I'm currently pregnant with twins, we had a scan at 16 weeks and were told that we were having 2 boys. It was only a 2d scan, but I think they were still too small to get a good clear indication. Then at the morph scan we were told that we had 1 boy & 1 girl. It was so much clearer to see the gender at 20 weeks. Needless to say I was desperate for confirmation that they were boy & girl, so I had a 4d private scan done. Costly yes, but we already have 2 boys so I was desperate to make sure we were having a girl! And yes 1 boy & 1 girl!

As much as the wait until 20 weeks takes forever, I would save your money and wait.