Small for gestational age babies are born smaller than expected for the number of weeks of pregnancy, often weighing below the 10th percentile compared to other newborns of the same gestational age. While many of these tiny infants are simply smaller by nature and grow up healthy, some face growth challenges that began in the womb, requiring careful monitoring and support.

Understanding Small for Gestational Age

When a baby is described as small for gestational age, or SGA, it means their birth weight falls below the 10th percentile for babies born at the same stage of pregnancy. To put this simply, if you line up 100 babies born at the same number of weeks, the smallest 10 would be considered small for gestational age. The term gestational age refers to how many weeks the pregnancy lasted, counting from the first day of the mother’s last menstrual period to the day of birth.^[1]

Not all small babies are unhealthy. Some babies are small simply because their parents are small, and this is perfectly normal for them. These infants may appear physically and neurologically mature despite their smaller size. They can be proportionately small, meaning their weight, length, and head size are all similarly small, or they may have normal length but lower weight and body mass.^[1]

Small for gestational age babies can be born at different stages of pregnancy. They may arrive prematurely, before 37 weeks of pregnancy, at full term between 37 and 41 weeks, or even post-term after 42 weeks. Most babies typically weigh more than 5 pounds and 13 ounces by the 37th week of pregnancy, and those born weighing less than 5 pounds and 8 ounces are considered low birth weight.^[2]

Epidemiology

Between 3% and 10% of all live births each year are diagnosed as small for gestational age. This means that out of every 100 babies born, roughly 3 to 10 will fall into this category. The variation in these numbers depends partly on which definition and measurement standards are used by healthcare providers.^[7]

Most newborns who are moderately small for gestational age turn out to be healthy babies who simply happen to be on the smaller side. About 80% to 85% of children born small for gestational age experience rapid catch-up growth during their first 12 months of life. By contrast, 10% to 15% do not experience this catch-up growth, and it is this smaller group that often requires ongoing attention and care.^[16]

Approximately 9 out of 10 infants born small for gestational age do experience catch-up growth by the age of 2 years, usually by 6 months of age. Catch-up growth typically means that the child’s growth curve moves upward, crossing the 3rd percentile line at minimum and ideally getting closer to the percentile curve the child should be at based on their parents’ heights. The smaller subset of children, about 1 in 10, who fail to achieve catch-up growth by age 2 are often referred to as “short SGA” children.^[7]

Causes

Although some babies are small simply because of genetics, meaning their parents are small, most babies who are small for gestational age are small because of growth problems that happened during pregnancy. Many of these babies have a condition called intrauterine growth restriction, often abbreviated as IUGR. This term refers to a situation where the baby in the womb does not receive the necessary nutrients and oxygen needed for proper growth and development of organs and tissues.^[1]

Intrauterine growth restriction can begin at any time during pregnancy. When it starts early in pregnancy, often during the first trimester, it is called early-onset IUGR and is often due to chromosomal abnormalities, maternal disease, or severe problems with the placenta. When a problem with the baby begins early in pregnancy, it tends to affect the baby’s whole body, resulting in what is called symmetric growth restriction, where the newborn is proportionately small in weight, length, and head size.^[3]

Late-onset growth restriction occurs after 32 weeks of pregnancy and is usually related to other problems. When a problem begins later in pregnancy, it does not affect the baby’s body equally because some tissues develop sooner than others. This is called asymmetric growth restriction, where weight, length, or head size are not equally affected. For example, the head size may continue to grow as expected while weight or length may be lower than expected.^[3]

Risk Factors

When the unborn baby does not get enough oxygen or nutrients during pregnancy, the baby’s body and organs do not grow as much as they should. Some problems that cause babies to be small for gestational age limit how much blood flows through the placenta, which is the organ that connects the developing baby to the mother’s blood supply. This can cause the baby to get less oxygen than normal, which increases the baby’s risks during pregnancy, delivery, and later in life.^[2]

Maternal Risk Factors

The risk of having a small for gestational age baby is increased for mothers who are adolescents or over 35 years of age, or who have had other babies who were small for gestational age. Several medical conditions in the mother can increase the risk of having an SGA baby. High blood pressure, also called hypertension, is one of the most significant risk factors. Other conditions include long-standing diabetes, chronic kidney disease, heart disease or lung disease, lupus, severe anemia, sickle cell disease, and abnormalities of the uterus, such as a uterus that has two parts.^[3]

Severe malnutrition in the mother can also contribute to having a small baby. Mothers who weigh less than 100 pounds are at higher risk. Infections during pregnancy can affect the baby’s growth, and substance use poses serious risks. Using alcohol, tobacco, or illicit drugs such as amphetamines, cocaine, or opioids during pregnancy significantly increases the chance of having a small for gestational age baby.^[1]

Medical conditions that develop during pregnancy also play a role. Having more than one baby, such as twins or triplets, is associated with smaller babies. Interestingly, twins typically grow at the same rate as single babies until about 32 weeks of pregnancy, after which they grow more slowly and may be small for gestational age at birth. For triplets, this slower growth begins even earlier, at about 28 weeks. Other pregnancy complications include use of assisted reproduction to conceive, preeclampsia (a condition involving high blood pressure and protein in the urine during pregnancy), and early separation of the placenta from the uterus, called placental abruption.^[3]

Taking certain medications during pregnancy can affect fetal growth. These include antiseizure medications and certain cancer medications. Even without these specific risks, cigarette smoking alone is a major contributor to having a small baby.^[1]

Placental and Uterine Risk Factors

Problems with the uterus and placenta can directly affect a baby’s growth. Decreased blood flow in the uterus and placenta limits the nutrients and oxygen reaching the baby. When the placenta detaches from the uterus or attaches too low in the uterus, growth can be affected. Infections in the tissues around the baby can also interfere with normal development.^[2]

Fetal Risk Factors

Sometimes the cause of small size lies with the baby itself. Genetic disorders that may include birth defects of the brain, heart, or kidneys can result in smaller babies. Certain infections in the baby, including Zika virus, cytomegalovirus (CMV), or rubella (also known as German measles), can restrict growth. Chromosomal abnormalities are another important cause, particularly when growth restriction begins early in pregnancy.^[3]

Symptoms

Despite their smaller size, many small for gestational age newborns may look and act similar to newborns of similar gestational age who are typical in size. Some babies appear physically and neurologically mature but are simply smaller than other babies born at the same stage of pregnancy. They may be small all over, with all body parts proportionately smaller, or they may have normal length and size but lower weight and body mass.^[3]

Some small for gestational age newborns appear thin and have less muscle mass and fat than expected. A number of these babies may have sunken facial features. The way a baby looks depends partly on whether the growth restriction was symmetric or asymmetric, and when during pregnancy the growth problems began.^[3]

It is important to understand that many small for gestational age newborns have no symptoms at all and do very well. The appearance and health of these babies varies considerably depending on the underlying cause of their smaller size and whether they experienced true growth restriction or are simply constitutionally small.^[3]

Prevention

While not all cases of small for gestational age babies can be prevented, especially those due to genetic factors or unavoidable medical conditions, there are several steps that can reduce the risk. Avoiding tobacco, alcohol, and illicit drugs during pregnancy is one of the most important preventive measures. These substances can significantly restrict blood flow to the baby and interfere with normal growth.^[1]

Maintaining good overall health before and during pregnancy helps reduce risk. This includes managing chronic conditions such as high blood pressure, diabetes, and kidney disease with the help of healthcare providers. Proper prenatal care allows for early detection and management of complications that might affect fetal growth. Women who attend regular prenatal appointments give their healthcare providers the opportunity to monitor the baby’s growth and identify any concerns early.^[2]

Adequate nutrition during pregnancy supports healthy fetal growth. Women who are underweight before pregnancy or who have severe malnutrition are at higher risk of having small babies. Eating a balanced diet with sufficient calories and nutrients, and taking prenatal vitamins as recommended by healthcare providers, can help support normal fetal development.^[1]

Pregnant women should work closely with their healthcare providers to ensure any medications they take are safe during pregnancy. Some medications, including certain antiseizure drugs and cancer treatments, can affect fetal growth. In some cases, medication adjustments may be possible to reduce these risks while still managing the mother’s health conditions.^[3]

Pathophysiology

The underlying physical and biochemical changes that lead to a baby being small for gestational age often involve the placenta not working as well as it should. The placenta is the organ that develops during pregnancy and attaches to the wall of the uterus. It serves as the connection between the mother’s blood supply and the developing baby, transferring oxygen and nutrients to the baby and removing waste products.^[1]

When the placenta does not function properly, it may not deliver adequate nutrients and oxygen to the growing baby. This can happen for various reasons, including problems with how the placenta attached to the uterus, maternal health conditions that affect blood flow, or structural abnormalities in the placenta itself. Reduced blood flow through the placenta means the baby receives less oxygen than normal, which can slow growth and development.^[2]

In cases of symmetric growth restriction that begins early in pregnancy, the problem often lies with the baby itself rather than the placenta. Chromosomal abnormalities or genetic disorders can affect how cells divide and grow throughout the baby’s body. When these problems occur early in development, they affect all body systems equally, resulting in a proportionately small baby.^[3]

Asymmetric growth restriction, which typically occurs later in pregnancy, reflects the baby’s adaptive response to limited resources. When oxygen and nutrients become scarce, the body prioritizes growth of vital organs, particularly the brain. This is why in asymmetric growth restriction, the head size may remain closer to normal while body weight and length fall behind. This protective mechanism, sometimes called “brain sparing,” helps preserve brain development even when overall growth is compromised.^[3]

Infections during pregnancy can cause growth restriction through different mechanisms. Some viruses and other pathogens can cross the placenta and directly infect the developing baby, damaging tissues and interfering with normal cell division and organ development. Other infections may cause inflammation that affects placental function without directly infecting the baby.^[3]

Depending on the cause of growth restriction and when it occurs during pregnancy, the development of the brain and other vital organs may be affected. These effects can result in long-standing and perhaps lifelong problems, though many small for gestational age babies develop normally. Some small newborns remain small as adults, while others eventually reach heights within the normal range.^[3]