1 or 2 Blastocysts?

As I await the start of my first IVF cycle (8 -10 days), I have one decision to make.  IF enough eggs are retrieved, IF enough eggs are fertilized, and then IF enough fertilized eggs make to to the blastocyst stage, we have to decide on 1 or 2 for transfer.  I was told there is an increased risk of birth defects with twins.  My fertility centre is not able to tell me how much of an increase in risk, just that "there is an increased risk and you have to decide if you want to risk it".  Thanks for that scientific answer.

I like to see stats.  I like to see the research.  I like to see opinions.  And I don't mind spending time trying to compile this information.  So, I used a medical search data base, google, and several blog sites to help me (us) with our decision.

Although the stats vary slightly, it seems to be that 50% of all twins will be born premature (and low birth weight)  and 9% will be very premature (and very low birth rate).  In comparison, singletons will be premature 7% very premature 4% of the time.
Infant mortality rate for twins is around 3% compared to 0.3% for singletons.
Many twins are delivered healthy.
Monozygotic twins have a higher rate of congenital anomalies than dizygotic twins.
There is an increased maternal risk for obstetric complications with twins.
Beyond increased risk of prematurity and mortality, the increased risk of complications for twins includes respiratory distress syndrome, necrotizing enterocolitis, sepsis, intracranial hemorrhage, congenital malformations, cerebral palsy and other long-term neurological complications.  Finding the specific increased risk for each of these complications was nearly impossible.  I did find the cerebral palsy risk among one or both of the twins at about 7% compared to 1.6% for singletons.

A few interesting papers discussed single embryo transfer (SET) with IVF/ICSI.  Basically, with double embryo transfer (DET) the percentage of pregnancy (this includes miscarriages) is 50% with 30% of these pregnancies being twins.  With SET the pregnancy rate is 30-35% with only 2% twins (obviously).    In theory, if you choose SET, the other "best" embryos can be frozen and used in the next cycle if pregnancy isn't achieved.  SO, these researchers are suggesting physicians encourage SET instead of DET to decrease the risks associated with twins.  The researches talk about the expense of IVF and that  most people will choose DET, even if it's not the "best" choice.
Not one paper that I read pointed out how depressing it is to get negative results cycle after cycle and by the time someone gets to IVF they have had so many negative cycles that they are desperate for a positive.

All of my hope is in IVF now.  I am hopeful and excited for the first time in 10 months.  I really need a positive result.  50% and 35% is a HUGE difference.  And I know that the embryo isn't wasted but it may not make it through freezing or thawing.  Even with a positive pregnancy, only 30% of those are twins.  I would, however, feel terrible if either twin had a disability because I couldn't wait another cycle or two.
As of today, with the above information, I'm leaning toward transferring 2.  In the end, we may not even have this decision, but I feel better being prepared for the decision.  Plus, it keeps me busy instead of reading all the TWW sites.