The growth of the fetus and the percentile of the ultrasound sonogram during pregnancy is dependent on many factors such as genetic, placental and maternal factors.
Under normal circumstances, the fetus' inherent growth potential yields a newborn of appropriate size with a wide range of normal sizes. The maternal-placental-fetal units act in harmony to provide the needs of the fetus while supporting the physiologic changes of the mother. Limitation of growth potential in the fetus is analogous to failure to thrive in the infant. The causes of both can be intrinsic or environmental.
Intrauterine growth restriction (IUGR) is usually diagnosed when the fetus falls below the 10th %tile curve for the gestational age. That's why many doctors order a sonogram around 32-34 weeks to make sure they know how well the fetus has been growing.