What Causes Accelerated Fetal Growth Risk?

The classic method for determining accelerated fetal growth is by determining fetal length and weight for a given age. The result of accelerated fetal growth is a baby large for gestational age (LGA). At birth, large babies (greater or equal to 8 lbs., 13 oz., or 4,000g) are also described as having macrosomia (literally, "large body"). Macrosomia is associated with pregnancy and birth complications, increased weight and fat in children, and obesity in adults.
Accelerated fetal growth is due to genetics in most cases (roughly 70 percent), but dietary and maternal health are also important factors. Risk factors that can predispose a fetus to accelerated growth include: a prior history of having big babies, maternal obesity, maternal diabetes, weight gain during pregnancy, repeated pregnancy (multiparity) and a male fetus.

Genetic Factors

The genetics of an infant accounts for approximately 15 percent of variations in fetal weight. These include gender and maternal genetic contributions. The mother's genetic influence on fetal growth is estimated at 20 percent, but the father's size does not appear to contribute significantly to fetal growth. Women tend to bear infants with comparable birth weights and gestational ages across successive pregnancies, so a history of large babies is most associated with future macrosomia. Approximately two percent of variations in birth weight (5 to 7 oz., or 150 to 200g) are attributable to gender, since male babies tend to grow faster and larger than female babies.

Diabetes

Women with pregestational or gestational diabetes have elevated sugar (glucose) levels. This is due to decreased insulin, a growth hormone important in glucose uptake, sensitivity and an inability to increase insulin secretion. The free glucose crosses the placenta, and the fetal levels are then elevated (70 to 80 percent of the maternal concentration). This results in a glucose surplus, which increases insulin secretion. Insulin is a growth hormone, so increased fetal insulin causes increased cellular glucose usage, increased deposition of fat, increased protein production and ultimately accelerated fetal growth.

Maternal Obesity

Birth weight generally increases with maternal body mass index. While obese women are more likely to have diabetes, approximately one-third of obese mothers without diabetes have infants with macrosomia.

Pregnancy Weight Gain

Women with excessive maternal weight gain (more than 35 pounds) during pregnancy are more likely to have LGA babies. In obese women, the risk is even greater than for nonobese women.

Multiparity and Grand Multiparity

Giving birth multiple times (multiparity) or giving birth five or more times (grand multiparity) increases the risk of having infants with macrosomia. While women bear infants with comparable birth weights across successive pregnancies, birth weights tend to increase slightly with each child. This can ultimately lead to an increased risk of macrosomia as the number of pregnancies increases.