Changes in the genital mucus around the oestrus are used by different diagnostic methods to determine optimal fertilisation time. In the current study, the authors evaluated the different arborisation patterns found in vestibular mucus, and also established its relationship with vestibular resistance changes during oestrus. Thirty multiparous sows were checked by transrectal ultrasonography to determine ovulation time every 12 hours. Vestibular resistance was measured with a commercial resistance probe, and vestibular mucus ferning was also evaluated every 12 hours during the oestrus. Significant changes (P < 0.05) in vestibular resistance were detected, registering high variation among individuals. Maximum resistance data was reached between 12 and 24 hours after ovulation time in 83 per cent of the sows. Crystallisation samples were classified into three different patterns according to the fern-like crystal degree. Arborisation peak occurred from 48 to 36 hours before the moment of ovulation, when vestibular resistance values increased gradually. In the optimal insemination moment, vestibular resistance increased significantly (P < 0.05) and vestibular mucus showed a low crystallisation pattern (P < 0.05). Combining several methods to measure genital mucus changes may predict the ovulation time and the best insemination moment.