Small for Dates Baby

Small for Gestational Age, SGA

When a baby is born smaller than expected for the number of weeks of pregnancy, parents naturally worry. While most small babies are simply on the smaller side and perfectly healthy, understanding what “small for gestational age” means can help families navigate this experience with greater confidence.

Table of contents

• What Does Small for Gestational Age Mean?
• Why Are Some Babies Small?
• Who Is at Risk?
• How Small Babies Look and Act
• What Being Small Means for Your Baby

What Does Small for Gestational Age Mean?

Small for gestational age, often shortened to SGA, is a term doctors use to describe babies who are smaller than expected for the number of weeks of pregnancy^[1]. These babies have birth weights below the 10th percentile (a way of comparing measurements to show where a value falls in comparison to others). This means they are smaller than 90% of other babies born at the same stage of pregnancy^[2].

Gestational age refers to the number of weeks of pregnancy, counted from the first day of the mother’s last menstrual period to the day of birth^[3]. This time frame is often adjusted based on early ultrasound scans, which provide additional information about how far along the pregnancy is. A baby is expected to arrive around 40 weeks of gestation.

Most babies normally weigh more than 5 pounds, 13 ounces by the 37th week of pregnancy. Babies born weighing less than 5 pounds, 8 ounces are considered to have low birth weight^[2].

SGA babies may appear physically and neurologically mature but are simply smaller than other babies of the same gestational age^[1]. They may be small in a balanced way, with weight, length, and head size all proportionately small. Or they may be of normal length and size but have lower weight and body mass^[1].

SGA babies may be born at different times during pregnancy. They can be premature (born before 37 weeks of pregnancy), full term (37 to 41 weeks), or post term (after 42 weeks of pregnancy)^[1].

Why Are Some Babies Small?

Some babies are small simply because their parents are small and this is normal for them^[2]. Most newborns who are moderately small for gestational age are healthy babies who just happen to be on the smaller side^[3].

However, most SGA babies are small because of growth problems that occur during pregnancy^[1]. Many of these babies have a condition called intrauterine growth restriction or IUGR, also known as fetal growth restriction^[4]. This happens when the unborn baby doesn’t get the nutrients and oxygen needed to grow and develop organs and tissues properly. This can begin at any time during pregnancy^[2].

Growth restriction can be classified in two ways. Symmetric growth restriction means the baby is proportionately small—weight, length, and head size are all similarly low. Asymmetric growth restriction means these measurements are not equally affected. For example, the head size may be normal while weight or length is lower than expected^[3].

Symmetric growth restriction is usually caused by a problem with the baby that begins early in the pregnancy, often during the first three months. When a problem begins early, it affects the baby’s whole body. Asymmetric growth restriction is usually caused by a problem with the placenta (the organ that provides nutrients and oxygen to the baby) or with the mother that occurs later in pregnancy, during the last six months^[3].

Early-onset IUGR is often due to chromosomal abnormalities, maternal disease, or severe problems with the placenta. Late-onset growth restriction (after 32 weeks) is usually related to other problems^[1].

Who Is at Risk?

When the unborn baby doesn’t get enough oxygen or nutrients during pregnancy, the baby’s body and organs don’t grow as much as they should^[2]. Some of the problems that cause babies to be small for gestational age limit how much blood flows through the placenta. This can cause the baby to get less oxygen than normal, which increases risks during pregnancy, delivery, and later^[2].

The risk of having an SGA baby is increased for mothers who are teenagers or over 35 years of age, or who have had other babies who were SGA^[3].

Several factors related to the mother’s health can contribute to having a small baby:

• High blood pressure
• Chronic kidney disease
• Long-standing or advanced diabetes
• Heart disease or respiratory disease
• Severe malnutrition or anemia (low blood count)
• Certain infections
• Use of alcohol, tobacco, or drugs
• Weighing less than 100 pounds^[1][2]

Problems with the uterus and placenta can also cause a baby to be small:

• Decreased blood flow in the uterus and placenta
• Placenta detaching from the uterus (placental abruption)
• Placenta attaching low in the uterus
• Infection in the tissues around the baby
• Abnormalities of the uterus^[2][3]

During pregnancy, certain conditions increase the risk of having an SGA baby, including having more than one baby (such as twins or triplets), use of assisted reproduction to conceive, preeclampsia (a serious pregnancy complication involving high blood pressure), or use of certain medications such as antiseizure drugs or cancer medicines^[3].

Factors related to the baby itself can also play a role:

• Multiple pregnancy, such as twins or triplets
• Genetic disorders that may include birth defects of the brain, heart, or kidneys
• Chromosome problems
• Certain infections in the baby, including Zika virus, cytomegalovirus (CMV), or rubella (German measles)^[2][3]

How Small Babies Look and Act

Despite their size, small-for-gestational-age babies may look and act similar to babies of the same gestational age who are typical in size^[3]. Small for gestational age babies may look mature, but they are smaller than other babies of the same gestational age^[2].

Some SGA babies appear thin and have less muscle mass and fat. Some have sunken facial features^[3]. They may be small all over, or they may be of normal length and size but have lower weight and body mass^[2].

What Being Small Means for Your Baby

Many small-for-gestational-age newborns have no symptoms and do well^[3]. About 80% to 85% of children born SGA experience rapid catch-up growth during the first 12 months of life. However, 10% to 15% do not experience catch-up growth^[7].

Between 9 out of 10 infants born SGA experience catch-up growth by the age of 2 years, and usually by 6 months of age. Catch-up growth typically means that the child’s growth curve moves upward, ideally getting closer to where the child should be based on their parents’ heights^[7].

Depending on the cause of growth restriction, the development of the brain and other vital organs may be affected, resulting in long-standing and perhaps lifelong problems^[3]. Some small newborns remain small as adults^[3].

Just because an infant is born small for gestational age does not mean additional care is needed at home. Infants will be kept in the hospital if there are any complications until they are deemed healthy enough for discharge^[18].