"My 9 month old daughter slept all night without waking for food (7 to 6) for the first time ever on the 23rd of December (8.5 months old). I was ecstatic. I didn't expect her to do it again but she did...she kept it up for 10 whole days. I thought it would last. But then the teething started...she pops them in pairs so it's always pretty painful. We had a hellish week and she cut both top teeth and then things settled down again but went back to one night waking to be fed. And she hasn't slept through
the whole night since.
I have problems with letting her CIO and have always believed that she would sleep through on her own when she was ready. I have reason to believe she is a good sleeper. While she slept for crap the first 6 months of her life, at 6 months she (with no sleep training at all, and after being rocked to sleep for every nap and every bedtime of her young life) began to fall asleep on her own in her crib without crying. She went from 3 bottles at night to 2 to 1 with no encouragement from me. So I think the kid qualifies as a good sleeper, at least from 6 months on.
Should I not have fed her when she started waking? What did I do wrong to mess up my straight-through-sleeper? And how do I undo it? In case this helps: we have a great and very consistent bedtime routine, her last meal is around 5:30, her bedtime is 7pm, and she takes 2 naps a day (one easy to get one at 9am which is generally 1-1.5 hours long, and one really hard to get one around 2pm which lasts about 1 hour).
Any insight you have as to why she regressed would be greatly appreciated."
You didn't have anything to do with this. You didn't do anything wrong. It isn't your fault, and there's only so much you can do to get her back on track until she goes back by herself. You've got a great routine going, and that's all you can really ask for. Release the guilt and second-guessing.
Babies are strange*, and there's stuff about them we'll never know. So there's no way any of us can know for sure why they're sleeping or not sleeping. But what I guess happened with your daughter was that she dragged herself through the 37-week developmental spurt (learning to see things in categories, according to The Wonder Weeks) and then slept through as soon as that spurt happened for those 10 days just as kind of a rebound effect from being awake during the weeks before the spurt. Those spurts are hard, and take a lot out of babies! Then the teething messed up her sleep, and now she's back to where she was before she slept through at 8.5 months. Does that make any sense?
If she's waking once a night that seems pretty normal and par-for-the-course for a 10-month-old. The next developmental spurt will probably happen around 46 weeks, and her sleep may slip a little right before then. Once she hits 14-15 months you'll probably see her one wake-up happening a little bit later progressively. She may hit the 18-month sleep regression (which really makes you think you've screwed something up royally), and then by 21 months or so if she's still waking up it'll start being a really early morning wake-up instead of a middle-of-the-night wake-up.
So I think you do have a "good sleeper" on your hands. (I don't like to assign value judgements to the way kids sleep--they sleep the way they sleep--so maybe we can call her an "easy sleeper"?) She's just not quite there yet with going all the way through the night. You could try the whole "mixing the formula with water and then decreasing the formula-to-water ratio thing until gradually she's just taking water in a bottle in the middle of the night" thing. It works for some people. But by the time you get her down to just water over the course of a few weeks she'll probably be hitting another developmental spurt.
I think if you can tread water for another few weeks she'll come out of the 46-week spurt (learning about sequences) and she may do another stretch in which she sleeps straight through. If that happens you can seize on that just to get rid of the last night feeding. But I don't think you can get rid of the night feeding just on your own unless you're willing to go through some serious planning and execution of the plan, and it may be an uphill battle to try to drop that feeding while she's having a developmental spurt.
It's excruciating (my son is 9 months now, so I'm feeling it with you), but it isn't going to go on forever. It feels like forever. Especially at 2 am. But a few months will make a huge difference, if you can stay sane on that long.
*Or maybe it's adults that are strange, and babies make perfect sense. Who's to say for sure?