Some of the most important events in your cycle now occur behind the scenes, in the
laboratory. The eggs mature for several hours before sperm are added, usually in the
afternoon of your egg retrieval. The addition of sperm to the culture media is called
insemination. Insemination is followed several hours later by fertilization,
when the sperm enters the egg. The stages that follow are very important to the future
embryo. After fertilization, the sperm loses its tail and its head enlarges, so that, at
this stage, the egg looks like a cell with two nuclei, called pro-nuclei. The pro-nuclei,
which hold the genetic material of the sperm and the egg, are called pro-nuclei because
they have not yet fused to form a single true nucleus. You may hear this stage referred to
as the two pro-nuclear or 2PN stage. Identification of the 2PN stage is very important to
determine if fertilization has occurred.
The 2PN stage can't always be identified. The pro-nuclei join or fuse within a few
hours, producing a fertilized egg, or embryo. When this happens, the early embryo looks
just like an unfertilized egg. Keep in mind that the embryos are kept in the dark and only
observed for brief time periods, so the 2PN stage might not be seen for some of your
embryos, and it may be difficult to tell how many embryos have fertilized.
After the pro-nuclei fuse, the embryo can begin cleaving, or dividing, first into two
cells, then into four. Cleavage to four cells generally takes 36 to 48 hours or more after
the egg retrieval. Embryo transfer typically occurs at 72 hours, three days after egg
retrieval. Transfer can also occur at 5-6 days after egg retrieval, when the embryo
develops to the blastocyst stage.
Problems can occur with fertilization and cleavage. Occasionally sperm are unable to
penetrate the egg in the first 24 hours, and a fertilization failure occurs. Most eggs can
fertilize only the first day, and a re-insemination the second day doesn't produce more
embryos. When multiple sperm penetrate an egg, polyspermy occurs. Although polyspermic
embryos are abnormal and cannot be transferred, polyspermy is sometimes a good sign, since
pregnancy rates with the remaining embryos appear to be slightly higher when this occurs.
Sometimes embryos do not divide or stop dividing at an early stage, and a cleavage arrest
occurs. These embryos may resume division in the uterus, and can be transferred.
Fragmentation or breakage of some of the cells in the embryo is also quite common; severe
fragmentation will reduce pregnancy rates, but milder fragmentation is not a serious
problem. Fragmented embryos are not thought to produce a greater risk for abnormal
babies.
As part of our continuing effort to produce the very best possible pregnancy rates,
several new techniques have been initiated in our lab. Some of these techniques may be
used in your case. Assisted hatching, in which a portion of the covering of the
egg is removed to help it stick to the uterus, is the most common technique we employ.
Please refer to "Assisted Hatching" in the "IVF Laboratory" section of
our web site for more details.
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