Lee Clark was a very relieved Birmingham manager after his side chalked up their first win of the season - but was not pleased with the performance of referee Gavin Ward.
The game was decided by a first-half own goal from Peterborough goalkeeper Robert Olejnik, when he punched a Wade Elliott free-kick into his own net.
Clark, who is not used to losing games after guiding Huddersfield to a record-breaking unbeaten run last season, had to endure an anxious final 10 minutes when Peterborough produced their best attacking play of the game.
Clark expressed his frustration with the referee's decision not to award Birmingham a first-half penalty when Peter Lovenkrands appeared to be bundled off the ball by Kgosi Ntlhe.
He admitted: "It is madness how we have started the season. I do not like talking about officials because they have a tough job. I have great respect for them but it is becoming nonsensical now. We didn't have two penalties we should have had against Charlton and Coventry and had one given against us at Watford which should not have been a penalty.
"Against Peterborough the argument was whether the incident took place inside or outside the box. It was definitely a foul. The Peterborough lad was the last defender and the laws of the game would have meant that he would go off. I am not asking for that but we have to start levelling things out.
"That could have cost us as a 2-0 lead would have enabled us to express ourselves. To be honest I thought some of the refereeing decisions were strange. I am getting frustrated because I try not to criticise these guys but it cannot keep happening."
Clark, who praised Paul Caddis for his man-of-the-match performance following his move from Swindon, said: "The result was a huge relief. The win was more important than anything else.
"If we had put in a fantastic performance and only drawn or lost it would have been frustrating."
Posh boss Darren Ferguson said: "I thought the second half was a very good performance. There was a little nervousness about our performance but when we did step up the tempo we caused them all sorts of problems. We just couldn't take our chances."