I was surpised too to hear that you should start being aware of how it feels etc form about 20wks (later on the link talks about making notes form when you first feel them, so 16wks usually).
The "Count the Kicks Campaign" suggests you start to get a feeling for when baby's active and not what these feelings are (mentally or write them down) from 20wkish:
"So try to recognise your Baby?s movement patterns by making notes every few days from around 20 weeks. It?s difficult in the early weeks as baby?s movements vary so much. But our advice is to always follow your instinct; if you have an uneasy feeling, something isn?t right ? call your midwife."
NICE guidelines: ?Any changes in fetal movements should be reported to a midwife?
My problem yesterday was that I hadn't felt anything since the night before. I'm usually woken in the night (about 4am) with her bashing about in there and wasn't. Intermittently through out the day (one of nudges etc). Then again just after lunch, then usually when I get home from work on and off. Not even a decent prodding elicited a wiggle.
At 1730 rang triage looking for them to tell me to not to worry and relax...and no, despite me having an anterior placenta they said to come in when I could get there (I live 40mins away). Was frankly a shock as I was just about to launch into a complex piece of spreadsheet work - was still at work - and was just ringing to stop the nagging. Everything's fine (LO's gone from being very low down and on her head, to hiding behind the placenta and being sideways we think), but MW did say she was pleased I'd decided to come in as if there had been something up (usually at this stage it's Mum's illness that sees a decrease in LOs movements), I'd have regretted it later.
Of course, LOs been very active since then. Feeling her bashing about frequently. She did wake me, but not for the usual 90mins. Will now be getting myself a note book to write things down in...
On and it's not only a 'lack' of movement to look out for either. Increased movements, especially post 27wks can also be a sign of distress.