Synchronizing oestrus in gilts is an important tool for pig producers to optimize the management of reproduction. Synthetic progestogens are used routinely in pig farms for this purpose, but there is a need for alternative non-hormonal breeding tools. Before puberty, gilts exhibit a “waiting period”, related to ovarian development and gonadotrophin secretions, during which an external stimulation, such as boar exposure, could induce and synchronize the first ovulation. Our aim was to characterize this “waiting period” and search for biomarkers of this period using non-invasive tools. During this period, an increase of urinary oestrone concentration has been observed, but urinary sampling is difficult in group-housed females. Salivary samples are non-invasive and easier to perform. Thus, the aim of this study was to evaluate steroid concentrations in the saliva of immature to pubertal gilts in order to search for biomarkers of the “waiting period”. Trans-abdominal ultrasonographies were performed over a period of 5 weeks on six 140-day-old Large White gilts until puberty detection. Urinary samples were collected for oestrone assay to detect the “waiting period”. Salivary samples were collected for steroidome analysis using gas chromatography coupled with tandem mass spectrometry to detect potential biomarkers of the “waiting period”. Urinary oestrone concentration increased 2 weeks before puberty. Steroidome analysis allowed the quantification of 28 steroids, 13 of them showing significant differences between weeks. Dehydroepiandrosterone concentrations decreased and estradiol-17 concentrations increased significantly just before the “waiting period”. These steroids could be biomarkers of this period. Present results confirm that non-invasive salivary samples could allow the identification of the physiological status of the gilts and the optimal time for application of the boar effect.