Pregnant women with diabetes tend to have abnormally large babies, but it seems that active fetuses manage to stay a relatively normal size, new research suggests.

While babies of diabetic women are prone to become overly big, a condition called macrosomia, it has been unclear why a substantial proportion do not develop this problem. The “fidgety fetus hypothesis” has been proposed as an explanation, suggesting that increased fetal activity can counteract the macrosomia-promoting effects of the mother’s high glucose levels.

As reported in the medical journal Diabetes Care, Dr. Lois Jovanovic, from Sansum Diabetes Research Institute in Santa Barbara, California, and colleagues tested this notion in a three-phase study involving 69 women with gestational diabetes.

In the first phase, involving 10 women, the researchers showed that fetal movements are pretty consistent from one day to the next.

The second phase, which included 13 women and involved the use of a home fetal monitoring device, showed that active fetuses, defined as having at least 4 heart rate accelerations in a 20-minute period, are always active, while inactive fetuses are always inactive.

Finally, in the third and largest phase, involving 46 mothers, the researchers showed that the average birth weight of the active fetuses was in the 37th percentile, significantly lower than the 62 percentile for inactive fetuses. Macrosomia is typically defined as a birth weight heavier than the 90th percentile for gestational age.

“The fetus appears to play a role in determining its own destiny,” Jovanovic and colleagues write.

They conclude: “Although normoglycemia is the goal of therapy in all women with diabetes, in those women whose fetuses manifest less movement, increased surveillance and treatment of hyperglycemia is of utmost importance.”

SOURCE: Diabetes Care, January 2006.