Work and pregnancy

So i am almost 26 weeks pregnant, and i have times where i can barely stay awake and i feel sick and dizzy... I was planning to work untill close to my due date which is 21st april, but omg i dont think i can!!!   Is it rude to ask my boss to start looking to replace me untill i come back? And as soon as he finds someone i will stay home!   I just feel like i cant do it anymore, work, very hot weather here, and i feel like i have no power at all!   And my work does not allow me to take breaks when i feel like.  I work from 8am till 6pm everyday and i only get to have a break at 1 pm for lunch :((   I feel sad and scared for my baby thats why i want to stay home asap.   What would you do!?