There is much difference between individuals in the development of these signs, and certainly age is a factor. The first-calf heifer, particularly in the milking breeds, develops udder for a very long time, sometimes for two or three months before parturition. The swelling and relaxation of the vulva can be highly variable, too.
Typically, in the immediate two weeks preceding calving, these signs become more evident, the udder is filling, and one of the things that might be seen is the loss of the cervical plug. This is a very thick tenacious, mucous material hanging from the vulva. It may be seen pooling behind the cow when she is lying down.
Some people mistakenly think this happens immediately before calving, but in fact this can be seen weeks before parturition and therefore is only another sign that the calving season is here.
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 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, relaxation 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 to 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. 
Glenn Selk is an Emeritus Extension Animal Scientist at Oklahoma State University.
—Excerpt from Cow/Calf Corner e-newsletter.
