Any transvaginal ultrasound experts on here could answer some questions for me ?

I had an early scan at 6+5 which showed twins. Now on the screen she showed me there was one big sac and one small sac. The big sac measured 28mm and the small sac 10mm. The small sac held the baby that had a heart beat but the baby was too small to measure apparently !? The large sac apparently had a baby but thats all i know they didnt tell me about a fetal pole or yolk sac or anything.

These are the pictures I got:

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I then had a second (private) scan 6 days later and there were still 2 sacs yet this time it appeared there was a healthy baby in the big sac with yolk sac and heart beat and a CRL measurement of 13.8mm making it 7+5. The smaller sac still had an embryo and a tiny fetal pole but no heartbeat measured 3.6mm
These are the pictures I got:

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Now my question is : Did the babys really swap over as it seems - did the one with the heartbeat die and the other one develop or is it likely that the one with the initial heartbeat is still the one living and the other one that had no heart beat just didnt develop ? Im so confused as they seemed to have swapped roles as the healthy baby was in the small sac to start - can sac sizes change ? all this pictures dont mean anything to me.

Im not being nieve and thinking the small one might still stand a chance as I know with such a big measurement difference there is no way the situation will chage b ut im just trying to get more of an understanding whats gone on as the EPU where I got seen are pretty useless as answering questions. They are just there for emergency bleeds etc and dont seem to have the time to spend answering questions or giving opinions.

Any advice would be greatly appreciated.
Im due back there again on Jan 4th so will see what happens then - fingers crossed all is well ... Would the other non viablebaby have dissapeard by then do u think - i would be 9+1/9+2 or is it likely to be there until nearer 12 weeks ?

I think what happened to you is really confusing. I am only guessing here but my logical mind thinks that at the first scan you had 2 live babies, 1 was in the big sac and the tech just couldn't see the heart beat yet (maybe it was just in a bad position or she wasnt skilled enough to be able to see properly). And she did manage to see the smaller ones heart beat. Then by the time you had the second scan the smaller one had passed and the bigger one thrived and the tech was better and the baby was bigger so the heart beat was easily detected. I could be totally wrong, but that is the only senario that makes sense to me.

I actually think the opposite of Cinss. I think the baby in the small sac just thrived and grew according to gestation and you were seeing the same heartbeat twice. I think the original sonogram's "small sac" was actually a "normal" sac for gestation and the larger sac was the unhealthy one. I think perhaps what happened is that the baby in the larger sac was trying to grow and did not have the genetic ability to continue. I think when you had the sonogram at 6+5, the baby in the sac was already behind, but still developing (hence the larger sac and still seeing an embryo) and by the time 7+4 rolled around, it had already failed to develop and therefore the sac began to shrink and the heartbeat/embryo was probably starting to dissolve. While, the smaller-sac baby from the beginning was thriving and the sac continued to grow accordingly. I notice that your sonogram pictures are not all in the same "transverse" or whatever it is in sonogram speak. Even your first sonogram, there is one picture (top one, left side) that the big sac is left and the smaller is right. But then two pics down, on the right side, the sacs are completely opposite with the smaller one being on the left and the larger one on the right. So the next sonogram, there is no way to really know which sac did what. But from my logic, the healthy heartbeat was likely to remain healthy and the one that was never seen was likely never healthy from the start.

As I posted on another thread, we had a similar thing happen in our second pregnancy, but the twins were in the same sac. One had a strong heartbeat at 6+5 and the other had one, but it was "thready" and difficult to get a bead on. They thought it might have been positioning, but did warn me that it was possible the second baby was not healthy and was actually beginning to miscarry instead of being just hard to get a good heartbeat on. Strangely, if you look at the picture, most people would assume the one who ended up passing was the healthy baby and the opposite was the unhealthy one. But the one on my left side was the strong one and the one on the right (who was clearer in the picture) was the unhealthy one.

As for the sac being gone, I don't know. Its possible. I could still see the baby we lost at my sonogram at 8 weeks even and at 9 weeks as well. By the time we had our NT scan at 12 weeks, the evidence of another baby was gone entirely. In fact, the tech never even mentioned it, when both the one who did the 8 week scan and the one who did the 9 week scan both knew what happened (and reassured me it would not affect my healthy baby). Given how small the sac is now, I would imagine it would be gone by 12 weeks at the latest. It is possible it will be gone by the 9th week as well. Since mine didn't involve a disappearing sac, I don't have experience with that.

It is hard to not know what exactly happened. And even harder to grieve something you are confused about. But I hope you start to feel better about things soon. You have one gorgeous healthy bean in there!

Thank you for taking the time to answer ladies it is very kind of you and i can kind of see where both of you are coming from.

Mum23boys...am thinking of u and wondering how your appt went?

Well the baby with no heartbeat is still there but had no improvement or growth so definately won't be viable but the other is lovely and wriggly and measured spot on at 9+1. Happy but gutted was hoping for a miracle but am lucky and blessed I know so am trying to focus in that. Thank u for thinking of me