Insert huge sigh of relief here.
The doctor scared us a bit at first, when the first thing out of his mouth was, "well, that's odd," but it turns out that both embryos implanted and one didn't develop. "Do we have one or two?" asked my husband. "Well, one and a half," said the doctor, and then explained, but our momentary disappointment (I was mostly relieved, to be honest) was quickly eclipsed by the one very healthy, perfectly-measuring, 152 bpm-heart-beating baby in sac A. We even got a little photo, complete with an arrow pointing as if to say, "THIS IS YOUR BABY." This is helpful, because right now, it looks more like a gummy bear.
Measuring right on track at 7 1/2 weeks, and due on October 28th.
Seeing the heartbeat really is amazing. Wow. I think this is actually happening.