Case Report: Contralateral Pregnancies Are Carried to Term in Heifers but Not in Lactating Holstein Cows.

The following case report describes two pregnancies contralateral to the side of the corpus luteum (CL) that were carried to term after artificial insemination (AI) in Holstein heifers. Pregnancy diagnosis was performed by ultrasonography at 32, 46 and 74 days after timed AI. Both heifers had the pregnancy located within the right uterine horn and only one CL on the left ovary. Another eight lactating cows with contralateral pregnancies were followed around the same time; however, all lost the pregnancies between 32 days after AI and 74 days after AI. Pregnancies contralateral to the side of the CL have been described after embryo transfer in heifers, but to our knowledge, this is the first report of contralateral pregnancies after AI that were carried to term in heifers. In previous reports, all seven cows with contralateral pregnancies lost the pregnancies by 49 days of gestation. It is not clear why contralateral pregnancies can be successful in heifers but not in lactating Holstein cows. In summary, we confirmed that contralateral pregnancies were not viable in lactating Holstein cows but that successful contralateral pregnancies after AI are possible in Holstein heifers. The mechanism for this finding deserves further investigation.