What is Large for Gestational Age (LGA)?

Medically Reviewed by Dan Brennan, MD on July 24, 2022
Written by Sarah Vallie
4 min read

The average baby weighs about seven pounds when they’re born. Sometimes, a baby will be born a lot larger than expected. In that case, the baby may be labeled “large for gestational age.”

If your provider tells you that your baby is large for gestational age (LGA), that means that your baby measures larger than the doctor would expect for how far along you are in your pregnancy or how far along you were when the baby was born. 

Gestational age is calculated by determining the first day of your last period. This means that at the time of ovulation, you may already be two weeks into your pregnancy. By the time someone misses their first period, they are already four weeks along. Therefore, even if you think you know the date of conception, your calculated due date may be different than your doctor’s.

If your periods are irregular, your doctor can also estimate gestational age with an ultrasound. The most accurate guesses for gestational age via ultrasound are from ultrasounds done sometime between the 8th and 18th weeks.

A fetus may be described as large for gestational age if they measure larger than their gestational age would usually indicate. A newborn baby is classified as large for gestational age if they are in the 90th percentile for their weight.

A few factors can cause a fetus or baby to be large for gestational age, and some are more a cause for concern than others.

Incorrect estimated gestational age. The simplest, least concerning cause of a fetus or baby being LGA is that your doctor miscalculated gestational age. This can happen if your menstrual cycle is irregular, if you don’t remember when your last cycle was, if you take hormones or hormonal birth control, or if the doctor miscalculated from the ultrasound or the ultrasound was difficult to see.

Diabetes. Diabetes is the number-one cause of LGA babies. This includes gestational diabetes and those who had diabetes before pregnancy.

Gestational diabetes is diabetes that occurs in pregnancy and often goes away after pregnancy. Researchers don’t know exactly why some pregnancies include gestational diabetes, and some don’t. The best guess is that, as hormones change within your body, sometimes those changes cause the body to have a difficult time managing blood sugar.

Someone with diabetes before pregnancy also has a higher chance of having an LGA baby, especially if their diabetes is not well managed. A person with diabetes may pass their high blood sugar on to their fetus, whose body then, in turn, makes extra insulin to compensate. This excess insulin can lead to fast growth and fat deposits.

Size of the Parents. Genetics may contribute to the fetus or baby’s size. Parents with a larger stature are more likely to have babies that are large as well.

Gaining too much weight during pregnancy can also contribute to an LGA baby. Some weight gain during pregnancy is expected, but how much weight you should safely gain will depend on many factors like your pre-pregnancy size and activity levels. Much of this weight gain comes from simply growing another human, but too much excess weight gain can result in a large baby.

In some cases, complications may arise if your baby is large for gestational age. The severity of these complications depends on what caused the LGA and the baby’s size.

Delivery Complications. Sometimes, the LGA baby is too large to fit easily into the birth canal, which can cause delivery complications. These include:

  • Prolonged labor and delivery.Prolonged labor creates its own complications. A baby stuck in a prolonged labor might not get enough oxygen. They may also get an infection, get sepsis, or aspirate meconium, the infant’s first bowel movement. 
  • Difficult birth. A difficult birth isn’t just a long labor. A difficult birth can also include high-stress labor and delivery. It may also include the need for extra intervention, which could be the use of forceps, a vacuum extractor, or an episiotomy to get the baby out.
  • Birth injuries. If the baby has difficulty getting through the birth canal, injuries like a broken collarbone or damaged arm nerves may occur. Other complications, like brain trauma or cerebral palsy, may occur if the baby doesn’t get adequate oxygen.
  • Increased risk of cesarean delivery. While many pregnant patients have successful cesarean deliveries, an emergency cesarean can be incredibly scary. C-sections also have their own risks, such as an increased risk of heavy bleeding and blood clots. They may lead to a longer recovery time than a vaginal delivery.

Blood sugar complications. Why are LGA babies at risk for hypoglycemia? Hypoglycemia, or low blood sugar, occurs if the LGA is caused by gestational or poorly-controlled diabetes. While the baby is in utero, its body produces excess insulin to accommodate the high blood sugar it’s getting. After birth, the baby no longer has that supply of high sugar but still has high insulin. This can cause blood sugar levels to plummet, resulting in hypoglycemia.

Other complications for babies born to diabetic patients may include breathing difficulties, increased risk of obesity and type II diabetes later in life, and can even be stillborn if the diabetes is left untreated.

If you have an LGA pregnancy or are concerned that your baby will be large, the best thing you can do is talk to your doctor. Together, the two of you can work out a plan for how you want your birth to go and what contingencies you may need to put in place.