • Ascites

    • Anasarca

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PREGNANCY MANAGEMENT

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Investigations and Consultations Traditional karyotyping with, or without, fluorescent in situ hybridization (FISH) analysis via amniocentesis or chorionic villi sampling (CVS) establishes the diagnosis. Once a cytogenetic diagnosis has been established, no further investigations or consultations are necessary. For patients in whom ...

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