Simple Summary Smaller than average sized pigs requiring extra time to reach target slaughter weight are often referred to as slow growing pigs. This subset of pigs poses a management challenge and may have economic implications for pig producers. The aims of this study were to investigate the effect of birth and weaning body weight on performance of grow-finisher pigs and to estimate cut-off values for birth and weaning body weight in order to identify slow growing pigs early in life. Pigs with low birth and weaning weight grew slower compared to pigs with higher birth or weaning weight, although feed conversion efficiency was similar for both groups. Pigs weaned at approximately 28 days weighing less than 3.7 kg would be identified as slow growing pigs within a batch. Moreover, a cut-off value of 1.1 kg of body weight at birth and 6.4 kg of body weight at weaning would allow pig producers to identify the pigs that are likely to reach target slaughter weight at 22 weeks of age. In conclusion, birth body weight does not always determine subsequent growth performance, and the cut-off values identified could be used on farm to design new management and nutritional strategies for slow growing pigs. Abstract This study aimed to (1) investigate the effect of birth and weaning body weight (BW) on performance indicators of grow-finisher pigs and (2) estimate birth and weaning BW cut-off values in order to identify slow growing pigs (SGP). Pigs (n = 144) were classified as SMALL (0.9 ± 0.13 kg) or BIG (1.4 ± 0.20 kg) at birth and re-classified as SMALL (5.4 ± 1.6 kg) or BIG (6.3 ± 1.91 kg) at weaning. Individual BW was recorded bi-weekly, and feed intake was recorded on a daily basis. Average daily gain (ADG) and feed intake, feed conversion ratio (FCR) and days to target slaughter weight (TSW) were calculated. SMALL–SMALL pigs had lower ADG ( p < 0.05) requiring 167.1 days (i.e., 14.2 extra days) to TSW ( p < 0.05) compared with BIG pigs at birth and/or weaning. However, FCR was similar between groups ( p > 0.05). Pigs weaned at <3.7 kg BW would likely be SGP. Pigs born at ≥1.1 kg BW or weaned at ≥6.4 kg BW are more likely to reach TSW at 22 weeks of age. The results suggest that birth BW might not be the best predictor for subsequent performance, as some small-born pigs were able to catch up with their bigger counterparts. The cut-off values identified could be used to design specific management and nutritional strategies for SGP.