What is the significance of size date discrepancies?
Size-date discrepancies are frequently the result of errors in dating, usually because of an unknown last menstrual date or irregular menstrual cycles. Implantation bleeding can be mistaken by the patient as a menstrual period and cause a dating error. US is used to check on the fetal size compared with that predicted from dates. This may require sequential US studies to demonstrate a normal growth rate or to show an abnormal one.
Large for dates fetuses can result from maternal diabetes mellitus, either gestational or type II. Assuming that an error in dating has been excluded, a large for dates pregnancy (i.e., when the fundal height measured by the obstetrician is greater than expected) can also mean multiple gestations, high amniotic fluid level (polyhydramnios), or fetal macrosomia. Small for dates fetuses can result from too low of an amniotic fluid level (oligohydramnios), intrauterine growth restriction, or constitutionally small fetus (a diagnosis of exclusion). US is useful for both large and small for gestational age pregnancies to determine the etiology and plan follow-up.