High and low birthweight: What the numbers mean for mom and baby
April 7, 2025
California mom Madison Peltzer’s video of her adorable, roly-poly son, Callum, who weighed 32 pounds at his six-month checkup, went viral earlier this year when the Detroit Lions called “dibs” on the “future linebacker.”
Baby Callum, who is in the 99th percentile for growth for infants his age, is healthy and exclusively breast-fed. His growth is expected to slow as he becomes more mobile headed into toddlerhood. You might be surprised to learn that he was only a bit larger than average as a newborn, weighing just over 9 pounds at birth. The average newborn weighs 6-9 pounds.
Globally, the average birthweight has been trending upward over time. Since 1950, the average weight of a newborn baby has risen 1.46 grams annually. That’s about 0.05 ounces, which doesn’t sound like much. But when multiplied by 75 years, it is a total increase of 3.86 ounces, or nearly a quarter of a pound.
A baby’s birthweight is one of the first things parents share about their new arrival, and we hear a lot of questions about it at every stage of pregnancy. Many parents-to-be are worried that if a baby measures large or small during pregnancy, there could be problems with delivery or the baby’s health.
While some health conditions are associated with fetal size, many babies who are labeled small or large for gestational age are healthy. UT Southwestern uses a combination of physical measurements and ultrasounds to estimate your baby’s size at each prenatal visit, then track those numbers against a standardized fetal growth chart to see how your baby compares to others at the same week of pregnancy.
Along with fetal growth charts and other statistical trends, we factor in the unique traits of every pregnant patient, including personal and family history, ethnicity, and lifestyle, to determine a healthy fetal weight range. From there, we work together to create a plan, if needed, to help improve the baby’s weight and prepare for labor, delivery, and beyond.
How we measure fetal growth
- Fundal height: This is the distance from the top of your uterus to your pubic bone. After about 20 weeks, fundal height will correlate with how long you’ve been pregnant. For example, at 30 weeks, fundal height will be about 30 centimeters.
- Ultrasound: Sonography shows the amount of amniotic fluid surrounding the baby and reflects fetal urine output. The closer your due date, the less accurate ultrasound will be in estimating birthweight – the range of error can be as much as a pound.
- Fetal growth charts: There are many growth charts designed by different health organizations. Talk with your prenatal care provider about how a particular chart reflects important factors such as twin pregnancy or your ethnicity.
If a baby is measuring big
If your baby is trending on the larger side, there are few phrases you might hear. While they have similar meanings and some wiggle room in the weights associated, some are specific to gestational age at birth, and others are used regardless of gestational age:
- Large for gestational age (LGA): This is when a newborn’s birthweight is in the 90th percentile or above compared to other newborns at the same week of gestation (the fetus outweighs more than 90% of similar-age fetuses).
- Fetal macrosomia: In Latin, “macro” means “large,” and “soma” means “body.” In the U.S. and many other countries, the threshold for this is 9 pounds, 15 ounces (4,500 grams), but some consider it to be anything over 8 pounds, 13 ounces (4,000 grams). This is independent of the gestational age at birth.
If you develop gestational diabetes or gain excessive weight during pregnancy, the baby is likely to be larger.
Labor and delivery
A bigger baby usually has no significant health problems in utero, but fetal size can affect how we approach labor and delivery. Larger fetuses can result in longer labor, especially during the final stretch when mothers are pushing. That may increase the risk of maternal infection or excess bleeding after delivery.
During a vaginal birth, a large baby has a higher risk of complications such as dystocia due to the wider shoulder girth not allowing for passage beyond the mother’s pelvic bone. This complication can have minimal impact. But the situation itself or efforts to resolve it may result in fracture of the newborn’s clavicle or nerve damage to the brachial plexus that serves the arm. When the dystocia cannot be resolved quickly, it can result in brain damage from oxygen deprivation or infant death when it is severe.
We also consider the maternal risks, which can include excess bleeding or a perineal tear, which commonly occurs when the tissue between the vagina and anus tears during delivery. In some cases, a cesarean section (C-section) delivery may be safer for both the patient and baby.
The first few days
Larger newborns – especially those over 9 pounds, 15 ounces (4,500 grams), might need additional surveillance and screenings even after an uncomplicated pregnancy and delivery. Some babies will have more challenges with breathing, or respiratory distress syndrome. Some will have low blood sugar, or hypoglycemia. The nursery team will take a blood sample to check for hypoglycemia, or low blood sugar. The baby may need a bit of formula or a mixture of glucose and water to stabilize the amount of glucose in the blood.
Larger babies are more likely to develop jaundice, a common condition that causes the skin and eyes to have a yellow tint. This happens when a newborn’s liver must work harder to eliminate bilirubin, a yellow substance that comes from the natural processing of red blood cells. Larger babies are more likely to have excess red cells (polycythemia), making this more likely. Jaundice often goes away on its own within a few days. If treatment is needed, the baby will rest for 12-24 hours in an incubator with special blue lights that help eliminate the bilirubin.
Long-term health
Fortunately, most LGA newborns go home with no immediate health worries. In the long-term, however, there is a greater risk of childhood obesity and diabetes. For fetal macrosomia, a pediatrician will watch for the development of metabolic syndrome, which causes high blood sugar, high cholesterol, and high blood pressure, which increases the risk of diabetes, heart disease, and stroke.
Pregnancy with a bigger baby can also have a long-term impact on the mother, specifically the risk of developing diabetes later in life. Research shows that women who deliver a large baby have a 21% increase in risk of developing diabetes or prediabetes 10-14 years after childbirth.
Work with your prenatal care provider
Every prenatal visit should include a discussion about your baby’s size, how you feel, and your general nutrition and exercise routines. Women who begin pregnancy at a healthy weight should gain about 25-35 pounds. You're not really eating for two – pregnancy only requires an extra 300-400 calories a day (the equivalent of about 1 cup of oatmeal with raisins and a banana).
For a well-rounded diet, focus on lean proteins with a goal of eating 75-100 grams of protein per day as well as brightly colored fruits and vegetables. More fiber will also help reduce constipation that often comes with pregnancy. As you get closer to delivery, discuss whether vaginal birth or a C-section might be best for you.
Related reading: Worried about having a big baby? Four things to know about birth weight
Healthy weight gain during pregnancy
- If you are underweight (body mass index less than 18.5), a healthy weight gain is 28 to 40 pounds.
- If you are at a healthy weight (body mass index of 18.5 to 24.9), a healthy weight gain is 25 to 35 pounds.
- If you are overweight (body mass index of 25 to 29.9), a healthy weight gain is 15 to 25 pounds.
- If you have obesity (body mass index of 30 or more), a healthy weight gain is 11 to 20 pounds.
If a baby is measuring small
There are several reasons why a fetus could be measuring smaller than expected during pregnancy, and they vary depending on whether it is early in pregnancy or near term.
Very early on, it is key to assess whether the pregnancy is appropriately dated. But once dating is established, it would not be altered later because a baby is measuring small. In the middle of pregnancy, your obstetric provider would consider chronic conditions like hypertension or lupus that can affect placental function as well as infections such as cytomegalovirus or maternal exposures such as tobacco or alcohol.
Sometimes fetal growth restriction is noted before obstetric complications like preeclampsia become apparent. And sometimes it is related to a poorly functioning placenta, which can be more common later in gestation, especially if there are other findings such as low fluid. We also consider the possibility that a fetus could have a chromosome or genetic condition leading to the growth lag. Further testing may be recommended.
Late in pregnancy, some fetuses are just constitutionally small but doing well.
We use some terms to compare babies with others of the same age as well as absolute weight values that are considered small regardless of gestational age:
- Low birthweight: In the U.S., newborns who weigh less than 5 pounds, 8 ounces (2,500 grams) are considered to have a low birthweight. A newborn measuring less than 3 pounds, 5 ounces (1,500 grams) has a very low birthweight. Babies born prematurely may be an appropriate size for how far along they are but still fall into this category.
- Small for gestational age (SGA): This is when a newborn’s weight is in the 10th percentile of the weight charts, meaning the fetus weighs less than 90% or more of similar-age fetuses.
- Fetal growth restriction (FGR): This is the medical term for poor fetal growth during pregnancy. This is a comparison of the sonographic estimated fetal weight to fetuses of the same age. It can be caused by maternal health conditions, substance use, viral infections, and fetal health conditions as well as by having twins or more. Some fetuses, especially those of smaller stature parents, can fall into this category and may be growing appropriately for who they are, falling at the low end of the sonographic growth curves. There is some overlap, and some fetuses who are lower than the 10th percentile in utero do not end up being in the low birthweight or SGA categories at delivery based on pediatric growth standards.
Labor and delivery
Very small babies have some of the same risks of complications during vaginal birth as large babies, including birth injury, breathing problems, and low blood sugar. Due to their small size, the risk of shoulder dystocia is low. SGA newborns can also have difficulty regulating body temperature, and their immune systems may not be robust enough to resist infection.
There is no specific risk to a patient who is carrying a very small baby during the pregnancy unless there is an underlying maternal or obstetric factor such as preeclampsia that is associated with the poor fetal growth. The key concern is the possibility of an early delivery or unplanned C-section due to concern for poor placental function and possible fetal intolerance of labor.
After the baby arrives
When an SGA baby is born, the neonatal team is on hand for any specialized care such as breathing support or heart monitoring. This might mean a stay in the neonatal intensive care unit (NICU). The tiniest babies can get specialized pediatric care through the Thrive Program at Children’s Health, which offers expert support for growth, nutrition, and vision and hearing impairments that can affect some SGA infants.
In addition to screenings immediately after birth, nurses will continue monitoring the baby for low blood sugar and oxygen levels as needed. Just like large babies, those who are small for gestational age may develop polycythemia and jaundice.
Over the long term
Very low birthweight babies sometimes have neurological delays that don’t surface until the child is older. A pediatrician will also assess whether an SGA baby may be immunocompromised.
Working with your doctor
When we suspect intrauterine growth restriction, we’ll ask the sonographer to measure the level of amniotic fluid during a regular ultrasound. We’ll also recommend a Doppler ultrasound for a closer look at the umbilical cord. These results help us determine a possible cause and how to manage the symptoms.
Depending on gestational age, other evaluations of fetal well-being such as a biophysical profile (BPP) or non-stress test (NST) may also be recommended. Based on the composite findings, hospitalization for more intensive surveillance of fetal well-being is sometimes recommended.
As your due date approaches, your doctor will explain the possible outcomes so you know what to expect for both delivery and your newborn’s health.
Related reading: What to expect if your newborn had fetal growth restriction
Research highlight
UT Southwestern researchers led by Elaine Duryea, M.D., created the revised reference guide that's used nationwide to calculate birth weight percentile for newborns between 24 and 42 weeks of gestation.
Every pregnancy is unique
Whether you’re “carrying big” or “barely showing,” measuring bigger or smaller does not automatically mean that something is wrong. The range of normal birthweights is broad, and the threshold for being too big or too small is higher than most people think. With regular prenatal visits and recommended screenings, you and your doctor can prepare for the healthiest pregnancy, delivery, and newborn possible.
To make an appointment to talk with a pregnancy care provider, call 214-645-8300 or request an appointment online.
