I honestly don't understand how more movement isn't perceptible to me (or any pregnant woman at this stage). The average length of a fetus Besido's age is 5 1/2 inches head-to-rump. I mean, common, that's HUGE. Add a couple inches of legs and arms on there, that's one big wriggly non-perceivable baby. Our bodies are such impressive things.
Below is a picture of a live 18-week fetus. Blow the picture up so the length from crown to rump is 5 1/2 inches, and there you have Besido. :)