I'm an avid Diet Coke drinker... and my DH loves coke. We would drink around 1 litre a day. I've drunk DC for as long as I can remember... and I didn't stop when I was TTC and I only cut down a little when I got pregnant. I am sure it had no effect at all on my ability to conceive. I was 37 when I first conceived and 39 this time.... and the odds should be against me - but I fell pregnant very easily without trying too hard. I don't drink tea or coffee or alcohol though, nor do I smoke. (my DH smokes).

I've also heard that low fat milk could impare conception.... but I only ever drink low fat milk - and have cereal every morning. So again; that was not a hinderence to my conception.

Personally I think losing weight would help conception more then cutting out diet coke. I lost about 8kg before getting pregnant the first time. Mind you.. I was 20kg heavier a year later - and still fell pregnant.