The objective of the present study was to evaluate environmental and dam factors associated with birth size of Holstein calves. Data of 1,594 births from dairy herds in Belgium and Germany were analyzed in a retrospective cross-sectional study. Immediately after birth, the birth weight of the calves was measured. On the next day, the heart girth (HG), withers height, and diagonal length of the dams and calves were measured. Parity, body condition score, gestation length (GL), and age at calving were recorded for all dams. For the cows, days open, lactation length, length of the dry period, and calving interval were also calculated. The magnitude and shape of the lactation that took place during gestation was quantified using the MilkBot model based on monthly milk weights. Using the same procedure, cumulative milk production from conception to drying off (MGEST) was calculated. After descriptive analyses, mixed models were used to identify factors that are significantly associated with the birth weight (most consistent measure of size at birth) of the calves born to both heifers and cows. Of the variables offered to the offspring birth weight model in heifers (n = 540), calf sex, season of calving, GL, HG, withers height, diagonal length, and age at calving were significant. The mean birth weight of the calves born to heifers was estimated to be 41.3 +/- 1.01 kg. In comparison to calves born to old (25.5 to 37.3 mo; n = 99) heifers, the birth weight was estimated to be 2.75, 3.29, and 2.35 kg heavier when the calves were born to very young (20.3 to <22 mo; n = 98), young (22 to <23.5 mo; n = 145), and standard aged (23.5 to <25.5 mo; n = 198) heifers, respectively. Of the variables offered to the offspring birth weight model in cows (n = 1,054), calf sex, season of calving, GL, parity, dry period, and MGEST were significant. The mean birth weight of the calves born to cows was estimated to be 44.1 +/- 0.99 kg. For cows having an identical HG, the birth weight of the calves was estimated to be 0.97 and 1.11 kg higher in cows with low (1,400 to <5,400 kg) and high (6,500 to <7,200) MGEST, respectively, compared with cows with very high (7,200 to 11,600 kg) MGEST. The decisive effects of age at calving in heifers and of high milk production levels during gestation in cows on the birth weight of their calves may provide a basis for developing managerial interventions to improve long-term health and productivity of the offspring.