Hi T.,
There are a lot of things that can cause a baby not to grow rapidly, ranging from simple genetics (for example, parents are small or had similar early slow growth) to serious health issues, such as kidney dysfunction, genetic syndromes, and metabolic disorders. Your child's doctor should consider a number of factors and may have had building a level of concern in him/her over the past month. The best person to ask about this is your pediatrician. Do not be afraid to ask lots of questions. Think about what you want to ask before the appointment and write the questions down. Be ready with paper and pen to record the answers so you can review them later, think about them and do more research if warranted. Don't be afraid to ask your doctor to slow down, explain things more clearly, and give you more information. Remember, YOU are paying the doctor. Demand satisfactory responses. If your doctor can't give them to you, you have the right to refuse testing or treatment and seek a second opinion.
Good medical practice should include good communication with the patient. In that vein, make sure you're giving the doctor complete information, too. Is your baby eating regularly? Is she falling asleep before finishing? Does she have good periods of wakefulness? Does she move her limbs a lot? Does she seem lethargic? Can she focus on an object held 8" in front of here? Is she reaching for things? Does she cry a lot/very little? Is she fussy? Is she wetting 10 - 12 diapers a day? Does she have regular stool/bowel movements? Is she spitting up a lot? Is she vomiting (especially projectile vomiting). All of these are important considerations. Also, you should let the doctor know if there is a family history of any kind of infantile disorders (in other family members or their children). Most important -- what is your gut feeling? You're with your daughter a lot more than the pediatrician -- learn to trust your instincts. Moms know when something is wrong!
Here are a few questions to start with:
What do you feel should be my child's growth rate at this point? Rate is often more important that actual growth. If you child is growing at a regular rate, but possibly just more slowly than the norm, this needs to be considered. As long as you child is within 2 standard deviations of the average (your doctor will know what this means) for growth rate for children your daughter's age, there may be less reason for concern.
Are there other symptoms or physical indications that concern you?
Have you had experience with whatever disorder you're concerned about?
Is it really necessary to test now? Is there risk in giving my baby more time to grow and/or display other symptoms? What are those risks?
What kinds of tests are you thinking of running? These are likely to be blood tests, so followup questions should be:
How much blood will you have to draw?
When will the results be back?
Will you call me, or will I need to check back? When should I check?
If results do suggest a problem, you should seek out a specialist in the appropriate field immediately.
Keep in mind that most true disorders are very rare. Think horses, not zebras! Other moms have already suggested some good, nonpathological reasons for slower growth. I would question the doctor about these first. As hard as it is, try not to worry!
All the best for you and your baby,
R.