artificial insemination is important, both for its efficiency in pregnancy rate and for the number of piglets born by litter. Objective: To determine the impact of sperm concentration of ejaculate and rest days between ejaculates on rate of pregnancy and number of piglets born by litter using cooled pig semen. Methods: The study involved the use of 29 ejaculates from 19 boars, which were 229–757 day old, with 5 to 16 rest days between ejaculations, for inseminating 315 sows. The effect of motility, ejaculate volume, ejaculate sperm concentration and diluted sperm concentration, morphology, acrosomal integrity, cell membrane functionality, boar age, rest days between ejaculations, days of semen storage, farrowing, and wean-to-estrus interval were assessed on pregnancy rate and number of piglets born. Linear and logistic regression analyses were performed using SPSS. Results: None of the variables provided a satisfactory explanation for the variation in pregnancy rate and number of piglets born using linear regression. The probability of achieving a pregnancy rate ≥93% was 10.4 times higher when the ejaculates came from boars with ≥12.5 rest days, 9.7 times higher when these ejaculates had a sperm concentration ≥941 million sperm per mL, and 2.7 times higher when the females inseminated were in heat between 4 and 7 days after wean. Conclusion: The ejaculate of boars with more rest days and high sperm concentration had a greater probability of improving the pregnancy rate. The number of piglets remained unaffected.