I am a Neonatal Nurse, not only do UTIs occur premies, but my son had a UTI @ 4 weeks and was diagnosed with renal reflux. It can become evident at any age. I do agree if she is drinking/eating enough then -- 5 pees can be ok, perhaps she is changing her habits. Does she appear to be bloated? IN pain when she pees? Is the urine dark? Any fever -- usually the # 1 sign of a UTI in a child/infant? 18 mos is early for potty -did you train your others early too. Most Dr offices do the urine cath in the office. I would recommend talking to your pediatrician more, if you go to the ER, they will most likely not do a US or VCUG right then, they will schedule, since it is not urgent. She will have her urine checked and the results read before any other diagnostic studies. If you have that instinctual feeling something is wrong then definitely get some follow up on this!