I think she did a great job - but I found it very distracting. I kept thinking she would go into labor any minute.
Never thought about the timeline. Human pregnancy is only nine month
Even thought she is a good actress - there are many other British actresses that could have done the job.

No. They really wanted Olivia for the role. She told them she was pregnant, and they decided to go with it anyway.
For me it disrupted the timeline of the story. She was visibly pregnant through most of the series. They tried to hide it with big clothes but you could still tell.
It made it seem that the story spanned only a few months during her pregnancy when in fact it was several years (i think).
There was even one point toward the end when she's talking with Jonathan and says it's her first. It would've made more sense if she said it was her third or that she had had several miscarriages.
The first time i saw the show, i was confused about the timeline. But the second time there were clues about how long time had passed. I think the characters mentioned things and i think the time span is displayed at the beginning of some scenes.
Fraaaank. FRANK! Get my jean bin. Susie wants my jeans.
No she doesnt.