I'm not sure about Ryanair. I would check their terms and conditions as they tend to be arsy about the smallest things.
I know Easyjet will accept a letter from the midwife. Mine was so reluctant to give me one and would not put the words "fit to fly" so my note was kind of crappy. It just said how many weeks I was and that I was healthy up until now. I didn't want to have to rely on it so I told the Easyjet people I was 27 weeks. They asked on both flights too!
I figure people have no idea how big a bump should be so I risked it. I was only 30 weeks at the time though. I probably wouldn't recommend lying and pretending to be a full month behind where you are.