Q.
We recently had PGD performed, and it revealed that two abnormal embryos were developing beautifully and two genetically normal embryos had ceased developing. Why would the genetically normal embryos not develop in comparison to the genetically abnormal embryos? Would this be due to egg quality? Would the same results be expected for a future PGD procedure? Is it unlikely that a six-cell embryo that had not developed in two days would result in a pregnancy?

A.
I don’t have all the information needed to give you a complete answer but I’m going to assume that you are a typical IVF patient (in your late 30′s) and were doing PGD to eliminate embryos with chromosomal abnormalities or aneuploidy. During your IVF cycle the eggs that were harvested from your ovaries were inseminated and those that fertilized and continued to develop were analyzed genetically. Depending on your (maternal) age, somewhere around 50% of your eggs would have been genetically abnormal. The genetically abnormal embryos look and behave in the same way as normal embryos.

Most genetic abnormalities cause an embryo to fail at the time of implantation (5 or 6 days old) or cause a pregnancy to fail early (miscarriage). When we look at embryos under the microscope in the days leading up to transfer, there is no way of knowing which are genetically normal or abnormal. Both types of embryos grow and develop similarly. In fact, some embryos that we know are abnormal (e.g. resulting from an egg that is fertilized by 2 sperm) often develop faster and look more beautiful than normally fertilized embryos.

The egg is a very large cell and when it is released from the ovary it has already been programmed to develop for 3 or more days after fertilization. Mom pre-loads her eggs with the necessary information for this early development. In most cells, including sperm, there are internal checks to make sure that the cell is functioning normally and that it is genetically normal. Cells that are abnormal, commit suicide in a process that we call apoptosis. Eggs however, seem to have a very poor internal surveillance mechanism, and even those that are grossly abnormal (e.g. with a whole extra chromosome) can fertilize and develop even to the point of giving you a live child. Down syndrome is the classic example, although at least 75% of embryos affected with this condition miscarry early in pregnancy.

So, eggs are endowed at ovulation with the necessary information to keep them going and looking normal for days, regardless of their genetic constitution. There is no relationship between their genetic status and how beautiful they look in our petri dish. If there were, we wouldn’t need to do PGD. We can keep embryos alive in the laboratory for 5 or 6 days and some of the abnormal embryos might stop developing by that time. However, our experience with PGD over the years tells us that about 50% of the genetically abnormal embryos will still look beautiful on their 5th day of life.

The pattern of development that we see with human embryos, regardless of their genetic status, is extremely variable. As you have witnessed first hand, normal embryos often arrest for reasons that we don’t always understand. This is true, regardless of whether the embryos are growing inside of you or in our lab, and this leads to a very inefficient process of reproduction in human females because she only ovulates one egg per month. We do know that the younger a woman is, the better the chance that the embryo will continue to grow. Embryos are more likely to fail in older women. In very young women, over 50% of embryos will implant in the uterus, but in women over age 40 less than 10% will implant. Although we can’t fully explain this phenomenon, a major contributing factor is egg age. Since women have all the eggs they will ever have when they are born, a 40-year-old woman is trying to get pregnant with a 40-year-old egg. And 40-year-old eggs just don’t perform as well as younger eggs.

Are PGD results consistent from one cycle to the next? The PGD technicians tell me that they get similar results for a patient 2 out of every 3 times.

Any embryo that has not developed in 2 days will not get you pregnant. If an embryo is to be ready for implantation, it must be alive and increasing its cell number every day. We expect a full round of cell division (e.g. from 4 to 8 cells) every 16 hours. Further, an embryo transferred to your uterus on day 4 or day 5, following your PGD analysis, should have enough cells to begin forming a placenta. It sounds like your embryo had arrested (i.e. it was dead).

Human reproduction is a very complex undertaking, and often patients feel like they’re left with more questions than answers after their fertility treatment. Don’t be afraid to ask your questions, no matter how simple or complicated they might be. Chances are, we’ve encountered your situation before.

– Joe Conaghan, PhD, HCLD

Tags: Genetic Testing, IVF - In Vitro Fertilization, PGD - Preimplantation Genetic Diagnosis

This entry was posted on Saturday, June 11th, 2005 at 1:49 pm and is filed under Ask The Experts. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.