Also you don't choose your symptoms. They vary greatly between pregnancies. My sister had a little bit of a problem in her first pregnancy with her joints. In her second pregnancy she ended up having to walk on crutches towards the end because she had awful sciatica in her hips. You don't expect and choose to end up walking on crutches when you're pregnant.
I think someone's mum telling them that being ill is easier than morning sickness is stupid. I'm still going to give my seat up for pregnant women though. And old ladies. Just like I'm going to carry on offering to help old ladies with their shopping and hold doors open for people with more bags than me. Yeah they made the choice to carry lots of bags but I'd like someone to do it for me.
Having said that I find it really annoying when I hear of pregnant women tutting and sighing rather than politely asking for a seat. "I'm really sorry but my feet are killing me, do you mind if I sit?" is so much nicer than *humph**huff**mutter* "people are so rude!" I know when I'm going to and from work I tend to get lost in a book and don't notice people who need the seat. I always happily give mine up when I see people who need it more than I do. I just often forget to scan for them.