Accurate assessment of gestational age is an essential component of good obstetric care. Many techniques of gestational estimation are available, ranging from calculations based on the date of the last normal menstrual period to sonographic measurements of fetal parameters. Each method has advantages and disadvantages and a stage of pregnancy at which it is most reliable. Early attention to appropriate indicators, reinforced by the monitoring of gestational parameters throughout pregnancy, leads to a more accurate estimated date of confinement than reliance on a single indicator. Incongruity between fetal size and dates should arouse suspicion of specific, serious clinical problems.