“Normal” has different meanings. In the context of physical or laboratory measurements, “normal” may mean “average,” “disease-free,” or “within a given statistical range.” However, it is important to know the characteristics of the population yielding “normal” values before deciding whether these values provide an appropriate reference range with which to compare an individual test result. Many laboratories now print reference ranges on their reports and highlight test values that fall outside these values as “abnormal.” When the test subject is a pregnant woman, a fetus, or a newborn, and the reference population is composed predominantly of middle-aged men, then comparisons are patently inappropriate. It is important to understand how the physiologic changes of pregnancy affect the results of various tests and measurements before deciding whether an out-of-range result is actually abnormal.