Intrauterine Growth Restriction

Fetuses with IUGR are those that fall below the 10th percentile for weight for a given gestational age. IUGR occurs when a fetus does not meet its growth potential (Vandenbosche and Kirchner, 1998). Some below-10th-percentile fetuses are not growth-restricted but constitutionally small, have reached their growth potential, and are healthy. Often, it is difficult to differentiate the two states.

Although mild growth abnormalities are generally tolerated well by the fetus, more severe growth abnormalities can be associated with poor outcome, including fetal distress, fetal demise, and postnatal developmental abnormalities (Botero and Lifshitz, 1999). The growth-restricted fetus should be identified so that appropriate management can be instituted to ensure the best outcome. Factors that influence fetal growth can be divided into the following three categories:

1. Maternal factors affecting nutrient availability to the fetus, such as poor nutrition and cigarette smoking (Chomitz et al., 1995),

2. Maternal factors influencing placental growth and function, such as maternal hypertension, diabetes with vascular involvement, and connective tissue disorders.

3. Fetal factors interfering with adequate utilization of nutrients despite their availability, such as fetal infection or genetic disorder.

Growth restriction has also been described as asymmetric and symmetric. Asymmetric or "head-sparing" IUGR occurs because of fetal autoregulation of blood flow. The initial response to lack of adequate delivery of oxygen and nutrients to the fetus results in shunting of blood flow to important organs such as the brain and adrenal glands. Muscle and other viscera such as kidneys are somewhat underperfused, resulting in smaller body than head, smaller muscle mass, and oligohydramnios due to underperfusion of the fetal kidneys and decreased fetal urine output. If inadequate delivery of oxygen and nutrients is early, persistent, or profound, all organs and tissues of the body will be affected and symmetric IUGR ensues. IUGR from fetal infection or genetic disorder is often symmetric as well because all the tissues of the body are often affected.

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