Originally Posted by star_world

+1 - good advice. I just checked on ITA and it appears it will do infant (on lap) fare calculations automatically. I tried on a few flights and it looked like it was working correctly. On a CO EWR-LHR flight it correctly left out the YQ for the infant resulting in a fare of $1377 for the adult and $183 for the infant.

There may well be some airlines that will charge the YQ on infant fares though..