Fetal growth terms

Small for Gestational Age (SGA)	Size (for example as measured by estimated fetal weight) falls below a particular cut off percentile for gestational age, such as below the 10th percentile for a 32-week-old fetus. This percentile would mean 90% of fetuses of the same gestational age are larger. The SGA term may or not indicate growth restriction. For example, in the case where a fetus is genetically predisposed to be a smaller size, its growth may be normal even if it measures below the 10th percentile for estimated fetal weight.
Large for Gestational Age (LGA)	Size (for example as measured by estimated abdominal circumference or weight) is above a specified cut off percentile for gestational age. For example, a fetus above the 90th percentile at 36 weeks would be larger than 90% of fetuses of the same gestational age. This may or may not indicate abnormal growth because it is possible for overgrowth to occur without a measurement being large for gestational age. Certain babies are larger in size due to normal genetic predisposition.
Fetal Growth Restriction	The fetal growth potential has been altered. For example, a fetus developing along the 75th percentile and then gradually dropping over the course of a number of weeks to the 40th percentile may not be considered small for gestational age, however, may still be growth restricted. Growth may also be restricted during gestation due to an influence like maternal smoking. It is possible for growth to be restricted even when fetal measurements are above the SGA cut off percentile.

Mayer, C., & Joseph, K. S. (2013). Fetal growth: A review of terms, concepts and issues relevant to obstetrics. Ultrasound in Obstetrics & Gynecology, 41(2), 136-145. http://doi.org/10.1002/uog.11204