BackgroundGenetic testing is required for the registration of foals of most equine breeds.ObjectivesTo describe two clinical cases of marked delayed embryonic development or delayed fertilisation in pregnancies generated by embryo transfer.Study designCase report.MethodsDonor mares were inseminated with semen from one stallion during one oestrous cycle and semen from a different stallion on the subsequent oestrous cycle. Embryo(s) were collected 8 days after ovulation during the second oestrous cycle and transferred into synchronised recipient mares. Genetic testing was performed to determine parentage of the two foals.ResultsFor both foals, DNA parentage testing excluded the second stallion as the genetic sire and confirmed that the first stallion, whose semen was inseminated on the previous oestrous cycle, was the actual genetic sire.Main limitationsRare event in horses; two clinical cases are described.ConclusionsIt is hypothesised that either marked delayed embryonic development or extended sperm survival occurred in the donor mares. Without genetic testing, parentage assignment based solely on breeding records would have been incorrect.