What Happens to Frozen Embryos?
By Lisa Glickstein, Ph.D., Triplet Mom
Originally printed in November, 2004

For many who undergo infertility treatment, the story doesn't end with the birth 
of their triplets or more.  Parents who have IVF cycles may have a few or many 
frozen embryos remaining.  A couple of years later, a decision has to be made - 
to try for another pregnancy, to let another infertile couple adopt the embryos, 
or to donate the embryos for research.  This article will summarize the science 
behind these choices.  The emotional and psychological issues about the choice are 
discussed separately in this newsletter issue.


How long are frozen embryos viable?

Embryos are frozen suspended in a special solution of salts, sugars, and fats, 
along with a dehydrating agent (usually glycerol or DMSO) so that large ice crystals 
don't form during the freezing process.  Ice crystals are like tiny swords that stab 
the cells of the embryo and pierce the outer covering (membrane), killing them.  
The embryos are frozen in liquid nitrogen, more than three hundred degrees below zero, 
to suspend all life processes and preserve them.  In spite of these precautions, 
cells will slowly die over time.  Depending on the type of solution used and the 
original quality of the embryo, embryos may survive freezing for ten years or more.  
However, for optimal viability, embryos should probably be used within three years.


What is the chance our embryos will survive thawing?

Many embryos survive thawing during the first year of cryopreservation.  In fact, in 
2001 14% of ART cycles were performed with thawed embryos.  In one study, about 60% 
of embryos survived the thawing process.  However, as many as half of the thawed 
embryos will have one or more damaged cells.  This would be expected to be higher, 
the longer the embryos were frozen.


What is the chance of pregnancy with thawed embryos?

The success rate of ART is highly variable between patients, and depends on diagnosis, 
age, and other factors including clinical expertise.  Overall, however, the pregnancy 
rate with thawed intact embryos ranges from 12-48% per cycle.  The rate is usually 
5-10% less than the pregnancy rate with fresh embryos at the same clinic.  The 
pregnancy rate with damaged embryos was considerably lower (range, 4-17% per cycle).  
In two series, removing the damaged cells and transferring the remaining healthy 
part of the embryo greatly increased the success rate per cycle, to a rate comparable 
to that of intact thawed embryos at the same clinic.


What is the chance of multiple births with thawed embryo transfer?

Overall, in age groups up to 40 years reported to the Centers for Disease Control and 
Prevention (CDC) in Atlanta, the transfer of just two embryos resulted in a multiple 
birth risk of 16-17%.  The multiple birth risk increased with the number of embryos 
transferred.


Will a baby born from a thawed embryo be normal?

Yes, the babies born from previously frozen embryos have not been shown to suffer 
any adverse consequences.


What choices do people make about their frozen embryos?

Many patients make no choice; they simply allow the frozen embryos to remain frozen 
indefinitely.  There are estimated to be approximately 400,000 frozen embryos in the 
United States.  Of these, 10% have been abandoned (no current contact information 
for the family).  The American Infertility Association reported that of the minority 
of patients who make a final choice for their embryos, just over half choose to 
destroy them.  Most of the remainder donates their embryos for research, with about 
10% choosing donation for adoption.  In a study of over one thousand couples in 
Australia, nearly 90% opted to discard their embryos.  The remainder donated their 
embryos for adoption by another infertile couple.

The pregnancy rates for donated embryos are currently lower than that for couples 
using their own embryos.  About one in five cycles with donated embryos results in a 
pregnancy.