Bea,

We always went with blastocysts and were fortunate to be able to achieve good embryo quality and numbers.

We had to sign something similar I think - it's a bum covering exercise because of the medical risks associated with twin pregs. But bear in mind people have been having twins for millenia, so don't get too concerned about that, apart from acknowledging the greater risk of problems.

Realistically, I think if you are going to conceive on a cycle, you'll probably do so regardless of whether you have one or two put back, along the lines of what Dusty said. It probably increases the chance a bit, but won't double it.

When you're starting out I'd be sticking with one for several cycles unless you receive medical advice to the contrary. In our case our FS was more keen to do two as we'd had several goes at one without so much as a bite, and theoretically FETs are supposed to have a little less chance of taking than fresh embryos. In hindsight though I think her advice was a bit silly as we always had nearly 100% cell recovery in the embryos upon thawing, and had had a chemical pregnancy the cycle prior.