Blastocyst Transfers

The popularity of day-5 blastocyst transfers began several years ago. For many years, embryos were transferred 2 to 3 days following the egg retrieval. Following fertilization, which is determined the day after the egg retrieval, the embryo begins to divide. The single cell embryo begins growing by splitting into two cells. Then each of those cells split again, resulting in a 4-cell embryo. Two days following the egg retrieval, most healthy embryos are usually at the 4-cell stage. Three days following the egg retrieval, healthy embryos are usually at or about the 8-cell stage.

2-PN

4 cell

With the development of improved culturing medias for embryos, IVF labs began ‘growing out’ embryos for two, and sometimes three, additional days, until they reached the ‘blastocyst’ stage. A blastocyst embryo contains over 100 cells. Many laboratories had difficulty culturing embryos to day-5 and returned to performing day-3 transfers. Some laboratories saw their pregnancy rates decline when they began to perform day-5 blastocyst transfers. These same laboratories then returned to doing day-3 transfers.

There are basic advantages of transferring embryos on day 5 as opposed to day 3. What appears to be a healthy 8-cell embryo on day three may in fact be an unhealthy embryo on day 5. For example, if a couple has ten 8-cell embryos on day 3, all of which look the same and are categorized as ‘excellent quality,’ it is quite possible that only 2 or 3 of these embryos would result in a pregnancy or be capable of growing and developing into good quality blastocysts on day 5 or 6. By allowing the embryos to grow 2 additional days in the laboratory, we are able to select the ‘best’ embryos to transfer. This enables us to transfer fewer embryos and have higher implantation rates per embryo. If a program performs only day 3 embryos transfers, they may be selecting the wrong embryos to transfer or may transfer too many embryos to compensate for lower implantation rates per embryo. Some embryos just stop growing or develop into poor quality blastocysts. These embryos, if transferred on day 3, would not result in a pregnancy. If the embryo is going to stop dividing, we want that to occur in the laboratory and not in the womb.

Pregnancy rates vary tremendously between different centers. Patients differ as do the skills of physicians. Of major significance is the skill of the embryology staff, the laboratory facilities, and the air quality within the laboratory. No two labs are alike. At Dallas IVF, we strive to have the best embryology staff, laboratories, and air quality so that we can continue to maintain our high pregnancy rates.

• Embryology staff
• IVF laboratory