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Is monosomy 21 rare? Seven early miscarriages including one mosaic 45,XX,‐21/44,X,‐21 in a single study population

True monosomy 21 has rarely been reported, with few patients either liveborn or miscarried, described in the literature [Joosten et al., 1997; Chang et al., 2001; Mori et al., 2004; Cheng et al., 2006; Manolakos et al., 2010]. This study describes seven patients with monosomy 21 detected in our referral group of 2,042 miscarriage samples, including one double aneuploidywith concurrentmonosomy X. Correct identification of monosomy 21 can be problematic and many of the patients initially reported in the literature were identified when short contracted chromosomes were the norm, prior to the synchronization of blood cultures which enabled improved band resolution. In addition, G-banding was unreliable prior to the early 1980s [Hertz et al., 1993]. Derivative chromosomes containing sequences of chromosome 21may bemissed due to similarity of banding patterns (particularly for chromosome 5p), or insertion of the sequences from chromosome 21 into another chromosome. Accurate identification would be difficult to achieve with poor chromosomal preparations. Six patients with a prenatal diagnosis ofmonosomy 21 have been reported: Five with complete monosomy, ending with two in utero fetal demise [Joosten et al., 1996; Chang et al., 2001], one elective pregnancy termination [Manolakos et al., 2010], two premature birth followed by death on the first day of life [Mori et al., 2004; Cheng et al., 2006]; while the sixth patient exhibiting mosaic monosomy 21 was electively terminated after 19 weeks [Ghidini et al., 1993]. A seventh patient, which was the product of an intracytoplasmic sperm injection (ICSI) conception identified after a spontaneous loss, was reported by Ma et al. [2001]. The single chromosome 21 was of paternal origin, supporting a maternal meiotic error. The seven patients with monosomy 21 described here were spontaneous miscarriage samples referred to our laboratory over a period of 14 years with no preselecting variables differing from other early miscarriage patients. All miscarriage samples were handled according to routine cytogenetic protocols [Priest, 1997]. Five of these patients were confirmed monosomy 21 using the Vysis Aneuvysion 13/21 probe set (Fig. 1), with a single hybridization signal observed for the chromosome 21 probe, and two signals observed for the chromosome 13 probe. Five of the seven pregnancies were spontaneous. The pregnancy type and previous obstetric history was not known at the time of analysis. The maternal age for the seven patients varied from 25 to 38 years old, with an average of 32 years. The gestational age ranged from 6 to 14 weeks with an average of 8.8 weeks. Investigation into the clinical background of these patients showed that five patients have had subsequent normal births, while one patient has a current ongoing pregnancy. Two of the patients had previous normal births. Another patient had a previous termination and no information on previous pregnancies was available for the other four mothers. Four of the monosomy pregnancies were spontaneous, another was spontaneouswith ovulation induction, onewas an IVF conception and information was not available for the last patient. These data are summarized in Table I. The data reviewed byManolakos et al. [2010] reported five male patients with full monosomy 21, three of which progressed beyond 30 weeks gestation. In contrast, five of the six patients in our cohort