Congratulations on your pregnancy! It all depends on you. I worked full time for my 1st 2 pregnancies (no. 2 was twins) and was ok. I was working in the City for my 1st so that was long hours. For my 3rd pregnancy i worked nearly full time (earlier start, earlier finish) but that was harder going as i had 3 kids at home to deal with. I went off at 35 weeks for pregnancies 1 and 3 as i knew when i was being induced and that felt right to me. I went earlier with the twins as I wanted to avoid a premature birth.
I had nighttime hypo unawareness ending in comas in my 1st and 2nd pregnancies. I didn't call in sick the next day as if i can keep going i will. Also I consider anything related to my control a personal failure so would hate to call in sick for that reason. That's my choice. With my 2nd and 3rd pregnancies my employer was a lot more encouraging about working at home so I did ask to do it one morning when i kept on vomoting every 10 mins and didn't fancy the tube in that state! I felt fine apart from that.
Ultimately it all depends on you and your circumstances. I never felt i was pushing myself past my limits. However I was also motivated with my 1st by the fact i had no paid sick leave, a big mortgage and had started a new job where i wanted to impress. I was in a better situation with my next 2 but also had a more flexible employer so worked at home if needed and used leave to work shorter weeks in the last few weeks.
I hope that helps. Good luck! I always found the first trimester hardest as you can't tell anyone and have ao many hospital appts to explain away.