Comparative sonographic fetometry in dairy cattle.

The aim of this study was to compare crown-rump length (CRL), trunk diameter and number of heartbeats of embryos and fetuses in heifers and pluriparous cows and also to compare the growth pattern of these structures and their correlation with fetal age. Six nulliparous heifers and seven pluriparous cows (third to seventh of gestation) of the Holstein breed were selected and impregnated. Ultrasonographic measurements were performed twice weekly during days 26-80 of their pregnancies. By day 80, there was no significant difference in CRL and trunk diameter of embryos and fetuses of heifers and pluriparous cows. All of the measurements had a highly significant correlation with fetal age (P 0.0001) except for pluriparous cows in which the fetal heartbeat was significant at p 0.05. The correlation coefficients (CC) of the CRL, trunk diameter and number of heartbeats in the fetuses of heifers and cows were as follows: 0.97,0.98, -0.74 and 0.97, 0.98, -0.23, respectively. The growth pattern of the CRL and trunk diameter was quadratic in heifers and cows. Also the heartbeat in the fetuses of heifers decreased progressively in a quadratic pattern from day 31 through day 80. However, heartbeat in the fetuses of cows changed in an approximately stable or intermittent range. Moreover, standard error of heartbeats in the fetuses of cows was high. It was concluded that all of the measurements in heifers and cows are suitable for estimation of fetal age, except for heartbeat in the fetuses of pluriparous cows. This measurement is not practically recommended, due to a low CC and high standard error. In addition, there was no significant difference between the CRL and trunk diameter in heifers and cows until day 80 and this difference probably will become more pronounced in late pregnancy.