I have considered the question of one vs two embryos twice, and ended up transferring one both times. Here was my logic. Both times, I really did not want to get pregnant with twins, but for different reason. The first time, I had pretty bad OHSS and a twin pregnancy would have increased the chances of OHSS continuing dramatically, and possibly getting worse and life-threatening. Also, my first cycle went very well, so the chances per embryo were very high, upwards of 50%. I did get pregnant and had a successful pregnancy. We had two frozen embryos from that cycle, and naturally, we went one by one afterwards, but they did not work out. The second fresh cycle, my concern was (and still is) that our life is going to change much for the worse if we have twins - neither of us are young or energetic (I have always been low energy person), and we do not have any family in the country, so there is really no break with just one child, let alone three. My second cycle went well until retrieval, when everything just went haywire - almost half of my follicles were “empty” (i.e. very poor eggs that would not separate from the follicle), and then out 7 retrieved, 4 were immature. The 3 good eggs we got, all fertilized, and embryos were of very good quality, at least on day 3. This when a lot of calculations went into the decision. As someone who works in analytics, one thing I noticed is that your embryo implantation chances are not independent of each other. Essentially, either you have 2-3 good ones, or none of yours are any good. This is the only way you can get a 56% live birth rate and a 45% twinning rate for those who transferred two+ embryos is to have a very significant covariance in the embryo outcomes from the same cycle, in other words, if one of your embryos is going to result in live birth, the second one has a much higher probability to do the same than if your first one does not implant. When I played with the numbers, it looked that if we split the universe of [younger] women [with a decent sample size] equally into high and low performers, the high performers run ~70% chances per embryo, and the low ones were around ~18% chances. I did not know which group I fell into, of course, but just seeing the numbers was pretty chilling. So, based on that, I chose a single embryo, and ended up with what looks like a chemical (the other two did not make it to blastocyst, which is a pretty good sign they would not have helped my chances). The last consideration is, of course, how expensive it is to raise a child. IVF may be an expensive one time outlay, but it is roughly equal to one year of daycare cost. And there will be more than one year of daycare, plus having to get a larger car and a larger house, and college fund… all in all, it would be cheaper to have 2 more IVF cycles to get a singleton. Of course, this is generally not a consideration for women who would like to have two or more kids. That’s it. Sorry for the lack of paragraphs, this new forum format would not let me insert any.