Fetal growth restriction (FGR, also called intrauterine growth restriction [IUGR]) is the term used to designate a fetus that has not reached its growth potential because of genetic or environmental factors. FGR results in the birth of an infant who is small for gestational age (SGA). Mortality and morbidity are increased in SGA infants compared with those who are appropriate for gestational age (AGA). (See "Fetal growth restriction: Causes and risk factors".)
The most common definition of small gestational age (SGA) refers to a weight below the 10th percentile for gestational age (table 1) . However, this definition does not make a distinction among infants who are constitutionally small, growth-restricted and small, and not small but growth-restricted relative to their potential. As an example, as many as 70 percent of fetuses who weigh below the 10th percentile for gestational age are small simply because of constitutional factors such as female sex or maternal ethnicity, parity, or body mass index; they are not at high risk of perinatal mortality or morbidity .
Moderate and severe fetal growth restrictions (FGR) are defined as birth weight in the 3rd to 10th percentile and less than 3rd percentile, respectively. Normal term infants typically weigh more than 2500 g by 37 weeks gestation .
Ponderal index — Ponderal index (PI) is a ratio of body weight to length expressed as :
PI = [weight (in g) x 100] ÷ [length (in cm)]