Sonographic Assessment of Fetometric Parameters in Pigs of Different Prolific Genotypes for Gestational Age Estimation

The slaughter of pregnant sows remains a relevant concern in modern swine production, with prevalence rates reported from 1.5–13% in Europe. Considering fetal sensitivity during late gestation and legal restrictions on transport and slaughter, reliable assessment of fetal age is of considerable practical, ethical, and legal relevance. In this study, 70 pregnancies from low-prolificacy (purebred German Saddleback) and medium-to-high prolificacy genotypes (purebred German Landrace and Duroc × German Landrace hybrids) were repeatedly examined using transabdominal ultrasonography, with a total of 15 examinations per pregnancy. Seven fetometric parameters—rosto-occipital distance, bi-parietal distance, orbital distance, sternum length, thorax diameter, body diameter, and crown–rump length—were measured in vivo, assessing two fetuses per pregnancy and calculating mean values to account for intra-individual variation. Parameter feasibility varied across gestation: during early gestation (gestation days 38 and 40), orbital distance, sternum length, and crown–rump length could be reliably measured; in mid-gestation, all seven parameters were measurable, whereas in late gestation (from gestational day 87 onward), crown–rump length was no longer measurable, and the remaining six parameters remained assessable for gestational age estimation. Crown–rump length (CRL) increased from a median of 3.2 cm (range 1.9–4.2 cm) at day 38 to 16.3 cm (range 14.0–18.2 cm) at day 77, representing the most practical parameter for determining the stage of gestation. Litter size had no significant effect on fetometric growth, except for a weak correlation with thorax diameter at day 77, and parity showed no measurable influence on any parameter. The results show that fetometric values in modern sow genotypes are smaller than those reported in earlier literature, highlighting the need for updated gestational age assessment. These findings provide practical guidance for gestational age estimation, supporting the enforcement of animal welfare legislation and potentially contributing to a reduction in the slaughter of highly pregnant sows.