I would say 28 weeks would be good. I had 8 scans during my pregnancy and the best 3D pics were taken earlier than that. The very best pic we got was at 12 weeks but it was on very advanced equipment. My scans after 30 weeks didn't show a lot of detail due to where the fluid was. I had a scan at 32 weeks and couldn't see the face at all due to position. I also had one at 34 weeks and got a good 3D pic of half of the face. I would have been very disappointed if I'd been paying for 3D but like all my scans they were done purely for medical purposes therefore they were at the regular cost.