The question was answered pretty promptly in the ultrasound room. The big sac contained an embryo measuring textbook perfect for dates (7 weeks 5 days), and it had a visibly beating heart. The little sac was still little. It was maybe slightly larger than last time, but it was still only measuring at about 5 weeks and to my untrained eye the yolk sac was less visible than before. So there it is as plain as you can have it. The big one looks like it's doing well, but it looks like there will be no twin.
Everyone was very quick to point out that a singleton is better. Much lower risk, much lower chance of being put on bedrest, much less work once I give birth. All these things are of course very true, and I've said them myself. But I know also that it's how you try to console someone when it turns out that fate has decided that you don't get the extra free bonus baby after all.
I am still happy, on balance. A bit disappointed as I had gotten really psyched (against my better judgement) at the thought of twins. On the other hand, I've got picture perfect (so far) baby A, with a nice heartrate of 148. Given that a couple of months ago I thought it might never happen, this is still well into the plus column. And I am telling myself a few of my own reasons why a singleton has some advantages, like a lower risk of needing a c-section, and no need to knit duplicates of the four sweater, blanket and three sets of booties that I've already made for the baby.
The big downside on not having twins is the loss of a guarenteed sibling for my baby. I have a really great sister, and so having a sibling for my child is important to me. Thankfully, Grace assures me that I can go straight to IVF next time around, possibly starting the initial testing about six months after the birth of the current baby. Obviously I'll be older then, but 39 is not an impossible age for getting pregnant.
Oh, and Grace brought up my blood type without my prompting her. This is further proof that she is great. This early, the second sac will likely just get reabsorbed. I won't be needing rogram at this point unless I bleed. This is good news. The last thing I need is my own antibodies getting worked up about the babies.