I really do think it depends on what you do, how far you commute & how well you are after the birth.
I was put under pressure to go back after only 10days - that was 3 days after I got out of hospital as they kept me in
I eventually went back at 11 weeks, though had done some work from home from week 2
Started off as a 4 day week, but was too quickly back up to full time & made very hard for me to get away on time. This was a very full on high pressure job, though not particularly physical.
Biggest mistake of my life - I was so ill by 9 months that I had to give up work (also walked out due sex discrimination, abusive boss & constructive dismissal & took it to a tribunal which I accepted an out if court settlement on as the stress of that was affecting me badly too)
I thought I had just burnt myself out, take a few months to recoup & start again - without even looking, I had a lot of great offers of work to go back to - I never recovered & 11 years later I'm still living with the fall out of not looking after myself properly
So think VERY carefully