Pregnancy establishment and early embryonic survival in lactating Holstein cows are differentially constrained by parity, body reserves, and embryo transfer.

Pregnancy success in lactating dairy cows depends on both pregnancy establishment and maintenance during early gestation. This study quantified the independent and combined associations of parity, days in milk at service (DIM), body condition score (BCS), milk yield, service number, and reproductive technology, artificial insemination (AI) or embryo transfer (ET), with pregnancy at 28 days (PR28) and embryonic and early fetal loss between days 28 and 50 of gestation (EEFL) in lactating Holstein cows managed under commercial conditions. After exclusion of heifers and incomplete records, 9178 services from 1818 cows and 2975 lactation episodes were analyzed. Clustered multivariable logistic models accounting for repeated services within cow were fitted separately for PR28 and EEFL. Advancing parity was consistently associated with lower PR28 and higher EEFL, indicating that parity was the main adverse determinant across both reproductive stages. Higher BCS was associated with greater PR28 and lower EEFL. Embryo transfer was associated with higher PR28 than AI, but also with greater EEFL, indicating improved early establishment followed by reduced early gestational stability. DIM ≤79 days favored PR28, whereas DIM was not independently associated with EEFL. Service number was not independently associated with either endpoint after adjustment. In AI-only sensitivity analyses, the main effects of parity and BCS on PR28, and of parity on EEFL, remained directionally consistent. Repeat-breeder lactation episodes, defined descriptively as cow-by-parity units with at least three services, showed lower PR28 and higher EEFL than non-repeat-breeder episodes. Overall, these findings indicate that reproductive efficiency in lactating Holstein cows is structured by two linked but biologically distinct stages: pregnancy establishment and early pregnancy maintenance. Parity, body condition, and reproductive technology were the main variables shaping reproductive vulnerability under commercial dairy conditions.