Signs of impending calving in cows or heifers
The immediate signs that usually occur within 24 hours of calving would be relaxation of the pelvic ligaments and strutting of the teats. These can be fairly dependable for the owner that watches his cows several times a day during the calving season. The casual observer or even the veterinarian who is knowledgeable of the signs but sees the herd infrequently cannot accurately predict calving time from these signs. The relaxation of the pelvic ligaments really cannot be observed in fat cows, (body condition score 7 or greater). However, relaxations of the ligaments can be seen very clearly in thin or moderate body condition cows and can be a clue of parturition within the next 12 - 24 hours.
These changes are signs the producer or herdsman can use to more closely pinpoint calving time. Strutting of the teats is not really very dependable. Some heavy milking cows will have strutting of the teats as much as two or three days before calving and on the other hand, a thin poor milking cow may calve without strutting of the teats. Another thing that might be seen in the immediate 12 hours before calving would be variable behavior such as a cow that does not come up to eat, or a cow that isolates herself into a particular corner of the pasture. However, most of them have few behavioral changes until the parturition process starts. Much more information about managing cows and heifers during the calving season can be found in the Oklahoma State University Extension Circular E-1006, “Calving Time Management for Beef Cows and Heifers”
Source: Glenn Selk, Oklahoma State University Emeritus Extension Animal Scientist