I had my morphology scan booked for right on 18 weeks. The sonographer took one look and said, sorry you'll have to come back in a week or two as I don't think the readings will be accurate and bubs needs to grow a bit. I went back at 20 weeks for that scan. I'm pretty sure you can go for that scan anytime between 18-22 weeks...is going in early January an option? If you're eager to know the sex of the baby, perhaps it would be better to go later rather than sooner?