As expected, for a single egg transfer, B1 has the highest implantation success rate—about 70%. B2s are less successful—around 50%. B3s really drop off—less than 20%. They also rate the morulas. For a single embryo transfer, M2s have about a 30% chance, and M3s have about a 20% chance. (They don’t rate M1s—probably if you’re perfect, you make it to a blastocyst!)
They also rate success of two-embryo transfer, depending on the exact quality of the two embryos. For example, the rates of implantation for TWO B1s is also about 70% (actually a hair lower than the implantation rate for a single B1). BUT the chance is 50% that the resulting implantation is twins. In other words, if you implant two B1s, you have about a 1/3 chance of no implantation, 1/3 chance of 1 embryo implanting, and 1/3 chance of two embryos implanting. BUT, if you just implant a single B1, you have the same chance of implantation (about 2/3) but no risk of fraternal twins.
The stats are similar when “lesser quality” embryos are implanted along with a B1. The chance of implantation does not increase significantly if you move from one B1 to a B1+B2/B3/M2/M3, but the chance of twins rises dramatically. So if you’re lucky enough to have a B1, it seems like you should just implant the B1. Implanting a second embryo does not appear to make pregnancy more likely, but it dramatically increases the incidence of twins.
Things do get a little dicier for people (most of us!) who don’t have a B1 to choose from. For people with two B2s (still a good result!), if both are implanted the implantation rate rises from 50% to almost 70%. BUT the chance of twins also rises dramatically—from almost zero (only identical twins) to about 1/3. In other words, if you implant two B2s, just like the B1s you have about a 1/3 chance of no implantation, 1/3 chance of 1 embryo implanting, and 1/3 chance of two embryos implanting. Not that we had two B2s to choose from, but if we did we still would have only implanted one—the risk of success with a single B2 is good enough (50%) and the risk of twins implanting from two (30%) is too high considering the implantation rate only rises from 50% to 70%.
Where I think the decision gets really hard is if you don’t have a B1 or B2, or even a blastocyst at all. Like I said, with a single B3 your odds are only around 20%. (And some of these are very small sample sizes—so take these numbers with a grain of salt.) But if you implant two B3s, your chance of implantation rises to 50%, and of that 50% the risk of twins is about 1/3. In other words, if you implant two B2s, you have a ½ chance of no implantation, a 1/3 chance of implantation of 1 embryo, and about a 15% chance of twins. Those numbers start to look like odds I would take—that is, the risk of twins with a double implant (15%) does not outweigh the risk of not getting pregnant with a single implant (20% success rate). The rates of success with a B3+morula are similar to the rates of success for B3+B3. So again, if I had a B3 and anything, I’d seriously consider adding the other one as well.
Another thing I found interesting—my lab did not break this data out, but I was able to calculate it—the percent of each kind (B1, B2, B3, M2, M3) they use in their transfers. About 10% of the embryos transferred are B1, about 50% are B2, and the remaining are split pretty evenly between B3, M2, and M3 (about 15% each). Obviously the ones they use are, on average, better than what they retrieve. I.e. I got a B2, which I used, and three B3s, which I may or may not use. So on average women are not obtaining 10% B1s, they’re just picking their best ones. Like I mentioned, the lab told me about 5% of what they retrieve is a B1, so it’s still pretty rare. But because people tend to USE the best embryo they have, 10% of the embryos transferred are B1s, and because people often transfer more than 1 embryo, about 15% of transfers involve at least a B1.
These are just averages observed based on a relatively small group, and not all factors can or have been taken into consideration. (Also, I don’t know if these include frozen transfers or if they are just fresh transfers.) As an example, I am sure very few women would choose to only implant a B3 if they had another B3 or morula to choose from. So it’s likely that many of the women implanting a single B3 were doing it because that’s all they had, not because it was choice. And if you do IVF and end up with only a single B3, that might suggest there are other issues (less quality egg/sperm) making that B3 less likely to survive. Compare the woman who has only a B3 to choose from to the woman who has several B3s. You’d think even if they both implanted a single B3, the woman who had more B3s might have a better chance of success. But in the real world, the multiple B3 lady probably implanted two B3s, so the multiple B3 numbers look even better when compared to the single B3 numbers…. Does any of this make sense??
Also keep in mind that these are just implantation rates—not pregnancy rates. There’s still about a 10-15% risk of miscarriage even after implantation, similar to the rate of miscarriage after a normal pregnancy.
Wow, this is a lot of math! Does anyone else’s head hurt?? I know this is a lot of math to follow without actually seeing the numbers. But if you’re thinking about how many embryos you might want to transfer, take a look at your clinic’s numbers and figure out what makes sense for you.
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