Actually, "easily end up with twins" isn't necessarily so. The great thing about IVF vs IUI is that you have some control over the number of fertilized eggs you introduce. When you decide how many embryos to transfer, consider all your options, and all possibilities. When we did our first IVF we transferred two embryos figuring if we ended up with twins that was ok. We were warned against higher order multiples, so we didn't transfer more than 2 because then the only way we would get more than twins was if an embryo split. As it is, we got a single (now 7!). When we went for number 2, we transferred only one embryo because I did not want twins with a 2 1/2 year old at home - that baby is now 4 1/2.
Embryo quality also comes into play here. My first transfer my embies were 8 cell on day three - good quality but one had some fracturing. When I transferred for my second baby we had a 6 day hatching embryo and were told there was a 90% it would implant becuase it was so strong.
There was a study done in Boston several years back that the overall success rates for IVF when transferring one embryo vs transferring multiples were almost even.
So, think carefully about how many to transfer. In addition to the issue of conceiving twins, think about what if you conceive more? Would you be able to carry triplets or more? What would your drs say? How would you feel if you were advised to reduce a high order multiple pregnancy? All issues everyone needs to consider. I have a friend who transferred two embryos and both split. She had an incompetent cervix to begin with and was advised to reduce the pregnancy to one or two. So how would you handle it?
IVF can be a great experience, but you do have to be sure to think through all the issues nad have a clinic you really trust. Emotions run high with these procedures and you need to have clear answers to all these questions before you face the issue as they are usually time sensitive.
Good luck!