Litter characteristics at birth were re- corded in 4 genetic types of sows with differing maternal abilities. Eighty-two litters from F1 Duroc × Large White sows, 651 litters from Large White sows, 63 lit- ters from Meishan sows, and 173 litters from Laconie sows were considered. Statistical models included ran- dom effects of sow, litter, or both; fixed effects of sow genetic type, parity, birth assistance, and piglet sex, as well as gestation length, farrowing duration, piglet birth weight, and litter size as linear covariates. The quadratic components of the last 2 factors were also considered. For statistical analyses, GLM were first considered, assuming a binomial distribution of still- birth. Hierarchical models were also fitted to the data to take into account correlations among piglets from the same litter. Model selection was performed based on deviance and deviance information criterion. Finally, standard and robust generalized estimating equations(GEE) procedures were applied to quantify the impor- tance of each effect on a piglet's probability of stillbirth. The 5 most important factors involved were, in decreas- ing order (contribution of each effect to variance reduc- tion): difference between piglet birth weight and the litter mean (2.36%), individual birth weight (2.25%), piglet sex (1.01%), farrowing duration (0.99%), and sow genetic type (0.94%). Probability of stillbirth was greater for lighter piglets, for male piglets, and for pig- lets from small or very large litters. Probability of still- birth increased with sow parity number and with far- rowing duration. Piglets born from Meishan sows had a lower risk of stillbirth (P < 0.0001) and were little affected by the sources of variation mentioned above compared with the 3 other sow genetic types. Standard and robust GEE approaches gave similar results de- spite some disequilibrium in the data set structure highlighted with the robust GEE approach.