3. Developmental delay

What it is: Any significant lag -- five or six months -- in a child's physical, cognitive, behavioral, emotional, or social development.

Why it can be hard to catch: Because there's such a wide range of infant development -- there are "normal" late walkers and late talkers -- often a doctor can't make a diagnosis until a child is well beyond the outer range. Plus, chances are your doctor sees your child for only 15 minutes every few months; it's hard to make a judgment with so little time to observe. And in order for developmental testing to be accurate, a child needs to be relatively calm and relaxed, and, of course, during a checkup your kid could be tired, cranky, scared, or simply not in the mood for a full exam.

How you can help: Try to gauge your child's progress objectively by keeping track of it in her baby book or health book. Note where she was three to six months ago when you're making entries; if she's progressing steadily, then you most likely have nothing to worry about. Above all, don't compare your child with other kids. If you notice a plateau in your child's progress, bring it up with your pediatrician.

4. ADD and ADHD

What they are: Attention deficit disorder (ADD) and attention deficit hyperactivity disorder (ADHD) cause some kids to learn, think, and act differently from others. These conditions are characterized by distinctive behaviors such as inattentiveness, distractibility, impulsiveness, and, in the case of ADHD, hyperactivity.

Why they can be hard to catch: Just as your doctor has only a few minutes during each appointment to check out your child's developmental progress, she also has a limited amount of time to evaluate behavior. And it's virtually impossible for her to get a sense of how your child behaves in a variety of situations (at school, on playdates) while doing a quick checkup.

How you can help: While it's often tough for both parents and pediatricians to tell whether a child is simply a quirky kid or really is struggling, there are clues to ADD and ADHD, including excessive and volatile tantrums, a short attention span that doesn't increase as he gets older, and inappropriate social behaviors (he interrupts other people a lot, for example). If you notice any of these, keep track of them in your child's health book, including how frequently they occur and how much they're affecting your child; you might notice that he isn't doing well in school or doesn't seem to enjoy learning, or that his self-esteem is low (that's one of the most important things your doctor needs to know). If you're concerned and aren't getting the help you need from your pediatrician, get a second opinion or a referral to a specialist.

5. Urinary-tract infection (UTI)

What it is: An infection of the bladder, kidneys, or urethra (the tube through which urine exits the body), caused by bacteria that enter the urinary tract -- either through the urethra or via the bloodstream from other parts of the body. The bladder is most commonly affected, with girls suffering these infections more often than boys due to their shorter urethras.

Why it can be hard to catch: UTIs are one of the toughest conditions to detect in babies and toddlers. When an adult has a UTI, she tends to have frequent, painful urination. But in kids under 3, the symptoms can be quite vague. The one that's most likely to prompt a doctor to test for a UTI is an unexplained fever. Vomiting and inconsolable crying are also clues.

How you can help: A UTI that goes undetected (and therefore untreated) can lead to serious kidney damage. Besides fever, vomiting, and crying, the most reliable signs of one in a very young child may be those that only a mother can pick up on: The child simply isn't acting like herself, but she's clearly sick and not feeling well. In that case, ask: "Could she have a urinary-tract infection?" Note that once your child can talk well (by age 3 or so), she'll be able to tell you straight out, "Mommy, it hurts when I pee."

Longtime contributor William Sears, M.D., is the author, most recently, of The Healthiest Kid in the Neighborhood.

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