Usually the meconium isn't released by the baby until they are born. Sometimes it can happen when your water breaks which is why your really want to pay attention to color, consistency, & smell when your water breaks. Be happy that you're as regular as you are while pregnant because typically pregnancy causes constipation because the hormones slow down the digestive track. Thankfully I go once every day. I drink lots of water & take prenatals that have natural probiotics in them.
Yes the baby's meconium starts to accumulate in his intestines at 20 weeks.