Idk I've wondered the same thing. Maybe when they get bigger and there's not so much room in the sac. Maybe they could then
Later on I'm sure they can, I'm 30 weeks and whenever my baby stretches I rub her foot or hand, don't know which one yet, but she either holds still or pulls it back lol
At 20 wks the baby won't necessarily feel your hand but if you happen to shake or push gently on your stomach the baby can feel that and may move in response. Sometimes, when doing an ultrasound on someone, if I need the baby to change position I will gently wiggle the mother's tummy and the baby will move.