The development of a chicken egg into a chick is a precisely timed biological event requiring near-perfect conditions to succeed. In nature, a hen’s instinct provides the necessary environment, but even under optimal care, not every egg is destined to hatch. Failure is a common, expected occurrence and an inherent part of the reproductive cycle. Eggs fail to hatch for numerous reasons, ranging from a lack of fertilization to subtle genetic defects or environmental missteps during the 21-day incubation period. Understanding these factors provides insight into the complex requirements for new life to emerge.
Failure to Start: The Role of Infertility and Storage
The most immediate reason an egg will not hatch is that it was never fertile, meaning the ovum was not united with the sperm cell. Infertility occurs when a hen has not mated with a viable rooster, or if the rooster is sterile, old, or unsuccessful in mating. The egg is essentially an unfertilized ovum; while it may appear healthy, it lacks the genetic blueprint necessary for development.
Even a successfully fertilized egg can quickly lose viability if not handled correctly before incubation. Eggs stored beyond seven to ten days experience a significant decline in hatch rate because the embryonic cells begin to degrade. Improper storage temperature, such as holding eggs too warm (above 75°F) or too cold (near freezing), can fatally damage the delicate cells of the blastoderm, the initial site of embryonic growth.
The physical integrity of the egg shell protects the future chick from the moment it is laid. Severe shell damage, such as a crack, compromises the egg’s natural defenses and allows harmful bacteria to enter and contaminate the contents. This breach in the protective barrier can kill the embryo before it is placed in an incubator.
Fatal Flaws in the Incubation Environment
Once incubation begins, the external environment takes over as the primary determinant of the embryo’s survival for the next three weeks. Temperature control is the most important factor, with the optimal range for chicken eggs generally between 99.5°F and 100.5°F. A temperature deviation of just a few degrees can prove lethal, with high temperatures accelerating development unsustainably and causing severe deformities.
Temperatures that are consistently too low cause the embryo’s metabolic rate to slow down, retarding growth and delaying the entire hatching process. Embryos that survive prolonged low-temperature exposure often emerge weak and are unable to successfully complete the final act of hatching. Maintaining a stable temperature throughout the 21 days is necessary for consistent embryonic development.
Humidity management regulates the necessary moisture loss from the egg over the incubation period. The recommended relative humidity is typically 50–55% for the first 18 days, then raised to approximately 70% during the final “lockdown” phase. If the humidity is too low, the embryo can dry out and become “shrink-wrapped” by the shell membranes, preventing it from breaking free during hatching.
Regular turning of the egg prevents the embryo from sticking to the inner shell membrane. In a natural nest, the hen turns the eggs frequently, and artificial incubators must mimic this action, typically turning the eggs at least four to eight times daily at a 45-degree angle. Failure to turn the egg, especially during the first week, can cause the embryo to adhere to the shell, leading to fatal deformation.
Adequate gas exchange is necessary, as the developing embryo consumes oxygen and produces carbon dioxide. The eggshell is porous, allowing for this natural exchange, but the incubator requires proper ventilation, particularly in the final days when the chick’s metabolic rate spikes. A lack of fresh air at this stage can lead to the embryo dying from suffocation.
Biological and Embryonic Development Issues
Many failures are rooted in problems inherent to the embryo or the parent stock that produced the egg. Genetic errors are a significant internal cause of mortality, often manifesting as lethal genes or chromosomal abnormalities that halt development, frequently in the first few days of incubation. Inbreeding or poor breeding practices can increase the incidence of these defects, resulting in non-viable embryos.
The nutritional status of the mother hen directly dictates the quality of the nutrients packaged inside the egg for the embryo. Deficiencies in the hen’s diet, particularly water-soluble vitamins like riboflavin (Vitamin B2) or Vitamin B12, translate into inadequate levels within the yolk and albumen. A lack of these micronutrients can cause distinct developmental failures, such as edema or malformed skeletal structures, resulting in embryonic death.
Infectious agents pose a threat, as pathogens can invade the egg either before it is laid or through the porous shell after it is set. Bacterial contamination can lead to a foul-smelling, infected egg, causing the embryo to die prematurely. These pathogens may originate from a dirty nest box, unsanitary handling, or a contaminated incubator environment.
A common late-stage problem is malposition, which occurs when a fully developed chick cannot orient itself correctly to hatch. The chick must position its head near the air cell, with its beak tucked under its right wing, to successfully pierce the inner membrane and then the shell. If the chick is positioned incorrectly, for instance with its head in the small end of the egg or tucked between its thighs, it is physically unable to pip and will die from exhaustion or lack of oxygen.