Our scan at 6.5 weeks showed our mo/di twins clear as day.
I'm no expert, but I would think you would see di/di twins earlier as they are two seperate placentas/sacs, ID twins sharing a sac (with or without a seperating membrane) might be harder to distinguish. Then the earlier you are you wouldn't be able to tell if they are viable twins (heartbeats) anyway? I hope that my ramble makes some kind of sense LOL
It isn't unusual for an embryo to be missed or not visible in the early weeks hun, so it's still possible there might be another. I didn't have my first scan til 10/11wks so they'd we're very obvious by then, but there have been a few ladies here where one was missed, or there was a second less obvious 'blob' which turned out to be two xx
im actually not a sonographer but rather a radiologist. but thanks for the compliments =) given how quickly scans change early on it is quite possible that an early scan where you barely have a sac could miss a second sac. often we do a scan and find no sac and a few days later there is a sac, yolk sac etc. many people have twins that measure different dates so if you catch it too early one might be visible the other not. by the time you start getting fetal poles, heart beats etc it becomes less likely (or less likely the other will be viable if it is that much smaller or less developed). theoretically one can be missed later on when they get large but that seems rather unlikely to me as all my scans (and trust me ive done a ton on myself despite my poor scanning skills) and never yet had trouble identifying that there were two boogers in there. only scan with a question was the very first... and that made it look like 3. that is why it is well worth waiting til 7 weeks or so to scan. by then it is pretty clear if there is a baby, and if so how many rather than worrying about empty sacs etc. good luck
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