In Act IV of "Macbeth," the witches brew a mixture of organs in their evil cauldron. For instance, there are cut-off human lips, a nose of a non-Christian, the finger of a "birth-strangled babe," a baby born of a prostitute who disposed of it in a ditch. Entrails, stomachs, and gullets are also tossed into this brew and "grease that's sweated from the murderer's gibbet." That is, after hanging for so long convicted criminal's bodies were caged so no one could retrieve them. After so long, the skin weakened and the bodyfat liquified, forming a pool of oil.
The reference to a child's finger suggests the future murder of Macduff's precocious son who tells his mother there are more evil men than good shortly before they break in and stab him. Macduff himself was cut from his mother's womb at the moment of her death. Whenever a child could not be birthed naturally, he/she had to be cut out of the womb before the mother went into shock. After the mother's abdomen was cut, she would die. Thus her stomach is destroyed, just as the witches speak of the gullet and stomach.
The horrid ingredients of the evil cauldron in "Macbeth's" Act IV foreshadow the heinous acts that Macbeth will soon follow as well as his past murder of Banquo.