Eggs, embryos and blastocysts

We spend a lot of time in our office talking to patients about quality of eggs, embryos and blastocysts. Patients want to know if their eggs are the “right size” or “big enough”; if their fertilized eggs “look good” and if the blastocysts are “good quality”. This post is to provide some insight into the world of the laboratory and those microscopic cells that mean so much to us.

Eggs, when they are retrieved during IVF or ovulated in the body, are all the same size. Follicle sizes may vary, but the eggs are all the same. Some eggs that are recovered during IVF are immature (meaning that they have not gone through their second meiotic division and cannot be fertilized by sperm or the result will be an embryo with too many chromosomes) or degenerate (meaning they are starting the process of dying off). The eggs we want are “mature” meaning they have gone through the meiotic divisions and are ready to be fertilized. The eggs come out in a halo of cells called the cumulus mass. The embryologist looks through the follicular fluid for this complex of cells and the egg under the microscope. At the time of retrieval, the embryologist cannot tell if the egg is mature or not. That comes much later as he or she prepares to either inseminate the eggs or inject sperm into them (ICSI).

Embryos are what result when a sperm enters the egg. In the first 24 hours of embryo life, the embryologist is looking for two structures that look like lunar craters or pronuclei (PNs). One crater is the egg DNA and the other is the sperm DNA. By 48 hours cell division occurs and on the third day of embryonic life, we are looking for 8 cell embryos. Embryos that do not make 8 cells by day 3, typically do not form blastocysts (there are some exceptions, especially in the case of sperm from testicular biopsy). So, the number of 8 cell embryos on the third day gives us an indication of the number of blastocysts.

Blastocysts develop on the 5th or 6th day of embryonic life and have over 100 cells by this point. A “good quality” blastocyst has a regular outer cell mass, a fluid filled cavity and an inner cell mass. There are many grading systems for blastocysts and they are largely used for the embryologists reference. They really do not always indicate the potential success of the blastocyst to make a baby.

In our practice, if you are 35 or under or are using donor eggs, transferring two embryos does NOT increase your chance of pregnancy, it just INCREASES the chance of TWINS. We are even finding that in our patients under 40 this may be the case, as well. We will only transfer and/or cryopreserve blastocysts that have a regular outer cell mass, a fluid filled cavity and an inner cell mass. These are the embryos that have the potential to make a pregnancy. These blastocysts freeze and thaw well, as evidenced by the rate of pregnancy in frozen embryo transfers (the same as fresh).

There is tremendous attrition from egg numbers to blastocysts for transfer or freezing. Typically, it goes something like this: 10 eggs, 8 mature eggs, 6 fertilized embryos, 4 eight cell embryos, 2 to 3 blastocysts. If the egg numbers are larger than 20, we typically have more immature eggs in the bunch. Cycles with between 5 and 10 eggs often yield the same numbers of blastocysts as those cycles with 15 to 20 eggs.

The ideal cycle is one that, regardless of the egg number, results in a blastocyst for transfer or cryopreservation.