A fertilized chicken egg cannot be reliably identified solely by examining the appearance, colour, weight or shape of its shell. The safest method that does not require breaking the shell is egg candling, also known as ovoscopy. After several days of incubation, candling makes it possible to observe the developing network of blood vessels, the shadow of the embryo and changes occurring within the air cell. The first clearly interpretable result is usually obtained between days 5 and 7 of incubation, although the exact timing depends on factors such as shell colour and thickness, the quality of the light source, the poultry species and the conditions under which incubation is carried out.
It is important to understand from the outset that the terms “infertile egg” and “egg with no visible embryo” do not always mean the same thing. An egg that remains clear during candling may indeed be infertile, but a similar appearance can occur when the embryo has died at a very early stage of development. In professional hatchery diagnostics, distinguishing true infertility from very early embryonic mortality may therefore require the egg to be opened and the germinal disc examined.
What is egg candling?
Egg candling is a non-invasive method of examining the contents of an egg using a concentrated light source. When the beam is directed through the shell, it reveals the air cell, yolk, blood vessels and developing embryo. This makes it possible to separate normally developing eggs from infertile eggs, eggs containing dead embryos and eggs displaying other abnormalities.
The name originates from the traditional practice of examining eggs over a candle flame. Today, the procedure is performed with dedicated candling devices equipped with powerful, cool LED light sources. This is important because excessive heating of the shell during examination can unnecessarily expose the embryo to thermal stress. Professional hatcheries also use automated candling systems capable of inspecting complete trays and removing eggs that show no signs of normal embryonic development.
According to the University of Illinois Extension, candling incubated eggs can be used both to assess fertility and to monitor embryonic growth. Initial changes may be detected earlier in eggs with light-coloured shells, while brown, thick or heavily pigmented shells reduce light transmission and make accurate interpretation more difficult.
Can you tell if an egg is fertilized before incubation?
Without opening the egg, it is not possible to determine with complete confidence whether a freshly laid egg has been fertilized. Candling before incubation can reveal the position of the yolk, the size and mobility of the air cell, shell cracks and certain internal abnormalities, but it will not yet show a developing circulatory system.
When a fresh egg is opened, the germinal disc located on the surface of the yolk can be examined. In an infertile egg, it usually appears as a small white spot known as the blastodisc. In a fertilized egg, it has a more organised structure resembling a pale ring with a central area, known as the blastoderm. This assessment requires experience and destroys the egg, which is why it is primarily used when analysing the fertility of a representative sample of hatching eggs.
In poultry production, the most important evidence is not simply that fertilization has occurred, but that embryonic development has begun and continues normally. This is why the first candling is generally performed only after the eggs have remained under controlled incubation conditions for several days.
When is the best time to candle chicken eggs?
An initial assessment can be performed from approximately day 5, although people with limited experience will usually obtain a more reliable image on day 7 of incubation. At this stage, a normally developing embryo is surrounded by a clearly visible network of blood vessels extending across the egg membranes in a pattern resembling a branching web.
Guidance from the University of Illinois Extension indicates that eggs may be candled from approximately day 5 until day 17 of incubation. Aviagen also states that, under commercial hatchery conditions, fertility may be assessed by examining eggs identified as clear during candling between days 10 and 14. However, distinguishing a genuinely infertile egg from one containing an embryo that died at a very early stage may require the egg to be opened and examined more closely.
There is no need to remove every egg from the incubator each day. Excessive handling increases the risk of mechanical shell damage, cooling, contamination and inaccurate conclusions based on an image that is not yet sufficiently clear. In a small incubator, two or three scheduled examinations are usually sufficient. In a commercial hatchery, the candling programme should be adapted to the production technology, equipment and quality-control procedures used by the facility.
Recommended schedule for assessing chicken eggs
| Examination time | What can be observed? | Primary purpose |
|---|---|---|
| Before incubation | Air cell, yolk position, cracks and major internal defects | Preliminary quality selection without confirming fertilization |
| Days 5–7 | The first clearly visible blood vessels, the shadow of the embryo and sometimes movement | Confirmation that embryonic development has begun |
| Days 10–14 | A large dark area, developed blood vessels and clear embryonic movement | Viability assessment and removal of clear eggs |
| Day 18 | An egg that is mostly dark, a clearly visible air cell and sometimes movement | Pre-hatch assessment and inspection of the air cell |
| After hatch completion | The developmental stage of embryos in unhatched eggs | Diagnosis of the causes of hatch loss |
How should an egg be candled correctly?
Candling should preferably be performed in a darkened room, as ambient light reduces contrast and makes delicate blood vessels more difficult to see. The candler should produce a strong, concentrated beam without causing excessive heating of the shell. Its opening or tip should be appropriately sized for the egg so that light does not escape around the edges.
Eggs should be handled with clean, dry hands, without vigorous turning or shaking. The broad end of the egg is normally placed against the candler because this is where the air cell is located. Gently changing the angle can make the shadow of the embryo and the vascular network easier to observe, but movements should remain controlled, particularly during the early stages of incubation.
The amount of time eggs spend outside the incubator should be kept to a minimum. The University of Illinois Extension recommends that small groups of eggs should not remain outside the incubator for more than approximately 5–10 minutes during inspection. This does not mean that each egg should be examined for this length of time. An efficient assessment normally takes only a few or several seconds, while a longer inspection may be necessary only when the image is inconclusive.
Egg candling step by step
- Prepare a clean, darkened room and a functioning candler that produces a cool, concentrated light.
- Wash and dry your hands thoroughly to reduce the risk of contaminating the shell.
- Remove only as many eggs from the incubator as can be examined efficiently.
- Place the broad end of the egg against the light source and assess the air cell.
- Gently change the angle of the egg while looking for blood vessels, a dark embryo and signs of movement.
- Record the result using consistent categories, such as normal development, inconclusive result, clear egg or blood ring.
- Mark eggs with an inconclusive appearance and examine them again after 24–48 hours instead of removing them immediately.
- After the inspection, promptly return normally developing eggs to the incubator.
What does a fertilized egg look like when candled?
The most characteristic sign of normal development is a red network of blood vessels radiating across the inside of the egg. A darker spot or shadow representing the embryo can be seen close to this network. As incubation progresses, the embryo gradually occupies a larger part of the egg. At approximately day 7, the blood vessels should be clearly visible, and under favourable viewing conditions embryonic movement may also be observed.
During the later stages of incubation, the interior of the egg becomes progressively darker as the embryo and extraembryonic membranes occupy more space. At the same time, the air cell at the broad end should gradually increase in size as the egg loses water at a controlled rate. Shortly before hatching, light passes primarily through the air cell, while most of the remaining interior appears almost completely dark.
Appearance of the egg at different stages of incubation
| Day of incubation | Appearance during candling | Interpretation |
|---|---|---|
| Days 1–3 | No clearly visible vascular network or only very subtle changes | Too early for a reliable assessment |
| Days 5–7 | Red blood vessels and a small dark spot | Usually a normally developing embryo |
| Days 8–10 | Extensive vascular network, larger shadow and possible movement | Clear signs of viability |
| Days 11–14 | A large part of the egg is dark, with blood vessels still visible | Continuing embryonic development |
| Days 15–18 | Almost the entire egg is dark, with a clearly visible air cell | Advanced pre-hatch development |
| Around day 21 | Extensive darkness, changes in the shape of the air cell and pipping of membranes and shell | Beginning of the hatching process |
What does an infertile egg look like?
An infertile egg remains relatively clear during candling because it contains neither a developing vascular network nor an enlarging embryonic shadow. The yolk may be visible only as a faint, mobile shadow, together with the air cell at the broad end.
During the first few days, however, this appearance is not sufficient grounds for immediately removing the egg. In eggs with dark shells, or when the candler is not powerful enough or has been positioned incorrectly, blood vessels may remain invisible despite normal embryonic development. If the result obtained on day 5 or 6 is inconclusive, it is advisable to mark the egg and repeat the examination after 24–48 hours.
If the egg continues to appear completely clear during a later inspection, the probability that it is infertile increases. In commercial hatcheries, however, the final assessment of flock fertility should be based on the examination of a representative sample of clear eggs because candling alone cannot reliably distinguish infertility from embryonic mortality occurring at the earliest developmental stage.
What does a blood ring inside an egg mean?
A blood ring is a complete or partial circular line visible during candling after the embryo has died during the early stage of incubation. When circulation stops, blood may move into the peripheral vessels of the extraembryonic membranes, creating the characteristic red ring. The University of Illinois Extension identifies blood rings and blood streaks as important signs of early embryonic death.
A single case does not necessarily indicate a problem affecting the entire batch. However, if the number of eggs containing blood rings increases noticeably, it is necessary to examine the quality of the hatching eggs, storage and transport conditions, the warming profile at the beginning of incubation, temperature stability within the setter, process hygiene and the health status of the parent flock. The appearance observed during candling indicates when the problem may have occurred, but it does not automatically reveal its underlying cause.
Does the absence of blood vessels always mean that the egg is infertile?
The absence of visible blood vessels does not provide conclusive evidence of infertility. At least four different situations can create a similar appearance: genuine infertility, embryonic death before incubation, very early mortality after setting, or poor visibility caused by the characteristics of the shell.
This distinction is particularly important in a professional hatchery. If every clear egg is automatically classified as infertile, the actual fertility level of the parent flock may be underestimated, while a problem related to storage, transport or the beginning of incubation may remain undetected. Aviagen recommends examining the contents of clear eggs to distinguish genuine infertility from early embryonic mortality.
Key diagnostic appearances during candling
| Appearance in the candler | Most likely meaning | Recommended action |
|---|---|---|
| Clear interior without blood vessels | Infertile egg or very early embryonic death | Repeat the assessment; in a professional analysis, open a sample of eggs |
| Clearly visible vascular network and dark spot | Live, developing embryo | Continue incubation |
| Blood ring or blood streaks | Early embryonic death | Remove after confirmation and include in hatch-loss analysis |
| Dark, motionless mass without clearly visible vessels | Possible later embryonic mortality | Compare with normally developing eggs and conduct an egg breakout analysis |
| Dark interior with a clearly defined air cell near the end of incubation | Advanced normal development | Avoid unnecessary handling |
| Irregular or displaced air cell | Possible transport damage or improper storage | Mark the egg and monitor further development |
Chart 1. Diagnostic usefulness of candling on different days of incubation
The chart below provides a practical, qualitative assessment of the usefulness of egg candling. It does not show embryo survival percentages; instead, it illustrates the stages at which the internal image becomes easiest to interpret.
During the first three days, the image is generally too inconclusive to justify removing an egg. A visible vascular network usually appears between days 5 and 7. During the middle stage of incubation, viability becomes easier to assess, although the increasing darkness inside the egg gradually limits the visibility of individual structures.
Chart 2. How to interpret egg candling results
The most common egg-candling mistakes
One of the most common mistakes is conducting the first selection too early. The absence of visible blood vessels on day 2 or 3 should not be used as grounds for rejecting an egg, particularly when the shell is brown or relatively thick. Development may have begun normally, while the relevant structures remain too small to identify without extensive experience and suitable equipment.
Another problem involves using a weak or dispersed light source. An ordinary torch may be sufficient for eggs with light-coloured shells, but it generally fails to provide adequate contrast when darker eggs are examined. Attempting to compensate for poor illumination by examining an egg for a prolonged period increases the amount of time it remains outside the stable incubator environment without guaranteeing a more reliable result.
Vigorous turning, shaking or repeatedly moving eggs also constitutes a serious error. The embryo and developing membranes are vulnerable to mechanical damage, so every movement should be controlled and gentle. Eggs should not be removed solely because their appearance raises doubts; it is safer to mark them and repeat the assessment at a later stage.
In commercial production, the absence of proper documentation is another significant problem. Removing clear eggs improves hygiene and allows incubator capacity to be used more efficiently, but it does not provide the information required to manage production results. The numbers of infertile eggs, early-dead embryos, late-dead embryos and eggs rejected for other reasons should be recorded in relation to the parent flock, flock age, laying date, storage period and individual setter.
Egg candling in a professional hatchery
Under commercial conditions, candling is not merely a way of checking whether an egg contains an embryo. It is part of a comprehensive process-control system that enables the hatchery to assess fertility, early embryonic mortality, developmental uniformity and equipment-capacity utilisation. Clear eggs are also removed because they should not occupy valuable space in the hatcher, while leaving them in the system for longer may increase the risk associated with decomposition and microbial contamination.
Candling results should be analysed together with the number of first-quality chicks, mortality at different developmental stages, hatch timing, navel quality and the proportion of unhatched eggs. Only by combining these data can it be established whether the causes of loss originated on the breeder farm, during storage and transport, or within the hatchery itself.
Veterinary Support provides specialist support for poultry hatcheries, including hatching-egg quality assessment, analysis of reduced hatchability, laboratory diagnostics, chick-quality evaluation and optimisation of incubation parameters. Regular candling provides valuable information, but its greatest value is achieved only when the results are correctly interpreted within the context of the entire production process.
Key conclusions
A fertilized egg cannot be reliably identified by examining its shell or by candling it immediately after laying. The clearest confirmation of embryonic development usually appears between days 5 and 7 of incubation, when a branching network of blood vessels and the shadow of the embryo can be seen. An egg that remains clear may be infertile, but it may also contain an embryo that died at a very early stage. A professional fertility assessment should therefore be supplemented by examining the contents of a representative sample of clear eggs.
Candling is a useful and safe diagnostic method provided that the examination is performed efficiently, in a darkened room, using a cool light source, and that inconclusive results are checked again. Within a hatchery, candling should form part of a broader quality-control programme rather than serving as the sole basis for diagnosing every cause of hatch loss.
FAQ – frequently asked questions
After how many days can you tell if an egg is fertilized?
The first signs of development may become visible after several days, but the most practical time for the first candling of chicken eggs is usually between days 5 and 7 of incubation. Eggs with dark or thick shells may need to be assessed later because pigmentation makes the delicate vascular network more difficult to see.
What does a fertilized egg look like after seven days?
A normally developing egg contains an extensive network of red blood vessels, a darker spot representing the embryo and, in some cases, visible movement. The air cell remains visible at the broad end of the egg.
What does an infertile egg look like when candled?
An infertile egg remains relatively clear and contains no visible vascular network. Only the shadow of the yolk and the air cell may be seen. However, a similar appearance may occur after very early embryonic death, so a definitive distinction sometimes requires the egg to be opened.
What does a red ring inside an egg mean?
A red ring, commonly known as a blood ring, usually indicates early embryonic death. It develops after circulation has stopped and blood has moved into the peripheral vessels of the extraembryonic membranes.
Can eggs be candled with an ordinary torch?
A powerful LED torch may be sufficient for examining eggs with light-coloured, relatively thin shells. However, a dedicated egg candler provides better contrast and limits light escaping around the edges. When brown eggs are examined, a weak torch may cause a normally developing egg to be incorrectly classified as infertile.
Can candling harm the embryo?
A brief and carefully conducted examination using a cool light source should not harm a normally developing embryo. The risk increases when eggs remain outside the incubator for too long, become overheated, are shaken or are subjected to unnecessarily frequent inspections.
How often can eggs be candled?
In a small-scale incubation system, two or three inspections are generally sufficient, for example around days 7, 14 and 18. Daily examination does not provide proportionate benefits but increases handling and the risk of fluctuations in incubation conditions.
Does a moving embryo mean that everything is developing normally?
Movement is a positive sign of viability, but it does not guarantee that incubation will be completed successfully. The vascular network, air-cell size, degree of internal darkness and conditions within the incubator should also be assessed.
Does a dark egg always contain a live embryo?
No. During the later stages of incubation, both a living embryo and one that has recently died can cause extensive darkness inside the egg. Viability is primarily indicated by intact blood vessels, the normal appearance of the membranes, observed movement and changes detected during a subsequent examination.
When should an egg be removed from the incubator?
An egg may be removed after the absence of development, the presence of a blood ring or other clear signs of embryonic death have been confirmed. Leaking or cracked eggs, eggs producing an unpleasant smell and those showing signs of decomposition require particular care because they may contaminate the remaining hatching eggs.
Sources
- University of Illinois Extension – Candling Eggs
- University of Illinois Extension – Incubation and Embryology Q&A
- University of Illinois Extension – Incubation and Embryology Checklist
- Aviagen – Identify Infertile Eggs and Early Deads
