A heifer cow is a young female domestic bovine that has not yet produced offspring. At birth, a female calf is often called a "heifer calf". Heifers are mature female cattle that have not given birth to any calves, while cows are adult female cattle that have had calves at some point in their lives. The main difference between a heifer and a cow is whether or not they have given birth to calves. A heifer in the final weeks of pregnancy is called a "springing heifer," and a heifer that is pregnant for the first time is known as a "bred heifer". Upon giving birth, heifers become cows. Heifers typically birth their first calves at just under two years of age, usually bearing one calf at a time. Although cows are known for their udders, heifers do not have visible mammary glands, as the organs develop in response to pregnancy. However, heifers do have visible teats.