I don't take MTR too often, but so far I've been quite lucky, almost got a seat from someone every times (maybe 5-7 times in total?). Actually quite a few times it takes a few stations before people realized that there's a pregnant woman in front of them. They are just too busy playing NDS/PSP, sleeping or reading.
It's safer for us to take a seat even if you're not tired; so if someone offer, pls take it as it will make them feel good too. =)