It was a "humiliating" experience and the flight attendant avoided her eye for the rest of the flight, Ms Andersen said on a Facebook post.
She posted about the embarrassing experience on Jetstar's Facebook page, saying "As a woman, it's not a good feeling to be asked if you're pregnant when you're not".
"I used to be quite heavily overweight and even then was never met by such a rude question and especially no remorse from the flight attendant," she wrote.
Since the post, hundreds of women have posted messages of support and similar experiences on social media.
Radio personality Kerre McIvor shared the story on her Facebook page, stating "I've been asked this before - and I firmly believed that no one should ask a woman this question unless you can actually see the head crowning."
Facebook user Anika Sanford said it was "just another reason to make women feel more self-conscious".
"That is absolute c*** from Jetstar! I have been asked far too many times when I was not pregnant," she wrote.
Another woman posted that she had been asked the same question by an air stewardess in the past.
"I laughed with them for a few moments and it was forgotten," she wrote.
"It's good for them to know who is pregnant on board for safety reasons."
Pregnant woman Hanna Marshall said she was asked the same question before her last flight, and was still embarrassed.
"I was asked before my last flight and even though I am pregnant I was still shocked and embarrassed," she wrote.
"I was only 19 weeks and thought I looked small not 28 weeks or more! It's hard and I know where they are coming from as they have to ask but still, most pregnant women will tell them or know the rules before they fly."
Another woman said she was told to get off the plane, despite having a medical certificate to show she was 28 weeks pregnant.
"We were seated on the plane, engines started. All of a sudden a hostess together with 2 security guards came on board and requested I leave the plane as my medical certificate was outside their allowed time frame," Melissa Thomas wrote.
"It cost me $80 for a new medical certificate...then an addition $700 for us to get to Bangkok. This could have been avoided at check in if they had an issue with my medical certificate."
A Jetstar spokesman told NZME News Service its customer care team had contacted Ms Andersen to apologise.
A flight voucher of $100 had also been provided to the passenger as a "goodwill gesture".
"Jetstar sincerely apologises for the distress the passenger experienced," he said.
Women more than 28 weeks pregnant must carry a letter from a doctor or midwife on Jetstar flights, he said.
Jetstar staff were trained to ask for a medical certificate if they believed a passenger may be more than 28 weeks pregnant, he said.
It's not the first time the airline has been in hot water for embarrassing a woman over pregnancy queries - in 2012, a Jetstar attendant demanded a medical certificate from 21-year-old Kelsey Hughes as she was boarding her flight in Wellington.
She was not expecting a baby and said, at 70kg, she did not look like a mother-to-be.
