Intrauterine growth restriction (IUGR) occurs when a baby does not grow at the expected rate during pregnancy.¹ It means the fetus is smaller than expected for gestational age, often due to limited nutrients or oxygen reaching the baby in the womb. While the diagnosis can feel overwhelming, many babies with IUGR are closely monitored and go on to do well with proper care.

In this article, we’ll explain what IUGR is, how it differs from similar terms like SGA and low birth weight, what causes it, how it’s diagnosed, and the treatment options available. Understanding the condition can help parents feel more prepared and supported throughout pregnancy.

What’s the Difference Between IUGR, SGA, and Low Birth Weight?

These terms are often used interchangeably, but they describe different aspects of fetal growth and newborn size.^2,7

A typical or “normal” neonate is one who doesn’t have malnutrition or growth restriction and whose birth weight falls between the 10th and 90th percentile, according to the fetus’s gestational age, gender, and race. Babies with IUGR are typically below the 10th percentile in weight.² This means that 90% of other babies in their category weigh more than they do.

Medical literature may sometimes use the terms “IUGR” and “SGA” (small for gestational age) interchangeably; however, these terms aren’t synonymous.⁷ SGA refers to babies who are smaller than a typical infant at their age, but the definition only considers birth weight.³ According to a “Clinical Medicine Insights: Pediatrics” review, SGA doesn’t consider “in-utero growth and physical characteristics at birth.” The review also notes that IUGR is a clinical definition of babies born with malnutrition and in-utero growth restriction, regardless of their birth weight percentile, though they’re usually underweight.⁴

According to this same review, a baby can be born SGA because it has a birth weight less than the 10th percentile. But sometimes, the baby may not be classified as an IUGR infant if there aren’t features of malnutrition or growth retardation. Therefore, the terms “IUGR” and “SGA” are related and overlap in terms of the baby being abnormally small, but they don’t mean the same thing.⁴

SGA babies can be proportionately small (equally small across their bodies). Or they may be of a normal length and size but still have lower weight and body mass. Sometimes, SGA infants are premature, full-term, or post-term. Some SGA babies are healthy babies born smaller than average, simply because their parents are of smaller stature. Low Birth Weight (LBW) is a different classification and shouldn’t be confused with IUGR or SGA. LBW is based on a baby’s birth weight unrelated to its gestational age, sex, race, or clinical features.⁸

What Are the Two Main Types of IUGR?

There are two main types of IUGR:^2,7

1. Symmetrical IUGR: This means an infant’s body is proportionately sized, although it is below the 10th percentile for weight in its category. Symmetrical IUGR (or primary IUGR) accounts for 20%-25% of all IUGR cases.
2. Asymmetrical IUGR: This is when an infant has a normal-sized head and brain, but the rest of their body or abdomen is smaller than normal. Asymmetrical IUGR (or secondary IUGR) isn’t evident until the third trimester of development.

What Causes IUGR?

IUGR occurs when a developing fetus doesn’t receive the essential nutrients and/or oxygen needed for proper growth and development of its organs and tissues.

IUGR can occur if a mother is underweight (less than 100 pounds), has poor health and nutrition, abuses drugs or alcohol, or smokes cigarettes. It can also occur due to genetics or a mother’s chronic illness.²

According to one study, IUGR is six times higher in underdeveloped or developing countries than in developed countries.⁵ This may be because malnutrition and poor maternal health are commonly linked to incidences of IUGR in neonates.¹² Good nutrition and supplements are harder to come by.

The condition of IUGR can develop at any point during a baby’s growth in the womb. Chromosomal abnormalities, maternal disease, or a problem related to the placenta’s health can cause early-onset IUGR. When IUGR appears late in pregnancy (after 32 weeks), it’s usually associated with other issues.³

Related: Alcohol During Pregnancy: The History and Dangers

Risk Factors for Intrauterine Growth Restriction

Let’s explore the risk factors for IUGR involving the mother, the uterus and placenta, and the fetus:³

Mother

Here are the maternal risk factors for IUGR:^2,3,7

• Conceived within 18 months of a previous birth
• Lives in impoverished conditions
• Malnutrition or anemia
• Is under age 17 or older than 35
• Pregnant with multiple babies
• Had a previously low birth weight baby
• High blood pressure
• PIH (pregnancy-induced hypertension)
• Chronic kidney disease
• Advanced diabetes
• Gestational diabetes
• Heart or respiratory/lung disease
• Preeclampsia or hyperemesis gravidarum
• Infection (rubella, cytomegalovirus, toxoplasmosis, syphilis)
• Substance use (alcohol, drugs)
• Cigarette smoking

Uterus and Placenta

Here are the risk factors involving the uterus and placenta:^2,3,7

• Constricted blood flow in the uterus and placenta
• Placental abruption (the placenta separates from the uterus too early)
• Placenta previa (the placenta blocks the cervical opening)
• Infection of tissues around the fetus
• Low levels of amniotic fluid

Fetus

Here are the risk factors involving the fetus:^2,3,7

• Infection
• Birth defects
• Chromosomal abnormality
• Umbilical cord abnormalities

How Is IUGR Diagnosed?

Your doctor or midwife will usually suspect IUGR during a routine prenatal exam. They’ll first assess your “fundal height,” or the distance from the pubic bone to the top of the uterus, measured in centimeters. This basic yet essential test lets your provider know if your uterus, and thus the baby, is growing properly. If your doctor determines that the fundal height is too small for the baby’s gestational age, they’ll likely follow with an ultrasound. The ultrasound checks your baby’s size and the umbilical artery’s blood flow. If the results are abnormal, it may indicate that the baby isn’t receiving enough blood or nutrients from the placenta, and further action may be necessary.^2,11

Are There Complications of IUGR?

Complications from intrauterine growth restriction can vary from infant to infant. Your doctor or midwife will likely have identified IUGR in advance and developed a plan of action with you for your baby’s delivery. At birth and beyond, your infant may experience the following complications from IUGR:²

• Polycythemia (high red blood cell count)
• Hypoxia (lack of oxygen when baby is born)
• Meconium aspiration (baby swallows their first bowel movement in utero)
• Hypoglycemia (low blood sugar)
• Difficulty with body temperature equilibrium
• Low Apgar scores at birth (unhealthy breathing, heart rate, muscle tone, etc.)
• Hyperviscosity (blood doesn’t flow well)
• Lifelong disabilities
• Stillborn

How Do You Treat IUGR?

The treatment plan for babies diagnosed with IUGR can vary from case to case, depending on how far along the pregnancy is and how severe the baby’s condition is:⁷

• Keeping track of baby’s growth: A doctor or midwife will monitor the baby’s growth, conduct frequent ultrasounds and umbilical blood flow tests, and regularly assess the baby, placenta, and mother’s overall health.⁷
• A nutrient-rich diet for mom: Though this sounds obvious, we can’t overstate this fact enough! Pregnant mothers need to have a nutrient-rich diet. A diet high in vegetables, fruit, fiber, and probiotic foods benefits the mother’s and baby’s health. Diets high in those nutrients have been associated with a reduced risk of IUGR.⁶
• Conventional medications: A mother may be given medications to improve placental blood flow or help treat another diagnosed problem that contributes adversely to IUGR.
• Intravenous feedings and steroids: In some instances, your medical provider may administer intravenous feedings or steroids to help your baby mature more quickly.
• Hospital stays: In the advanced stages of IUGR, a pregnant mother may be admitted to the hospital for round-the-clock care and attendance.
• Induction or C-section: If the infant’s lungs are mature and the placenta or in-utero environment can’t be improved, a doctor or midwife may deliver your baby early at 32-34 weeks of gestation so they can get the nutrients and care your baby needs in a medical setting. Depending on the severity of the growth restriction and the results of testing, it may warrant delivery at even earlier gestational ages.⁷

Can You Prevent IUGR?

The cause of IUGR varies. In the case of chromosomal or genetic issues, it can be nearly impossible to prevent such a condition. However, when IUGR appears as a result of a mother’s poor health unrelated to genetics, it may be possible to prevent it. Avoiding smoking, maintaining a nutrient-dense and high-fiber diet, engaging in regular exercise and adequate sleep, and managing stress levels all contribute to improved maternal health and healthy fetal development.⁷

According to one study, maternal diets high in refined or processed foods were associated with poor pregnancy outcomes. Meanwhile, diets high in vegetables, fruit, and probiotic foods were associated with a reduced risk of FGR (IUGR) and other pregnancy complications.⁶

Related: Exercise During Pregnancy: The Dos and Don’ts

The Bottom Line

Receiving an IUGR diagnosis can feel scary, but many babies grow and thrive with careful monitoring and appropriate care. Working closely with your healthcare provider can help you understand your options and feel supported throughout your pregnancy journey.

FAQ: Intrauterine Growth Restriction (IUGR)