I won't tell you you're being a knob because I can understand your worries and think they are totally justified in the circumstances. Is it worth ringing your midwife and asking her/him? Maybe some scans are better done when the baby is still smaller? I don't know, but if I was you I would be finding out why so I could put my fears to rest. Here's hoping it's totally normal and you have a happy and healthy pregnancy.
I do seem to recall the screening mw telling me some time ago when I was pregnant with dd1 that they can see signs of spina bifida from 16 weeks. I would suspect in your case she is simply booking you in (as she would anyone) as early as possible, based on your past history and NOT due to anything suspicious she saw on your 12 week scan xx
I've already been told I'll be under the care of a consultant and will have additional growth scans, based on what happened in my previous pregnancy, and I'm only 8 weeks. I had iugr last time and was induced at 36 weeks. There's no evidence yet that I'm at risk of it happening again, so they're just being cautious. Try not to worry, but call your midwife if you want more reassurance - they should be happy to address your concerns.
Thanks everyone, why calling my MW for reassurance hadn't crossed my mind is beyond me. I will give her a call tomorrow. And i've spoken with my DP which I hadn't done before. Thanks again for your posts, i'm already feeling a bit calmer xxxx