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Large for gestational age (LGA) is a term used to describe infants that are born with an abnormally high weight, specifically in the 90th percentile or above, compared to other babies of the same developmental age. Macrosomia is a similar term that describes excessive birth weight, but refers to an absolute measurement, regardless of gestational age. Typically the threshold for diagnosing macrosomia is a body weight between , or more, measured at birth, but there are difficulties reaching a universal agreement of this definition.

Common risks in LGA babies include shoulder dystocia, metatarsus adductus, hip subluxation and talipes calcaneovalgus, due to intrauterine deformation. Newborns with shoulder dystocia are at risk of temporary or permanent nerve damage to the baby's arm, or other injuries such as fracture. Both increased birth weight and diabetes in the gestational parent are independent risk factors seen to increase risk of shoulder dystocia. In diabetic women, shoulder dystocia happens 2.2% of the time in babies that weigh less than , 13.9% of the time in babies that weigh to , and 52.5% of the time in babies that weigh more than
LGA babies are at higher risk of hypoglycemia in the neonatal period, independent of whether the mother has diabetes. Hypoglycemia, as well as hyperbilirubinemia and polycythemia, occurs as a result of hyperinsulinemia in the fetus. High birth weight may also impact the baby in the long term as studies have shown associations with increased risk of overweight, obesity, and type 2 diabetes mellitus. Studies have shown that the long term overweight risk is doubled when the birth weight is greater than 4,000 g. The risk of type 2 diabetes mellitus as an adult is 19% higher in babies weighing more than 4,500 g at birth compared to those with birth weights between 4,000 g and 4,500 g.

Complications of the pregnant mother include: emergency cesarean section, postpartum hemorrhage, and obstetric anal sphincter injury. The risk of having a macrosomic fetus is three times greater in mothers with diabetes than those without diabetes.

Obesity prior to pregnancy and maternal weight gain above recommended guidelines during pregnancy are another key risk factor for macrosomia or LGA infants. It has been demonstrated that while maternal obesity and gestational diabetes are independent risk factors for LGA and macrosomia, they can act synergistically, with even higher risk of macrosomia when both are present. Genetic disorders of overgrowth (e. g. Beckwith–Wiedemann syndrome, Sotos syndrome, Perlman syndrome, Simpson Golabi Behmel syndrome) are often characterized by macrosomia.

How each of these factors leads to excess fetal growth is complex and not completely understood. Traditionally, the Pedersen hypothesis has been used to explain the mechanism in which uncontrolled gestational diabetes can lead to macrosomia, and many aspects of it have been confirmed with further studies. It has also been shown that different patterns of excess fetal growth are seen in diabetic associated macrosomia compared to other predisposing factors, suggesting different underlying mechanisms. Variability of fetal weight estimations has been linked to differences due to sensitivity and specificity of ultrasound algorithms as well as to the individual performing the ultrasound examination. In addition to sonography, fetal weight can also be assessed using clinical and maternal methods. Clinical methods for estimating fetal weight involves measuring the mother's symphysis fundal height and performing Leopold's maneuvers, which can help with determining the fetus position in utero in addition to size.

LGA and fetal macrosomia associated with poor glycemic control can be prevented by effective blood glucose management below a mean blood glucose level of 100 mg/dl before and during pregnancy; additionally, closely monitoring weight gain and diet during pregnancy can help to prevent LGA and fetal macrosomia. Women with obesity that undergo weight loss can greatly decrease their chances of having a macrosomic or LGA infant. Two dimensional ultrasound can be used to screen for macrosomia and LGA but estimations are generally not precise at any gestational age until birth. A number needed to treat analysis determined that approximately 3,700 women with suspected fetal macrosomia would have to undergo an unnecessary cesarean section in order to prevent one incident of brachial plexus injuries secondary to shoulder dystocia. The use of metformin to control maternal blood glucose levels has shown to be more effective than using insulin alone in reducing the likelihood of fetal macrosomia. There is a 20% lower chance of having an LGA baby when using metformin to manage diabetes compared to using insulin. Modifiable risk factors that increase the incidence of LGA births, such as gestational weight gain above recommended BMI guidelines, can be managed with lifestyle modifications, including maintaining a balanced diet and exercising. Such interventions can help mothers achieve the recommended gestational weight and lower the incidence of fetal macrosomia in obese and overweight women.

In healthy pregnancies without pre term or post term health complications, fetal macrosomia has been observed to affect around 12% of newborns. Thus, women in Europe and the United States, with higher gestational weight gain, tend to have higher associated risk of LGA infants, macrosomia and cesarean. In general, rates of LGA infants have increased 15 25% in many countries including the United States, Canada, Germany, Denmark, Scotland and more in the past 20–30 years, suggesting an increase in LGA births worldwide.