Hi L. I am in the same boat as you! I had fraternal twins 14 months ago and got pregnant 7 months ago. Not what my hubby and I were anticipating. I had an ultrasound at 20 weeks and it was decided that there was one but the tech didn't spend as long looking around as I thought she should have and in the results it said that certain views were unavailable due to 'large maternal habitat'. So I am not convinced that there isn't one hiding. I have not been as sick as I was the first time around but I am as tired, of course chasing twins already could be part of that.
When I went in for my last check up 28 weeks, I measured at 35cm. Which in the grand scheme of things is 7 over what I should have been measured at. This was the first measurement though so I am trying not to worry until the next appointment and see if that difference has grown or not. If it has grown then we will assume there are two as I will not submit to another ultrasound (personal preference). We tested me for Gestational Diabetes which came back negative so that is where I stand.
I am sorry I can't help you with a definite but I can tell you that you aren't alone! shoot me an email and let me know if you want to talk having someone elses shoulder is always nice especially when they have been there done that and completely understand.
S.