Imagine taking two supposedly identical pages from two telephone books, then suppose that there's one single letter or number that's different. How do you locate the error without going crazy?
Here's a suggestion. Take a photocopy of each page on a transparent overhead, one in red ink and one in green ink. Carefully align the two sheets and project them. Almost every character will appear brown -- except where they don't exactly match.
Standing out like a little beacon will be a small bit of unmatched red and/or a small bit of green.
That's basically how CGH works. The sample DNA, or genome, is compared with a standard genome, one is dyed red, one is dyed green, and the two are fused together, or hybridized.
Any significant amount of DNA (such as a piece of a chromosome) that's missing in the sample will give a red signal; any significant amount of excess chromosome will appear green.
Not only can all the chromosomes be studied, it should also be possible to study the polar bodies of the eggs.
The advantage of this is that what's in the polar bodies is the complement of what's in the egg itself.
For instance an egg with an extra chromosome 21 (which will cause trisomy 21 upon fertilization by a normal sperm) will have one or other polar body missing that chromosome. So, by studying the polar bodies with CGH and finding that a chromosome 21 is missing, we can infer that the egg is abnormal -- without having to biopsy the embryo, possibly weakening it.
Of course polar body analysis will not discover trisomy 21 embryos caused by a sperm cell with two copies of chromosome 21, but these are a small minority. Most age-related trisomies take origin in the egg.
Likewise, if the woman is the carrier of an abnormal gene, locating the abnormal gene in a polar body using PCR can mean that it's not in the embryo -- genetic detective work that leaves the embryo alone.