You can literally feel like baby has descended. Even looking in mirror you can make out upto some extent. And as baby's head gets engaged, problems like acidity starts to diminish.

Its different for everyone. 
Some get engaged quite early while some don't until end. 
Mine got engaged at 36th week. I am not sure exactly which is the earliest week of head engagement though!. 
&#9752;gynac usually does a pelvic examination/2 finger test in later stage of pregnancy and tells you if the head is engaged.or you can ask her if it is.

<a href="http://app.babychakra.com/article/2086">Pregnancy Week 35: Physical Development</a>

<a href="http://app.babychakra.com/article/2084">Pregnancy Week 35: Baby Growth</a>

By which week does baby engage its head to the pelvis? Does it feel different. How can it be differentiated?