Mhari-Claire Doolan, 29, from Stockport, got on the train last because a busy rush barged her out the way, but then found a business man in his late 30s sitting in her seat.
Doolan said she the man "refused" to give up the seat on the one hour 30 minute train, even though she showed him her ticket with the seat reservation.
The mum-to-be told Manchester Evening News: "I was already tired and in pain and to be honest I’m so heavily pregnant I should probably not be travelling by train, so I made sure I had a seat booked.
"I told him I had that seat booked and positioned myself so that he could see I was pregnant but he just ignored me."
Doolan said the electronic booking system on the train was not working, but when she got her ticket out to show him he "laughed in her face" and turned away.
She added: "I couldn’t believe it. I am the size of a small house at the moment so he couldn’t have failed to see I was pregnant, he just didn’t care at all.
"Even worse, no one did anything to help or intervene and no one offered me their seat. People just averted their eyes and pretended they hadn’t seen it."
Doolan said she didn't push for an argument with the man, as he was sat with his friend and she didn't want to have to row with him then sit next to his friend.
The mother-to-be said, according to the Metro: "I was left standing for more than half an hour, which was almost unbearable but I didn’t want to ask for someone else’s seat and get laughed at again.
"My back ached, my feet ached and I felt sick. I was in a lot of pain but luckily a seat eventually came free."
Alerting the ticket inspector, Doolan was told although the electronic reservation system was broken, all reserved seats had been given paper reservations, so the man must have removed this.
The incident happened on a Cross Country 15.31 service from Birmingham to Piccadilly.
Cross Country trains have been approached by HuffPost UK Parents for comment.