An infant whose weight is below the 10th percentile for gestational age at birth. A low birth weight may be suspected before delivery if the size of the mother's uterus is small, or if a small fetus is shown by ultrasound . Although the overall size of the infant is small, the organ systems are mature for age.
Several factors can cause delayed growth of a fetus.
Undernutrition of the fetus may occur as a result of insufficiency of the placenta, multiple pregnancies, heart disease in the mother, preeclampsia or eclampsia , and high altitude. Babies with congenital anomalies or chromosomal abnormalities are often associated with a below-normal birth weight. Infections during pregnancy that affect the fetus, such as rubella , cytomegalovirus, toxoplasmosis , and syphilis , may also affect the birth weight. Risk factors in the mother that may contribute to intrauterine growth retardation include multiple gestation, poor nutrition, heart disease or high blood pressure , smoking , drug addiction , and alcohol abuse . If the mother is small, it may be normal for her to have a small fetus .