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What Does Low Birth Weight Mean?

Low birth weight contributes to 60 to 80 percent of neonatal deaths worldwide. Approximately 20 million low birth weight babies are born every year, most of them in developing countries. Preterm births, those occurring before the 37th week of pregnancy, are also on the rise. They account for over 10 percent of births globally. Most premature infants also have low birth weights.

Complications from prematurity and low birth weight are the leading cause of death for children under age five around the world. Many survivors of preterm birth and low birth weight may suffer from disabilities such as learning problems, hearing impairment and vision issues.

The normal range of birth weight is from 5.5 to 8.75 pounds. Delivery after the 37th week of pregnancy is considered “at term.” Any baby born weighing less than 5.5 pounds is low birth weight, and a baby born before 37 weeks is premature.

Newborn Baby Being Weighed on Scale

About one in 12, or eight percent, of babies in the United States is born underweight, and about two-thirds of these low birth weight babies are also premature. A small percentage, about 1.4 percent of babies have very low birth weight – under 3.5 pounds. Almost all of these babies are premature, and some are extremely premature – born at 25 weeks or earlier.

The number of babies born with low birth weight is increasing along with the number of multiple births in the United States. More than half of multiple birth babies have low birth weight, and six percent of single birth babies are born with low birth weight.

Can a Doctor Diagnose Possible Low Birth Weight Before Delivery?

During a prenatal visit, the doctor will check for medical problems that might indicate the baby is not growing properly. Conditions of the mother, such as preeclampsia and gestational diabetes, may limit the nutrition the baby gets through the placenta.

The doctor will measure the height of the fundus, or the top of the uterus, from the pubic bone. The fundal height measurement in centimeters should correspond with the week of pregnancy from the 20th week onward. If it is less than expected, the doctor may worry that the baby is not growing as it should. The doctor may also estimate a baby’s size through ultrasound.
Low birth weight babies can be born at term or prematurely. Any baby born who weighs at or below the 10th percentile for weight based on their gestational age at birth is small for gestational age (SGA) or low birth weight.

What Causes Low Birth Weight?

The main cause of low birth weight is premature birth, or any delivery before 37 weeks. The baby has not had enough time in the uterus to grow and gain weight. Babies born at term who are underweight may still be small and weak. Premature babies will be both small and not yet fully developed.

Premature Newborn Baby Gaining Weight in a NICU IncubatorIntrauterine Growth Restriction (IUGR) and SGA are not the same thing, although they often appear together. IUGR is when the baby’s weight is under the 10th percentile for their gestational age before they are born. It is diagnosed by ultrasound. IUGR indicates the fetus has not reached its full potential for growth because of some environmental factor in the womb or some genetic factor. SGA refers to a baby who weighs less than the 10th percentile for their gestational age at birth.

With IUGR, the baby may be developing too slowly due to infections in the uterus or due to birth defects. Babies with birth defects are more likely to be born prematurely and also with low birth weights.The doctor will want to establish an accurate gestational date for the pregnancy, starting the count from the first day of the last menstrual cycle. Once the gestational age of the fetus is known, the doctor can more easily determine whether the fundal height coincides with the expected size of the baby. The doctor can also compare ultrasound findings with expectations for a fetus of that age.

Some women are more at risk for having babies with IUGR. Risk factors in the mother include:

  • Weight of the mother under 100 pounds.
  • Poor prenatal nutrition
  • Use of alcohol, cigarettes or drugs
  • Pregnancy-induced high blood pressure or preeclampsia
  • Gestational diabetes
  • Chronic health conditions like diabetes, heart problems, lung or kidney problems, and high blood pressure
  • Taking medications for conditions including blood clots, seizures and high blood pressure
  • Inadequate weight gain during pregnancy
  • Birth of a previous low birth weight baby.
  • Age under 17 or over 35
  • Racial and ethnic factors: Women of color in the United States are between seven and 13 times more likely to have a baby with low birth weight.
  • Socioeconomic factors, such as poverty, low education and exposure to domestic violence.

Conditions inside the uterus may also cause the fetus difficulty in getting the nutrition it needs to grow and develop. These include:

  • Birth defects and chromosomal abnormalities
  • Problems or abnormalities with the placenta
  • Problems with the umbilical cord.
  • Being a fetus with a twin or triplet (or more) in the uterus.
  • Inadequate amniotic fluid (oligohydramnios)
  • Infections in the uterus, such as rubella, chicken pox, sexually transmitted infections, cytomegalovirus or toxoplasmosis
  • Birth by preterm labor shortening the time available to grow and develop.

What Can Be Done to Prevent Low Birth Weight Before Delivery?

The best way to avoid having a low birth weight baby is to get early prenatal care and continue it throughout the pregnancy. The doctor tracks the growth and development of the baby and knows of any warning signs early enough to allow interventions. If the mother develops health problems, these can be treated before they become major problems for the baby. Ensure good maternal nutrition and adequate weight gain during pregnancy and avoid alcohol, drugs and cigarettes to improve the odds of delivering a healthy baby.

Medical science is allowing the survival of smaller and smaller babies, and babies are born much earlier in the gestational period. These babies are very underdeveloped and face a lifetime of struggles, much of which could have been prevented by being kept in the womb longer.

A pregnant woman should control her diabetes or high blood pressure and take prenatal vitamins with folic acid throughout her pregnancy. She should follow her doctor’s recommendations regarding how much weight to gain during her pregnancy. She should alert her medical team immediately if she has any signs of preterm labor.

What Are Some Complications for Babies With Low Birth Weight?

Premature babies born around 32 weeks may struggle with breathing, staying warm and eating. Babies born very early, after fewer than six months or 26 weeks gestation, are the ones most likely to have more serious problems.

Premature babies may have problems breathing, called respiratory distress syndrome (RDS). Babies with RDS lack a protein called surfactant, which helps keep small air sacs in their lungs from collapsing. They receive this surfactant in the neonatal intensive care unit (NICU) to help their lungs work better and mature. They may also get supplemental oxygen.

Some premature babies and babies with low birth weight develop intraventricular hemorrhage, or brain bleeds. Most are mild and fix themselves, and leave no ongoing problems. More severe bleeds cause fluid buildup and pressure in the brain, which can cause brain damage. The baby may require surgery and the insertion of a tube to drain the excess fluid and prevent damage.

One problem seen more often is patent ductus arteriosus, in which an opening between two of the major blood vessels leading away from the heart has not yet closed properly. This leads to extra blood flow into the lungs. In some cases, the hole closes on its own after a few days. In other babies, surgery or medication may be required.

Tiny babies also have immature intestines that are more prone to necrotizing enterocolitis. This leads to swelling of the belly, feeding problems, and possibly surgery to remove damaged parts of the intestine.

Many premature infants are born with the retinas of their eyes not yet fully developed. Retinopathy of prematurity affects both eyes and requires immediate treatment for the baby to keep their vision.

Low birth weight and premature infants may have livers that are not working well or are not fully developed. They cannot process the chemical bilirubin out of their body and develop jaundice. Their eyes and skin turn yellow because of the excess bilirubin in their blood. Treatment includes time under special lights to transform the bilirubin into a chemical that is easier to rid from the body, IV treatment with immunoglobulin blood proteins, or even blood transfusions.

Because many low birth weight babies are also premature, doctors may have trouble telling which problems are due to the prematurity and which are due to the low birth weight. Regardless, the smaller a baby is at birth, the more likely he or she will have complications. Their survival depends on how well they gain weight and avoid infections.

Babies born with IUGR are statistically at higher risk for:

  • Birth by cesarean section.
  • Lack of oxygen (hypoxia) at birth.
  • Meconium aspiration, where the baby swallows part of its first bowel movement. The alveoli sacs in the baby’s lungs become distended and a pneumothorax, or collapsed lung, can occur. The baby may develop bacterial pneumonia.
  • Hypoglycemia, or low blood sugar, at birth.
  • Polycythemia, an increased number of red blood cells, related to lack of oxygen.
  • Decreased blood flow (hyperviscosity) due to excess red blood cells.
  • Increased risk of neurological and motor disabilities and delays.
  • Higher risk of social delays and learning disabilities.

How Do Doctors Treat Babies With Low Birth Weight?

Treatment for the baby is determined by the doctor depending on the baby’s gestational age, health and ability to tolerate treatments. Many times, care of a low birth weight or preterm baby includes spending time in the NICU, a temperature-controlled incubator, and special feeding through a stomach tube or IV line. Typically, low birthweight babies can catch up in growth if they have no other complications.
Low birth weight babies need to receive enough nutrition to grow in the NICU at about the rate they would have if they had stayed in the uterus. For a tiny baby born at 24 weeks, this may mean gaining about five grams (0.20 ounce) per day at first. A larger baby born around 33 weeks may need to gain 20-30 grams (0.7 to 1.1 oz) per day. Generally, a low birth weight baby should gain about ¼ ounce per day for every pound they weigh. This is the average rate a fetus grows throughout the third trimester of pregnancy.

Very premature babies may have trouble sucking from a breast or bottle because they cannot coordinate sucking, swallowing and breathing. Less premature babies often do best breastfeeding. A bottle nipple may cause them problems because it is harder to control.

Even the smallest low birth weight and premature babies benefit from breast milk, whether it comes from breastfeeding, a bottle, or via a tube feeding. Breast milk contains antibodies from the mother that help protect the baby from infections, Sudden Infant Death Syndrome (SIDS), and necrotizing enterocolitis (NEC). If the mother cannot produce enough milk, many NICUs will give babies breast milk from a milk bank because breast milk has benefits that cannot be replicated with formula.

After birth, it is important to guard low birth weight babies against infections, breathing problems and dehydration. These babies lose more water than babies born at term or normal weights because their kidneys are not developed enough to control water levels. The nurses in the NICU keep close watch on the baby’s fluid intake and output to ensure they stay properly hydrated.

What are Some Long Term Implications for Babies Born with Low Birth Weight?

Low birth weight babies are more likely to have health problems later in their lives. These issues may be related to also being born prematurely, or to failing to get the nutrition they needed at critical times during their gestation. Early intervention and treatment are critical to helping growing kids develop normally.

As medicine allows smaller and more prematurely born infants to survive, we see these children developing a range of health outcomes. Some have no illnesses or negative outcomes at all, while others continue to have slower growth, more illnesses, and other problems throughout their lives. Babies with low birth weight born into situations where they are at risk socially or economically are more at risk for health problems

About 80 percent of low birth weight infants suffer some long-term side effects, from impaired immune systems or lung problems to learning disabilities, behavior problems or even cerebral palsy. About 20 percent of premature and low birth weight babies go on to have no health problems at all. However, parents of all low birth weight infants must provide good nutrition and health care throughout childhood to ensure the best outcomes for these children.

If you are concerned about your baby’s risk for low birth weight, talk to your obstetrician at Kansas City ObGyn. Contact us at This email address is being protected from spambots. You need JavaScript enabled to view it. or (913) 948-9636.