No Stupid Questions

What if it was actually you that vanished and your twin survived? Have you even thought about that?

She's raising your twin in the attic to keep as a spare in case you decide to become a "prompt engineer"

I'm no obstetrician but something the size and complexity of a 7 month fetus does not get absorbed, it comes out. There is likely some misinformation involved along the way.

Well, the key word is usually

Late term vanishing twin syndrome is a thing. It comes with its own set of issues as well. Since its also extremely rare, you'd have to be some kind of nerd to know it exists unless you're an obgyn or at least a maternity nurse. I am neither an obgyn or a maternity nurse.

When it happens late term, and 7 months is very late term for it, you get an increased rush of complications, some of which can negatively impact the development of the remaining fetus. Hell, from what I remember, late term absorption tends to happen because there's something going wrong already. Iirc (and don't try to cite me on a test or anything), just being a little too cramped can trigger it, though it would be a very rare trigger for an already absurdly rare thing.

So, my best guess as a non doctor with zero access to the records of the pregnancy in question is that something happened to put the pregnancy at risk, and either your mom's body or yours set off the cascade leading to the failure of the other fetus. It isn't something that happens that late without some triggering event that's outside of a normal pregnancy. When it happens early on, it's a different story, it can happen for no detectable reason at all. But late term? Something went wrong that made it happen.

I'd have to go digging, and I'm currently brain fried, but one of the more common triggers worldwide is/was malnutrition. When the mother isn't getting resources to grow both critters, either her body shifts to support one exclusively, or one of the two essentially cannibalizes the other. That one (again, I'm old and tired, so the iirc factor is iffy here) is most likely to happen when the twins share a placenta, or something like that (see, old man brain missing details).

Since you've said in comments that you were placed in an unusual orientation and/or location, that would point to some kind of issue with the uterus not having enough room for both fetuses (fetii? I think I like that better despite it not being duet correct lol). I seem to recall a case in India where a woman prone to twins had a pregnancy where this happened because her uterus had lost the ability to stretch the way they normally do. Something about scar tissue maybe? Been ages since I read about this stuff.

Anyway, late term vanishing twin syndrome is the terminology I know of. If there's another, more formal terminology, iam not aware of it.

There's a few things that don't add up in your recollection of events.

You are being lied to, not necessarily directly by your mother.

If that's if going to be a problem to deal with you might want to consider dropping the whole thing.

Your mom said the doctor told her the ultrasound showed "two babies" but I had an ultrasound at 5 weeks and they looked at the tiny embryonic speck and said, "there's the baby!" In fact no babies are shown in ultrasounds, only fetuses. But my point was, they may have seen one 7-month fetus and one little shriveled thing and said "there's two babies."

I guess it's also possible she brought two babies to term, made a "Sophie's Choice" to give one up for adoption, and didn't want you to be heartbroken or afraid of being given away too. And since other people had heard about her being pregnant with twins, she gave you a story you could accept as a child.

And another possibility is that the doctor gave her anesthesia or rohypnol or something and took the twin to put up for adoption, telling her it was absorbed. She'd know she had given birth but not remember it was twice. It's terrible but not unheard of.

You could ask her, if you feel you're both adult enough now to handle the emotional implications of all the possible explanations. Remember that you can still trust her love no matter what, but she might not know the whole truth and/or might be ashamed to admit it.

You are The One

"Fetus papyraceous occurs in 1 out of 12,000 pregnancies and 1 out of 200 twin pregnancies."

Instead of absorption, it could have been papyrification, but apparently that leaves a kind of unmistakable trace for a doctor, which you mentioned so I assume was involved during birth. If pregnancy was non-standard in more ways then just finding out late, maybe those remains were so small that they were harder to notice?

I couldn't find a written record of a similar situation where a second twin that was seen at 7 months and just vanished afterward, so this could be a unique mix of circumstances. That also makes it statistically a lot more likely that somewhere along the lines, information was missing, or got garbled in chaos, or was misheard, not unlikely during extreme situations like birth. I'm not even thinking about bad intentions, just all the places where one human error could be the missing puzzle piece.

Can you think of any extra information regarding your time during pregnancy and your birth you are willing to share?

It sucks that you couldn't live with your twin, although I can imagine you have made you peace with it since. Having siblings can be a lot of fun, I know I love it :)