The sow's placenta and umbilical cord are thick tissues that selectively transfer nutrients to the developing fetus. Water-soluble nutrients are more effectively transferred than those that are fat soluble. The pig is born with a low body-fat content and a low reserve of fat-soluble vitamins, particularly vitamin E. There is also a relatively low supply of iron and selenium transferred to the fetus. The neonatal pig must therefore be supplied with these and other nutrients, either from colostrum, milk, or an exogenous source.
The major dietary energy source provided to the fetus during its development is glucose, which is supplied from the maternal blood supply through the umbilical cord. Fetuses positioned toward the center of the uterine horn receive the largest supply of nutrients, resulting in pigs that are larger at birth, whereas those pigs positioned toward the end of the horn receive less glucose and other nutrients and thus generally have a lower birth weight. Birth weight of the pig is therefore largely determined by the amount of glucose transferred to the fetus. Pigs that have heavier birth weights have also demonstrated a more rapid growth rate from birth to market weight. An excess as well as an inadequate quantity of dietary energy can, however, be detrimental to the sow.
Amino acids cross the placenta for the development of muscle cells, organs, and other protein-containing tissues. Although the number of muscle cells is largely genetically determined by the parental genotype, fetal pigs that receive an inadequate supply of amino acids will have cells that are less physiologically mature by the time of birth. Under this condition, the subsequent growth rate of these pigs to market weight will be reduced.