A cow's gestation period is about nine months, just like a human's. However, just like humans, it is not unusual for a cow to have her baby seven to ten days on either side of her due date. When you are excitedly waiting for that calf, you can grasp on any small change at all and think, "This is it!"
Adding to the uncertainty can be farmers that use bulls for breeding. When a farmer uses a bull, he knows the general time period when the bull was put in with the cows. However, unless he directly observed the mating, he will have no ideas of his cows' due dates.
While it is hard to pinpoint exactly when a cow will calve, there are some specific signs that a calving is coming soon. These changes indicate that the cow's body is getting ready to give birth.
Around the middle of the seventh month of pregnancy, the udders of first-time mother cows will start swelling. However, cows that have calved before may not make an udder until just a few days before calving. Udder swelling is one of those things that can fool you as a sign of impending birth. One sign that calving is going to be very soon is that the teats of the udder will begin to get swollen, full and strutted out. Even if the cow's udder is large, the teats generally won't fill up until calving is just a day or two away.
In the last three weeks of pregnancy, the cow's vulva will begin to relax. Hormones will make this doorway to the world get really loose and floppy to accommodate the calf that will be passing through soon. The vulva may also swell a bit and become enlarged. It is also common for a cow to pass mucus from her vulva during the last few weeks of pregnancy. Many people get excited at the sign of mucus, but that is not a sign that calving is imminent.
The ligaments connecting the pin bones to the spine will relax about 12 hours before calving. On dairy cows, you can easily see this change because the tailhead will appear much more pronounced. However, on both beef and dairy cows you will be able to feel the difference. Before your cow gives birth, run your hand along the tailhead directly on either side of the spine above the hips. Early in pregnancy, you will feel a little bump, but when the pelvic ligaments begin to relax, that small bump will disappear. When the pelvic ligaments are undetectable, start checking on your cow every few hours because calving will begin soon.
In the weeks before your calf comes, you will spend a lot of time standing around staring at the rear end of a cow. By understanding the signs, you can be ready to assist your cow if she needs help with calving.