Carriership of reciprocal chromosome translocation (RCT) in a family may be the reason for malformation at birth, stillbirth, early neonatal death, and miscarriage due to unbalanced karyotype (monosomy/trisomy). The size of chromosome segments determined by the breakpoint position, kind of chromosome involved and the carrier’s gender may influence the probability rate for each category of the unfavorable pregnancy outcome in the family of the carrier of a particular RCT. Until now, the literature lacks reports on the risk values for particular forms of pregnancy outcomes in case of single segment imbalance, both the short (p) and the long arm (q) of chromosome 20. Objective: The aim of the study was to evaluate individual risk rates for unbalanced offspring at birth for single segment imbalance in the form of trisomy/monosomy and a separate evaluation risk figures for different pregnancy outcomes, depending on the size of the involved chromosome segment, its origin and carrier gender in families of RCT carriers involving chromosome 20 (RCT-20). In addition, practical application of the obtained results in the family with unique RCT t(13;20)(q14.1;p11.21) carriership has been shown. Material and methods: Total empirical data of 50 families (219 pregnancies) were collected from 19 pedigrees of RCT-20 carriers coming from different collections of RCT and available references. Cytogenetic studies were performed by GTG technique. The probability rates of particular type of pathology related to the total number of pregnancies after ascertainment correction have been done by segregation analysis according to the method of Stengel-Rutkowski and Stene. Results: The probability rate for unbalanced offspring at birth for carriers of RCT-20p was calculated as 5.5±1.8% (9/164) (medium risk). Considering parental gender of the carrier for maternal (MAT) and paternal (PAT) carriers, the probability rate values were similar i.e. 4.8±2.3% (4/84) and 4.9±2.8% (3/61), respectively. The risk figures for stillbirth/ early neonatal deaths were found as 0.6±0.6% (1/164) (low risk), but separately for MAT and PAT carriers they were: 1.2±1.2% (1/84) and <0.8% (-/61), respectively. Risk figures for miscarriages were estimated as 28.6±3.5% (47/164) (high risk), with 32.1±5.1% (27/84) for maternal carriers and 32.7±6% (20/61) for paternal carriers. The risk figures for unbalanced offspring at birth for carriers of RCT-20q were calculated as about 2.6% (0/20) (low risk), for stillbirth/early neonatal death about 2.6% (-/20) (low risk) and for miscarriage 50±11.2% (10/20) (high risk). Conclusions: 1. The probability rates for unbalanced offspring at birth and for different categories of unfavorable outcomes show differences depending on the origin and the size of chromosome 20 segment. 2. There are no differences in the value of risk figures for particular form of pregnancy pathology in relation to carrier’s gender .