From Giggle Glow:
Your baby is adding more reflexive movements to his or her repertoire — "reflexive" meaning those things they instinctively know how to do. If your baby's hand is near his mouth, he might instinctively suck his thumb. That's how babies know how to feed without being told: when something comes near their mouth, their instinct is to latch on. Also present is a startle reflex, and you might notice your baby move if there's a loud noise on the outside.
From Baby Center:
Your baby's the size of an cauliflower! During month six, the average fetus measures about 13.6 inches to 14.8 inches and weighs about 1.5 to 2.5 pounds. She's enjoying her new sense of equilibrium -- she now knows which way is up and which is down. She's growing more fat and more hair too!
From What To Expect:
Your baby is growing by leaps and bounds, reaching nine inches in length and passing the pound and a half mark. Under his skin, capillaries are forming and filling with blood and by week's end, air sacs (also lined with capillaries) will develop in his lungs, getting them ready for that first breath. Mind you, those lungs aren't ready for prime time just yet — but they are developing surfactant, a substance that will help them expand after birth. And speaking of breathing, your baby's tiny nostrils, which have been plugged up until now, are starting to open, and his vocal chords are getting ready to roar.