This study aimed to estimate heritabilities of piglet birth-weight traits and to investigate the relationships between piglet birth-weight and litter-size traits using field data from Czech Large White and Landrace sows. Data including 86 241 piglets from 5 536 litters were analysed using repeatability, genetic, and genomic models. The investigated piglet-weight traits included arithmetic mean birth weight (MBW), within-litter standard deviation (SDBW), coefficient of variation in birth weight (CVBW), and the proportion of small piglets in the litter (SMALL). Heritability rates of piglet-weight traits were low, with the highest estimates for MBW (0.06-0.10) and SMALL (0.08-0.17), depending on the model used, while the other within-litter variability traits showed heritability rates from 0.03 to 0.05. The heritability rates of litter-size traits, including the number of piglets born alive (NBA) and the number of piglets weaned (NW), were low and ranged from 0.05 to 0.07. The realised accuracies of prediction were moderate to high, ranging from 0.50 to 0.72 for piglet-weight traits and from 0.54 to 0.77 for litter-size traits, with ssGBLUP improving realised accuracy by 4-15% in comparison to BLUP. Genetic correlations estimated using BLUP and ssGBLUP indicated that litter size at birth (NBA) tended to be negatively associated with MBW and positively or weakly associated with piglet-weight variability traits (SDBW, CVBW, and SMALL), whereas NW tended to show positive correlations with MBW and negative correlations with variability traits. However, none of the correlations were statistically significant.