When eggs go into lockdown on day 18, they prepare to hatch by positioning their beaks towards the wide end of the egg where the air cell is. For much of the journey thus far, the embryo has been supplied with oxygen through the eggshell’s pores via a network of blood vessels called the allantois. Just prior to hatching, the chick pokes its beak through the membrane (pips) into the air cell and begins breathing with its lungs.
Chicken eggs are expected to hatch on or around day 21 into incubation and should generally hatch within 24 hours of the first pip in the batch. Various factors play into the process that can either advance or delay that schedule, primarily temperature fluctuations. Temperatures slightly higher than ideal can result in a premature hatch and physical challenges for the chick. Temperatures slightly lower than ideal can cause a delayed hatch and physical impairments.
It is reasonable to expect a chick to hatch within 12-24 hours of pipping.
If a pipped egg does not make progress by expanding the hole and chipping around the circumference of the shell within ~12 hours of the first pip, they may be unable to accomplish the job alone. Whether or not to assist in a hatch is a matter of personal preference. If the decision is made to help, the chick must have absorbed all of the blood in the membrane as well as the egg yolk in order to survive outside the shell. Helping risks severing a blood vessel in the membrane, hemorrhage and death. Any assistance rendered should proceed very slowly and cautiously, stopping at the first sign of bleeding.