The three lines on the graphs here represent different growth rates. The red line in the center, referred to as the 50th percentile, shows an average pattern of growth. The thinner lines above and below, the 90th and the 10th percentiles, show the top and bottom ranges of normal growth. Since the head and abdominal circumferences are measured over time and plotted on a graph, a pattern of growth becomes clear. Here, the head is growing normally, but growth has slowed in the abdomen, possibly due to a placental problem that in itself may be caused by a condition in the mother. For example, conditions such as high blood pressure (see Blood pressure checks) or diabetes can affect placental blood flow. If the blood flow to the baby is restricted, this can result in the oxygen and nutrients carried in the blood being diverted to the baby’s most vital organs, the brain and heart, rather than to the abdominal organs, which results in unequal growth patterns in the head and abdomen.
