The gestation period of a cow lasts 280 to 290 days. For a heifer calving for the first time, it can take up to three hours for her to give birth once labour starts. In older cows that have previously had calves, the process should take about two hours from the onset of labour. Prior to labour, a cow or heifer will exhibit physical and behavioural signs signalling that she is about to give birth.
Physical Signs First Stage
Up to 24 hours prior to calving, the teats will fill and become swollen and full. The plug at the end of the teat may come out and a small amount of secretion will be visible at the end of the teat. Some cows may start to drip milk. About 12 hours before labour begins, the pelvic ligaments, which are located at the back end of the cow between the top of the tail and the peak of the buttock, will relax and appear sunken and loose. At the first sign of labour, the cervical plug will soften and expel, releasing a discharge from the vulva. The vulva will become relaxed, soft, flabby and large, as the birth canal adapts for birth. The cervix relaxes, dilates and then contractions begin. The contractions soften the placenta and encourage the calf to move into position for birth. The first contractions last a few seconds and take place about every 10 minutes. Gradually the contractions increase until the calf enters the birth canal, and then the contractions occur about once a minute.
Behavioural Signs First Stage
In the first stages of calving, the cow may become anxious as the contractions begin and she may leave the herd to find an isolated spot to calve. As uterine contractions occur, she will show uneasiness and may kick at her stomach, look at her side, swish her tail or alternately stand up and lie down. She may continue to graze or chew her cud between contractions. When a contraction occurs, the cow will feel an intense pain travelling along the uterus, which puts pressure on the calf and helps move it into position for birth.
Behavioural Signs Second Stage
If standing up, the cow will arch her back and extend her tail. The cow will become more attentive to her pain during contractions, and most cows will lie down and begin to strain hard intermittently. Once the forehead of the calf begins to emerge through the vulva, the cow will generally stay lying down and finish the delivery with a succession of hard contractions.
Physical Signs Second Stage
As the calf moves through the cervix to enter the birth canal and pelvic cavity, the pressure on various nerves stimulates the cow to begin straining with her abdominal muscles, which looks similar to a bowel movement. The water bag will break and protrude through the vulva as it pushes ahead of the calf in the birth canal. The calf continues to push through the cervix, opening it up completely, until the front feet and head appear; the rest of the calf will soon follow due to elevated contractions.