Mature vs immature eggs in IVF: GV, MI, MII, and what the maturity report means

On retrieval day you hear a number, perhaps "eleven eggs." The next day the embryology update says "eight mature, two immature, one degenerate." For many patients that is the first moment the cycle feels like it is shrinking. The drop is normal. Understanding why it happens, and what the maturity report is actually telling you, makes the rest of the cycle easier to follow and the next plan easier to discuss.

The three stages on your report

An egg does not finish developing until just before ovulation. During the final hours it completes the first stage of meiosis, the cell division that halves its chromosomes, and pushes out a small packet of spare chromosomes called the first polar body. Embryologists classify retrieved eggs by how far along that process they are. The ASRM committee opinion on in vitro maturation (2021) describes the stages this way:

Your report may also list eggs as degenerate, atretic or abnormal. These are eggs that were damaged, had already broken down, or had features that rule them out. Different labs use slightly different wording.

How maturity is checked

Straight after retrieval, eggs are surrounded by a cloud of supporting cumulus cells, which hide the polar body. If the eggs are going to have ICSI, embryologists strip those cells away first. At that point they can see each egg's stage, which is why ICSI patients usually get a maturity count the same day. ICSI is performed on MII eggs.

With conventional IVF, where eggs and sperm are simply placed together, eggs are often left in their cumulus. Maturity may only be confirmed indirectly the next day, when the lab checks which eggs fertilized. If your report shows "inseminated" rather than an MII count, that is likely why. Our guide to ICSI and when it is needed explains the choice between the two methods.

Mature on the outside is not the whole story

MII describes nuclear maturity: the chromosomes have reached the right stage. ASRM points out that eggs also need cytoplasmic maturity, a build-up of the internal machinery needed for fertilization and early embryo development. The two normally progress together, but not always. An egg can reach MII without fully maturing in its cytoplasm. ASRM's conclusion is that MII status on its own does not guarantee developmental competence. That partly explains why some mature eggs do not fertilize normally or stop developing a few days later.

Why a retrieval includes immature eggs

Follicles grow at different speeds

Stimulation recruits a group of follicles, but they never grow in perfect lockstep. When the trigger is timed for the lead follicles, smaller ones may hold eggs that are not ready. A wide spread of follicle sizes on the final scan usually means a mix of mature and immature eggs.

Follicle size is only a proxy

Ultrasound measures the fluid-filled follicle, not the egg inside it. Larger follicles are more likely to hold mature eggs, but no size guarantees it. Sometimes a large follicle yields an immature egg, or no egg at all.

The trigger

In a natural cycle, ASRM explains, the surge of luteinising hormone (LH) sets off the final steps of nuclear maturation. In IVF, the trigger injection does that job. Its type, dose and timing relative to retrieval all affect how many eggs complete the process. If maturity is unexpectedly low, a clinic will usually check whether the trigger was given exactly as instructed before changing anything else. Our fertility medications guide covers the trigger options.

Can immature eggs be matured in the lab?

Sometimes. Some MI eggs will reach MII if they are cultured for a few more hours, and some labs will inject those late-maturing eggs. ASRM's terminology calls maturing leftover immature eggs from a standard stimulated cycle rescue IVM. The updated ESHRE/ALPHA Istanbul Consensus says babies born from rescue IVM should be followed up, which signals that the evidence on this practice is still developing (ESHRE, 2025).

This is different from planned in vitro maturation (IVM), where eggs are deliberately collected immature, often with little or no stimulation, and matured in the lab as the main strategy. ASRM's 2021 opinion says IVM is no longer considered experimental, though it should be offered by trained clinicians with proper counselling. Planned IVM has historically been used mainly for people at high risk of hyperstimulation. We cover it separately in our IVM guide.

Unusual-looking eggs

Embryologists sometimes note features such as dark or granular cytoplasm, vacuoles or an irregular shell. The 2025 consensus says the evidence supports excluding only "giant" eggs from use. Many other features correlate with outcomes on average, but are not reasons by themselves to discard an egg. If your report lists dysmorphisms, ask whether they changed how any egg was used.

Reading the maturity numbers as a fraction

The maturity count means most when you compare it with the total retrieved. Here are two illustrative cycles:

• Cycle A: 10 retrieved, 8 MII. Most eggs were ready. Attention moves to fertilization and embryo development.
• Cycle B: 20 retrieved, 8 MII. The same number of mature eggs, but a much larger immature share. That pattern is more likely to prompt questions about follicle spread and trigger timing.

Both cycles give eight eggs to work with. The second suggests something may be adjustable next time.

Where maturity sits in the bigger picture

Maturity is the first of several points where egg numbers fall. It helps to know what comes next, so a low number at one stage is not mistaken for the end of the story. After the maturity count comes fertilization: not every mature egg will fertilize normally. Then comes embryo development: some fertilized eggs stop growing before day 5 or 6. Then, if you choose testing, there is a genetic result. Each stage has its own causes and its own possible fixes.

That is why it matters where a cycle lost eggs. A cycle with many immature eggs points toward stimulation and trigger questions. A cycle with plenty of mature eggs but poor fertilization points toward sperm and fertilization method, which our guide to fertilization failure discusses. A cycle where everything looked good until day 3 points elsewhere again. The maturity report is the first clue, not the whole diagnosis.

What your embryology report should list

• Total eggs retrieved
• MII, MI and GV counts, listed separately
• Degenerate or abnormal eggs
• Whether immature eggs were cultured further, and how many matured
• How many were inseminated or injected, and by which method
• How many fertilized normally (2PN) the next day

Keep this breakdown for every cycle. It is exactly what a second clinic will want to see, especially if you are planning treatment abroad.

Questions for your clinic

• How many of my eggs were MII, MI and GV?
• Was maturity checked before insemination, or inferred afterwards?
• Were any immature eggs cultured further and used?
• Does my maturity rate suggest changing the trigger type, dose or timing next time?
• How wide was the spread of follicle sizes on my last scan?

The retrieval count tells you how many eggs were found. The maturity count tells you how many could start the next step.

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