Fetal pigs are mammals, and therefore have all the mammalian characteristics that are displayed at the fetal stage. Externally, fetal pigs have a small amount of short hair on their bodies and nipples on both sexes. These characteristics quickly define them as mammals.
During dissection, it is possible to observe more of the mammalian characteristics of the fetal pig. Female fetal pigs have mammary glands connected to their nipples for nourishing their young. Only mammals exhibit mammary and sweat glands. Fetal pigs also have three layers of membranes over their brain, another mammalian characteristic. A specific fetal mammalian characteristic is a special short blood vessel that allows the blood to bypass the lungs until after birth, when the mammal begins breathing on its own.
Some characteristics of the fetal pig are common to most but not all mammals. One is having a separate urogenital opening from the anal opening. Another is having two types of teeth in the jaw: incisors in the front and cheek teeth in the back. A third is the folded surface of the cerebrum, the large bulk of the brain. Fetal pigs are often used for school dissection exercises because they are mammals and display many of the same physical characteristics and systems that humans do at similar stages of fetal development.