As with the topic of ladies first, it is more often than not proper etiquette to offer your seat to a standing lady or an elderly person. My personal rule is that I prefer to not sit as long as a woman is having to stand. Always ask them with a choice, "Would you care to have a seat?" This is especially true for a woman who is pregnant but never assume this either.

   Example: There were three women who boarded and it was standing room only. I was seated and immediately offered the first woman my seat. Four other men seeing my gesture quickly offered their seats as well. One of them was an elderly yet sprite gentleman. The point was well made to all those who saw.