Corrie McKeague, 23, from Fife, was last seen in the early hours of September 24 in Bury St Edmunds in Suffolk.
His girlfriend April Oliver, 21, told BBC Look East she discovered she was pregnant in October - just weeks after Mr McKeague’s disappearance.
Miss Oliver, from Norfolk, said: “I’ve had to make a massive decision by myself. I was hoping and praying that he’d come back so we could make the decision together.”
Mr McKeague, a gunner and team medic, was separated from friends while leaving the Flex nightclub on St Andrews Street South.
He was last seen in Bury St Edmunds town centre on CCTV at 3.25am wearing a light pink Ralph Lauren shirt, white jeans and brown suede Timberland boots with light soles.
The last sighting shows him walking from a shop doorway and into a horseshoe-shaped area in Brentgovel Street, with no sign of him emerging.
Miss Oliver said she saw him shortly before his disappearance and was on holiday in the US when he went missing.
She said she returned to the UK as soon as she heard the news.
“I have the support of my family and friends around me, which is great, but it’s still not the most pleasant thing to go through on your own, especially when the person you love is missing,” Miss Oliver said.
“I’ve given it a great deal of thought. Obviously there is the element of ‘what if he didn’t come back?’ what would I do, and that was something I had to take a sensible approach to.
“When I found out, although it was a horrible time and something I wished I could experience with him, it was also something I was excited about.”
Asked about when she discovered he was missing, she said: “I had only been there a few days when I got the call from the RAF boys asking if I’d seen him or heard from him, that’s when I knew that he was missing. I very quickly got a plane home.
“At that point I wasn’t entirely sure what had happened to him. It was completely out of character.
“I found out I was pregnant a couple of weeks after I came back.”
She said the baby is due in late spring/early summer and that Mr McKeague did not know about the pregnancy.
The pair had been together for about five months after meeting on a dating site.
Mr McKeague’s mother, Nicola Urquhart, who is a police officer from Dunfermline, told the BBC: “It’s something that somebody would normally want to keep quiet to make sure everything’s okay but it’s getting to the stage now where there are far too many people starting to notice and ask questions.
“That’s why we felt now is the time that we do have to let people know so that we can draw a line under it so April can enjoy her pregnancy safely without any additional stress.
“It’s incredibly difficult to bounce my head from the excitement of a new baby to what we’re actually trying to focus on, which is finding Corrie.”
Despite exhaustive police and RAF searches, there have been no clues to the whereabouts of the gunner who was based at RAF Honington.