A total of 246 357 measurements of birth (BW) and probe (PW) weights of purebred Yorkshire and Landrace pigs were used to compare the fitting of two alternate models including either common-litter effect or cross-fostering group effect to account for common environmental variation. PW, is a live ultrasonic weight measurement taken when the pigs are 100 ± 30 kg, following national standards. The common-litter effect was defined as piglets born into the same litter, and the group effect was used to account for cross-fostering, and defined as the effect common to piglets raised by the same nurse-sow, regardless of whether that piglet was born into that litter or not. It was found that the cross-fostering group explained 5% more environmental variation in BW when compared with the common-litter effect, indication that BW is a criterion in cross-fostering. Cross-fostering also explained 1% more environmental variation in PW in both the Yorkshire and Landrace. When the cross-fostering group effect was included in place of the common-litter effect, the direct and maternal genetic heritability estimates were similar, but residual variances were reduced. This study advanced the understanding of the effects of cross-fostering on PW, its association with BW and its implications in modern pig breeding programs.