MaryT,

actually recent advances in IVF technology has closed the gap between success rates for single and double embryo transfers, and you have almost as much chance of falling pg with 1 embryo as with 2 nowadays. Also because you are young and have only had a 2 transfers previously, and you're doing a blast transfer, I would perhaps take your doctor's advice this time. You are not yet classed as having issues with failure to implant, so you're taking a big risk of twins with a double blast transfer. Hopefully you'll fall soon and then it will be a moot issue, but if that doesn't happen, transferring two embryos is still something you can do in latter cycles.