Amanda Burch, 41, was wearing the loose top over her tankini to cover up her bump as she treated her daughter to a day out.
But she said she was left humiliated when she was asked to take it off – despite the fact that two other women were allowed to cover up on religious grounds.
Amanda, who is eight months pregnant reluctantly took off her vest so she could stay in the pool with her nine-year-old daughter Sophie.
The mum, from Havant, Hampshire, said: "Sophie and I were playing in the pool when a young male lifeguard told me I had to take the top off.
"I was a lifeguard for eight years so when I was told to take the vest off I knew it was nothing to do with health and safety.
"I took it off anyway because I didn't want to have an argument, but the next moment two other women walked in with vests and full-length leggings.
"I asked another lifeguard why they were allowed and he said it was on religious grounds.
"He said the only way you were allowed to wear extra clothes was on medical or religious grounds. I was 32 weeks' pregnant.
"I complained and put the top back on, but the supervisor said I couldn't wear it and I had to take it off again for health and safety reasons.
"I felt very humiliated and embarrassed. I don't want to come across as racist but I don't see why there should be one rule for them and one for me.
"It's definitely discrimination and I don't think it's fair."
The incident happened at the Pyramids Centre in Southsea, Hampshire.
School cook Amanda said: "I wore the top because even my tankini did not cover everything. The vest top is a maternity vest top.
"This was very unfair. I wouldn't have worn the top if I wasn't pregnant and I wanted the pool to show a little bit of discretion."
Amanda is due to give birth to her and partner Tony Saunderson's baby girl on October 25.
Gary Milne, the chief operating officer of the Pyramids Centre, said policy only permits swimwear in the pools, except for bona fide medical reasons or on religious grounds.
Mr Milne said: "In these circumstances, the swimmer is made aware of the potential risks and even in these cases if the lifeguard is not satisfied that it is safe to continue, then they will not permit this.
"The swimmers' safety is always the first concern."
But he added that common sense needed to be used and Amanda's situation seemed 'perfectly reasonable' and the incident will be investigated.
He added: "We would like to apologise for any upset caused to Miss Burch."