++
Key Points
Occurs in 1% to 2% of clinically recognized conceptions but only 1/10,000 livebirths. Not associated with advanced maternal age.
Most cases are 69, XXY (60%) or 69, XXX (37%); only 3% of cases are 69,XYY.
In the type I phenotype the fetus is relatively well grown with a large cystic placenta.
In the type II phenotype the fetus is markedly growth restricted with a disproportionately large head and a small, noncystic placenta.
One third of cases survive beyond 15 weeks and are associated with abnormal maternal serum screen results.
The longest postnatal survival for an affected infant was 10frac12 months.
++
Triploidy is defined as the presence of three complete sets of the normal haploid genome found in gametes. Triploidy occurs in one of three ways: (1) failure of division in meiosis I or II in the spermatocyte, resulting in an extra set of paternal chromosomes (diandry); (2) failure of division in meiosis I or II in the oocyte, resulting in an extra set of maternal chromosomes (digyny); or (3) double fertilization of a normal haploid ovum (dispermy). Using special chromosome-staining techniques, it has been shown that the extra set of chromosomes is paternal in origin in three quarters of cases (Jacobs et al., 1982). Most of the paternally derived cases are due to dispermy (Kajii and Niikawa, 1977). The distribution of karyotypes seen in triploid conceptuses is 69, XXY (60%), 69, XXX (37%), and 69, XYY (3%) (Jacobs et al., 1982). The infrequent occurrence of XYY suggests that either diandry must be uncommon or that XYY carries a disadvantage for survival.
++
Triploid fetuses can present with a broad spectrum of phenotypic features that can range from near normalcy to multisystem involvement. McFadden and Kalousek (1991) have described two distinct fetal and placental phenotypes that appear to correlate with the parent of origin in the extra set of chromosomes. In the type I phenotype, the fetus is relatively well grown and has a proportionate head size or microcephaly. The placenta is large, with cystic changes. At the microscopic level there is trophoblastic hyperplasia, scalloping of the villus surface, and focal hydropic change (Sergi et al., 2000). These cases are generally associated with diandry (Figure 132-1A). The type II phenotype predominates in the cases diagnosed after the first trimester; it consists of a markedly growth-restricted fetus with a disproportionately large head and a small, noncystic placenta (Figure 132-1B). In their series, McFadden and Kalousek were able to demonstrate that one case of type II triploidy originated from an error in maternal gametogenesis.
++