Non- Disjunction of Chromosome 21 in the Young Mother at Conception

Background: Maternal age influences the type of chromosomal anomaly in Down syndrome. The older age is associated with non-disjunction while the younger age is associated with translocation of chromosome 21. Aim of the Work: To study the cytogenetics of children with Down syndrome born to the young mother who presented to Pediatrics Genetic Unit at Cairo University Children Hospital during one year (2019), and compare with those born to an older mother. Materials and Methods: This descriptive retrospective study analyzed records of 210 children with Down syndrome who presented to Pediatrics Genetic Unit at Cairo University Children Hospital during 2019. Results: Among the total 210 studied children with Down syndrome, 21(10%) were born to young mothers aged 21 years or less at conception. Seven (33.34%) were males and 14 (66.6%) were females. Twenty (95.2%) were first born to a young mother, only 1 was a second born. Karyotyping proved that 6 (28.6%) had Robertsonian translocation of trisomy 21, and 15 (75.4%) had non-disjunction. Only 1 (4.76%) of children born to younger mothers had associated cardiac anomalies, while among those born to the older mother 40 (21%) had associated anomalies, of them were cardiac 36 (19.04%), hydrocephalus in one (0.5%), and ambiguous genitalia in 3 (1.58%) (p=0.072). First born trisomy 21 were 109 (51.9%) among the whole studied cohort, with a mean maternal age +/- SD of 30.2 years +/- 7.35 years, their karyotyping was non-disjunction, Robertsonian translocation and mosaic in 100, 5 and 3 respectively. Cardiac anomalies were encountered among 18 (16.5%). Conclusion: Children born with trisomy 21 to mothers younger than 21 at conception are mostly due to non-disjunction. First born children with trisomy 21 comprised half the studied cohort of trisomy 21. Further studies to define preventable causes of non-disjunction in the young mother, are needed to reduce the incidence of Down syndrome.