Every time your doctor measures your fundal height (the distance from your pubic bone to the top of your uterus) beginning halfway through your pregnancy, they are estimating your baby’s size and weight.
Ultrasounds are another more accurate way that your practitioner tracks your baby’s growth — but neither method is foolproof. You’ll have to wait until your baby arrives to know her actual size.
While the majority of babies in the U.S. arrive on or around their due dates and at a normal weight (on average 7.5 pounds), about 1 in 10 infants are born prematurely and about 8 percent have a low birth weight. Worth noting is that premature babies can still have a normal weight or even be considered large, while babies born at term can be "small for gestational age" (SGA).
The good news is that babies born just a few ounces short of a normal birth weight can quickly catch up. The teeniest infants, however, may require up to months of intensive care at the hospital before they’re ready to go home.
Fortunately, many recent medical advances are helping ensure that the majority of low birth weight babies develop into perfectly normal, healthy kids.
What is considered low birth weight?
When your baby is born, she’ll be categorized into one of the following types of birth weight:[1]
- Normal birth weight: Between 5.5 pounds and 8.8 pounds
- Low birth weight: Less than 5.5 pounds
- Very low birth weight: Less than 3.3 pounds
- High birth weight: More than 8.8 pounds (technically called macrosomia)
As of 2020, 8.24 percent of all babies in the U.S. were born at a low birth weight, and 1.34 percent were born at a very low birth weight, according to the Centers for Disease Control and Prevention (CDC).[2]
Read This Next
What causes low birth weight in a baby?
Babies are born at a low birth weight because they arrive early or because they have poor growth during pregnancy — or a combination of both.[3]
Poor growth during pregnancy is known as intrauterine growth restriction. IUGR happens when a fetus is below the 10th percentile for her gestational age because she hasn't reached growth projections due to prenatal conditions.
There are several potential causes for a low birth weight, including:
- Preterm birth. About two-thirds of all low birth weight babies are preterm, or born before the 37th week of pregnancy. Nearly all very low birth weight babies are premature.
- Genetics. About one-third of a baby’s birth weight is determined by genes, so if you and/or your partner was a small baby, there’s a greater chance your baby will be born small too. Chromosomal abnormalities, along with heart defects, can also cause IUGR and low birth weight.
- Problems with the placenta. If the placenta doesn’t get enough oxygen or nutrients to a baby or the placenta’s blood supply is impaired, she may grow at a slower rate during pregnancy.
- Carrying multiples. If you’re carrying more than one baby, the space inside your uterus can get cramped quickly, which is why 60 percent of twins are born early and frequently at a low birth weight. On average, twins are delivered at around 35 or 36 weeks and weighing about 5.5 pounds.
- Not gaining enough weight during pregnancy. Gaining less than the recommended amount of weight during pregnancy can result in a baby who’s born at a low birth weight. Talk to your health care provider if you have a history of an eating disorder, are struggling to gain the recommended amount of weight in the second or third trimesters, are losing weight in the first trimester, or have severe morning sickness and can’t keep down any food at any point during pregnancy.
- Substance abuse. Drinking alcohol, smoking cigarettes, using illicit drugs and abusing prescription medications during pregnancy can inhibit a baby’s growth in the womb, leading to IUGR and a low birth weight baby.
- Chronic medical conditions and infections during pregnancy. Having high blood pressure or heart disease during pregnancy — or infections like the flu, rubella, cytomegalovirus, syphilis or toxoplasmosis — may cause IUGR, which can result in a low birth weight.
- Certain medications. Be sure your practitioner knows if you’re taking any prescription or over-the-counter medicines, supplements or herbal remedies during pregnancy. Some medications, including those used to treat epilepsy, high blood pressure and blood clots, can impact a baby’s growth in the womb, so your doctor may want to switch you to a more pregnancy-friendly option.
What are the risk factors for a low birth weight baby?
The following risk factors during pregnancy can increase the chances of having a low birth weight baby:
- Living with extreme stress. Having a low socioeconomic status (including a low income) or experiencing domestic violence (physical, emotional or sexual abuse) during pregnancy are linked to low birth weight in babies.
- Your age. Women who are under the age of 15 or over the age of 35 are more likely to have a low birth weight baby.
- Your race. In the U.S., Black mothers are significantly more likely to have a low birth weight baby than women of other races. About 14 percent of all babies born to Black moms are low birth weight, compared to 6 to 8 percent of babies born to moms of other races.[4] Babies born to Black mothers are also significantly more likely to be born preterm, even when other potential risk factors (like smoking and obesity) are taken into account. Researchers continue to study the cause for these trends. Access to prenatal care, discrimination and systemic racism are thought to play a role.
Other risk factors for having a low birth weight baby include having a chronic medical condition (like high blood pressure or heart disease), getting sick with certain infections during pregnancy (like cytomegalovirus or toxoplasmosis), carrying multiples, drinking, smoking, or taking illicit drugs or certain medications (including those for high blood pressure and epilepsy).
How does low birth weight affect infants?
If your baby is born a few ounces under a normal birth weight, chances are very good she’ll quickly catch up to the curve with few if any health problems.
But the earlier and the lower birth weight a baby is born, the more likely she is to have health and developmental problems.
In the short-term, low birth weight babies — especially those who are born premature or are very underweight — are more likely to have:[5]
- Feeding struggles. While babies born near a normal birth weight often quickly master eating basics, those who are born before 34 to 36 weeks often struggle with breastfeeding or bottle-feeding because of an immature or underdeveloped sucking reflex.
- Breathing problems. Babies who are born early sometimes haven’t had enough time in the womb for their lungs to develop, which can lead to treatable breathing problems like respiratory distress syndrome (RDS).
- Jaundice. This common condition happens when a baby’s liver isn’t fully functional, causing a substance called bilirubin to build up in the blood that makes a baby’s eyes or skin look yellow.
- Infections. Because preterm babies’ immune systems aren’t fully developed, they’re at greater risk for infections.
- Patent ductus arteriosus (PDA). This heart defect occurs when two major blood vessels in the heart don’t close properly. It may resolve on its own within a few days of birth.
- Bleeding in the brain. Known as intraventricular hemorrhage (IVH), most bleeding in the brain is mild and clears up on its own, but more severe bleeding can potentially lead to brain injury and developmental disabilities.
- Vision or hearing problems. Retinopathy of prematurity (ROP) is an eye disease in many extremely premature babies when the retinas develop abnormally after birth. Low birth weight babies may also be at risk of hearing problems.
Babies born at low birth weight are also more likely to have other longer-term health problems later in life than those born at a normal birth weight, including:
- Intellectual and developmental disabilities (trouble or delays in learning, communication or physical development)
- Diabetes, high blood pressure or heart disease (or metabolic syndrome, a combination of all three)
- Obesity
Keep in mind that your low birth weight baby might not have any significant short- or long-term health problems, especially if she’s born close to a normal weight and her due date. And as they get older, most preterm babies catch up to their full-term counterparts.
Treatments for a low birth weight baby
Doctors and nurses in the neonatal intensive care unit (NICU) treat premature and low birth weight babies all day, every day, and they have many high-tech tools at their disposal to help ensure that their tiniest patients get the care they need for a healthy start.
In the hospital, low birth weight baby management includes:
- IV fluids or gavage feeding. For babies who struggle to feed, an IV or a small tube placed in the stomach via the mouth ensures delivery of the vital nutrients, vitamins, minerals and fluids necessary to grow and thrive.
- Light therapy. Babies with jaundice may be placed under special lamps that help the body process and excrete bilirubin. If light therapy doesn’t work, other more intensive treatments involving blood exchange transfusions or IV injections of antibodies may be required, but these are rare.
- Supplemental oxygen or surfactant. Babies with immature lungs may require supplemental oxygen or surfactant (a protein that helps keep the air sacs in the lungs from collapsing).
- Surgery or medication. If your baby has ROP (eye problems) or PDA (heart problems), your doctor may recommend medication or surgery.
- Removal of fluids in the brain. Severe bleeding in the brain can be treated by removing excess fluid with a small tube.
Once you return from the hospital, you may need to keep your home warmer than usual (at least 72 degrees Fahrenheit or so), sterilize your baby’s bottles, give your baby a vitamin supplement and feed her extra and often.
Kangaroo care (skin-to-skin contact) in the hospital and at home can help your low birth weight baby succeed at breastfeeding and gain weight, among many other benefits.
Be sure to ask your doctor if you have any questions about low birth weight baby care and development. Ensuring your child attends all her doctor appointments, gets the recommended vaccines at the appropriate times, and undergoes therapy to treat any developmental delays (if they occur) can help facilitate the best outcomes.
If your baby was born preterm, the American Academy of Pediatrics (AAP) suggests calculating her “corrected age” for the first two years of life.[6] This takes into account how early your baby was born to estimate when she should hit certain milestones.
While preterm and low birth weight babies may have slower developmental progress for a while at first, it’s usually not cause for concern. It's a good idea to be in close touch with your doctor, though, just to make sure your baby is growing as expected.
Over the long term, more than 2 out of 3 babies born prematurely will turn out to have no developmental problems, and most others will only have mild or moderate disabilities or delays. The risks of permanent development issues are much greater in babies who weigh less than 1.6 pounds at birth or who are delivered before 25 weeks. Most preterm babies have normal IQs, although some may have learning issues.
Can you prevent low birth weight in babies?
While there’s no way to prevent low birth weight in babies, there are a few steps you can take to reduce your child’s risk:
- See your doctor. Visit your practitioner as soon as you find out you're pregnant, and attend all your prenatal checkups.
- Arm your practitioner with info. Let your doctor know if you had a low birth weight baby in the past, as there may be specific steps you can take to reduce the risk in your current pregnancy. Also tell your doctor if you have any chronic health conditions and if you take any prescription medications.
- Don’t drink or smoke. Avoid alcohol and cigarettes while you’re expecting, and don’t take any drugs that aren’t okayed and/or prescribed to you by your practitioner.
- Eat a healthy, nutritious diet. Be sure to follow your OB/GYN's advice if you’re having trouble gaining the recommended amount of pregnancy weight.
- Get the flu vaccine. Catching the flu during pregnancy has been associated with an increased risk of having a low birth weight baby. During the fall and winter, the CDC recommends all pregnant women get the flu vaccine, which can also reduce the risk of hospitalization and other complications.
Yes 
No 
Trending On What to Expect
