The objective of this study was to examine the genetic basis of homogeneity in piglets and the genetic correlations with litter size and growth during lactation. Genetic parameters for variation in piglet BW within litters at birth and at 3 wk of age, and in the BW of individual piglets at 3 wk (BW3) were estimated from the Norwegian Landrace nucleus population. Data on BW3 were collected from 146,572 piglets from 14,045 litters in 58 herds. Body weight at birth and at 3 wk of age was recorded for 13,318 piglets from 5 nucleus herds. Litter data were evaluated using multivariate trait models. The heritability estimates for the SD of BW at birth and at 3 wk (SDBW3) were in agreement with the literature (0.10 and 0.08, respectively). The genetic correlation for the number of piglets born alive and the mean BW at 3 wk was negative (−0.40), and the correlation of number of piglets born alive with SDBW3 was close to zero (−0.03). The genetic correlation between the SD of BW at birth and SDBW3 was moderate (0.51). The mean BW at birth was genetically correlated with mean BW at 3 wk (0.59) but was independent of SDBW3 (0.08). The estimates of direct and maternal heritability for BW3 were 0.03 and 0.07, respectively, and the genetic correlation between the 2 components was negative (−0.43). The genetic correlation of SDBW3 with the maternal effect on BW3 was positive and strong (0.66), whereas a negative correlation was found with the direct effect on BW3 (−0.18). These results suggest that it is possible to select for mean BW at birth without an increase in within-litter heterogeneity at 3 wk of age. A more efficient strategy would be to consider both the direct and the maternal effects on BW3 in the genetic evaluation, together with SDBW3. Thus, it is possible to avoid the increase in within-litter heterogeneity that would occur as a result of selection performed at 3 wk on a litter trait such as mean BW.

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