Egg Count Mathematics: Why the numbers change between retrieval and transfer.

February 28, 2011Carole 203 Comments »

Recently, I got some really good questions from a reader who wanted to know why there is sometimes a discrepancy in the number of eggs initially reported at the retrieval and the final egg count. This reader also asked how does the embryologist know that eggs are mature–is it the size of the egg?-  and what does a “bad” egg look like?  She also wanted to know why the number of mature eggs and the number of eggs fertilized isn’t always the same. I thought that other readers might have similar questions so I thought I’d post my answer to her here:

Regarding egg count at retrieval: The embryologist’s first look at the egg is obscured by a cloud of cumulus cells that surrounded the egg inside the ovarian follicle as it developed. The oocyte cumulus complex or OCC consisting of both the egg and surrounding cumulus cells is sucked out during the egg retrieval. These cumulus cells make it hard for the embryologist to see an egg structure called a polar body which is produced and extruded by the egg when it is mature. The polar body is a cell membrane enclosed sac of excess chromosomes that the egg must remove before it can successfully combine with the sperm DNA during fertilization. The polar body is extruded by the egg into the perivitelline space, the space between the egg and the surrounding shell (zona pellucida).

So, how and when does the embryologist know the egg is mature? Immediately at retrieval, the most the embryologist knows is the number of oocyte-cumulus complexes retrieved. Occasionally, clouds of cumulus cells are recovered at retrieval that don’t contain an egg. That is why the doc and patient may get a report of say- 20 eggs- retrieved and later the report is revised to less than that. If the eggs are to be injected, then the cumulus cloud is removed from the egg by the embryologist to allow visual verification of the polar body and hence maturity of the egg.

You will get the most reliable count of mature eggs at the time of cumulus removal for ICSI- sometimes cumulus removal is called “stripping” the egg. Only eggs with a polar body (=mature eggs) can be injected. Eggs do mature naturally in vitro if they are real close to maturity at the time of retrieval, so sometimes you might gain a mature egg the next day and have a late egg injected at that point, again causing a change in the number of mature eggs.

Regarding size and maturity, egg size at retrieval is usually not a distinguishing factor. It is true that over the months it takes an egg to mature in your body, it expands dramatically in size but by the time of retrieval, eggs that are retrievable are usually very similar in size. Sometime extra large eggs double the size of the others are recovered but these jumbo eggs invariably are chromosomally abnormal, usually containing multiple extra sets of chromosomes.

“Bad eggs” at retrieval are usually dark in color, and may appear very grainy,  shrunken or actually broken, with or without leaking cytoplasm. The number of eggs that fertilize is very frequently less than those that are initially retrieved for all the reasons above. The reduction in egg number at each step is due to the fact that not all cumulus complexes retrieved contain eggs, not all retrieved eggs are healthy or mature, and even some mature eggs may not fertilize. You can see pictures of normal cumulus eggs and later stages of embryo development at this previous Fertility Lab Insider post:…s-progression/

Maybe it’s just February doldrums or a slow news cycle but I am struggling to come up with interesting posts. I would love to hear from you if you have any burning embryology questions or topic ideas that you’d like me to discuss.

© 2011, Carole. All rights reserved.

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