He's been inching and unching for a long time now, but within the last day, B has finally gained his mobility.
Yesterday evening, we placed him on the changing mat on our living room floor, on his back. Within seconds he was pulling his knees up to his chest, and then pounding them back down at the floor. It was like watching a spring being compressed and released. It took him a few more moments to figure out how to gain purchase on the surface with his feet, but as soon as he had that, he was off!
He pushed himself back a full body's length, but unevenly, so that he had also turned himself 90 degrees to his left. Over the space of the next five minutes, he continued around this circle, eventually ending up with his head where his feet had been, and this time pointing to his right. (See diagrams below.)
___ ___ \_|_ ___
|_o_| | |/ | | |
| O | | | | |
|/^\| | | | |
|___| |___| |___|_|_/
| \
(1) (2) (3)
And then this evening, I put him down on his front, and he pushed himself forward about twenty centimeters.
With just his legs. His legs are amazingly strong.
Because he found it harder to gain purchase with his feet, he was using his knees for propulsion, rubbing them red in the process. He's obviously got some way to go here before he can move freely on his front. When he feels like it, he can use his arms to raise his upper body enough that his face is pointing straight forward, and his tummy is almost off the floor. This proto-crawling behaviour would be easier if he raised his upper body while pushing off with his legs, but he hasn't figured that one out yet.
And he may forget how to do all of this. He was turning himself over from his front to his back at only six weeks, but then he forgot how to do that for a while.
One thing is certain: insofar as we've had an easy life with him so far, this is now over. We can no longer put him down, go away for a moment, and expect him to be in the same place when we come back. We never turn his back on him while he's on his changing table, but now we'll have to be doubly careful.
On the other hand, this is yet another fantastic step to watch him take. Babies are truly amazing!
(For the record, B is twelve weeks old today.)