Milk lines are precursors to the mammary glands and nipples
. All mammals, male and female, have a pair of symmetrical milk lines along which breast tissue, nipples, and areolas
Humans develop milk lines as early as week six of fetal life. Babies develop milk lines that go from the axillary region (armpits) down the torso to the groin, and sometimes to the feet.
By week nine of fetal life, most milk lines fade away, except for those in the chest area. Occasionally, milk lines will persist and may be connected to extra breast tissue and extra nipples.
Also Known As: milk ridges, milk streak, mammary ridges, mammary fold, ventral epidermal ridges
Mammals such as dogs and cats have milk lines with multiple nipples along the body of their trunk, so they can feed litters of babies. Hooved mammals have milk lines collected in one place as udders. Humans usually have two nipples, but sometimes develop a third or fourth nipple, which will appear along the milk line.