I had posterior placenta with my first son, and now anterior placenta with my second. It's crazy what a difference it can make!!
With my first, I felt movement from 15 weeks to the day (externally - against my hand - if my husband hadn't felt it then I would have dismissed it). I then felt the movement internally from 16 weeks, again to the day, and it just kept getting stronger and more frequent from there.
This time... I still felt first little "kicks" just after 16 weeks, but it was much less obvious, and less frequent. Now at almost 25 weeks, I feel obvious kicks/movement everyday, but nowhere near the consistency I was feeling it with my first. Some days, as recent as this week, I can realise I've got 3/4 of the way through the day and havent felt him - but then he'll make himself known in the afternoon/evening.
So different to my first at this point, who I'd feel at least once every 1-2 hours. It makes me nervous about when they start telling us to monitor movement, as I just don't feel its ever going to be as consistent this time.