Baby milestone sessions don't have to be held at the traditional 3, 6, 9, and 12 months. I actually prefer to go by the actual milestones a baby experiences during their first year - and every baby reaches them at different ages!
• lifting head and smiling consistently: 3-5 months
• sitting without assistance: 6-9 months
• the first birthday: 11-13 months
• the second birthday: 23-25 months