Objectives were to evaluate effects of feeding a calcium salt rich in linoleic and trans‐octadecenoic acids (LTFA) on synthesis of prostaglandin F_2α based on its metabolite (PGFM), uterine involution and pregnancy rates in lactating dairy cows. Five hundred and eleven Holstein cows were blocked according to parity, body condition score and milk yield in the previous lactation. Primiparous and multiparous cows were randomly assigned to one of the two treatments consisting of calcium salt (2% diet dry matter) of either palm oil (PO) or LTFA from 25 days prepartum to 80 days of lactation. Cows were time‐inseminated at 70 ± 3 days postpartum. Feeding LTFA tended (p = 0.08) to decrease the incidence of puerperal metritis (15.1% vs 8.8%). Primiparous cows supplemented with LTFA showed larger increase in plasma PGFM concentration at day 1 postpartum (17018 vs 6897 pm). Pregnancy rate after first insemination tended (p = 0.07) to be greater at 27 days after insemination (37.9% vs 28.6%), and was greater (p = 0.05) at 41 days after insemination (35.5% vs 25.8%) for cows fed LTFA compared with PO. These results indicate that unsaturated fatty acids fed in a rumen inert form have the potential to modulate reproductive events and improve pregnancy rates in lactating dairy cows.