During the sojourn in the womb the unborn baby is supposed to grow at a rate that falls within a predefined range. Any deviation from this growth pattern may place the fetus in jeopardy and can have lifelong implications.
When physicians classify growth, they often speak in terms of percentile growth. What does this mean? If you were to study 100 men who were 36 years of age and measure their height, you would find that there was a distribution in height from the shortest to the tallest. If you chose any one individual and evaluated his height and compared it to the remainder of the group, you could categorize him by the percentile his height would equate to. For example, if his height placed him at the 49th percentile, this would mean that 51% of the 100 men studied were taller than he was and 48% were shorter. If he were at the 10th percentile, this would mean that 90% of men were taller, and 9% were shorter. If he were at the 1st percentile, it would mean that 99% of the other men were taller than he is. When we evaluate fetal growth we use the term percentile. By definition, INTRAUTERINE GROWTH RESTRICTION (previously known as Intrauterine Growth Retardation) or IUGR, is a weight for a specific age below the 10th percentile. This means that if a fetus had IUGR and was at the 9th percentile, 91% of fetuses that were of the same age would weigh more. It is important to realize that the 10th percentile is not a fixed weight, but is different for different weeks of pregnancy. For example, if a baby were born at 36 weeks and weighed 5 pounds, this would be normal. However, if a baby were born at 40 weeks an weighed 5 pounds, it would be classified as having IUGR because this weight would be below the 10th percentile for an age of 40 weeksThere are two major classifications of IUGR based upon which parts of the body are altered by abnormal growth.
This is characterized by inadequate growth of the head, body and extremities and occurs in 25% of IUGR fetuses. The growth problem is the result of a decrease in the rate of cell reproduction, resulting in fewer cells. This usually has its onset prior to 32 weeks of pregnancy and has a 25% risk for chromosomal abnormalities (Down syndrome, trisomy 13, trisomy 18).
This usually occurs early in the third trimester and is associated with impaired growth of the body, with normal growth of the head and extremities. This is the result of failure of the cells to increase in size resulting in less fat and smaller abdominal organs. These fetuses often appear to be "long and skinny."