What does ‘small for gestational age’ mean?

Small for gestational age (SGA) is a term that describes a baby who is smaller than most other babies of the same gestational age. The gestational age of a baby needs to be thought about because the earlier the gestation, the smaller and lighter a baby tends to be. For example, a baby at 34 weeks could still be small for gestational age if it has the same weight and measurements as an average baby at 30 weeks gestation. The physical and neurodevelopment of an SGA baby reflects their gestational age rather than their size, so if born at term, a small baby will be just as mature as a normal-sized full-term baby.

How is a small baby detected?

An SGA baby is detected in the same way as a baby who is large for gestational age.

During most antenatal appointments, your midwife or doctor will check the size of your uterus by feeling your abdomen (this is called an abdominal palpation) and estimate where the top of the uterus is in relation to your pelvis and rib cage. This gives an indication of the size of the baby. If there is a suspicion that a baby may be growing too slowly or too quickly, then an ultrasound scan can be performed. Most women during pregnancy have routine ultrasound scans, but extra scans may be needed. These scans are conducted to find out if babies are growing and developing normally. Measurements of a baby’s bone length, head and abdominal circumference are taken, which are compared to the measurements of the ‘average’ baby (of the same gestational age).

The average measurement will fall on the 50th percentile (percentile means that the measurements are plotted on a scale with 100 points). Usually, a measurement that falls on or below the 10th percentile, meaning that the baby is smaller than 90% of babies at the same stage of development, suggests a baby is small for gestational age (or growth restricted, see below). This is the same for a baby’s measurements and weight following birth.

Your doctor may want to assess how well your placenta is functioning if your baby seems small. This can be done using a machine called a Doppler; sound waves measure the amount and speed of blood flow through vessels in the placenta.

What is intrauterine growth restriction?

Unlike a baby that is small because of genetics (its biological parents are small), a baby described as growth restricted has not reached its full growth potential for its gestational age. Growth-restricted babies will be on or below the 5th percentile.

On palpation and ultrasound scan (see above) a growth-restricted baby will seem smaller than expected. Women may notice that their baby’s movements are less frequent or rigorous than expected. Because growth-restricted babies do not have the same ‘reserves’ as normally grown babies, they do not have the same ability to cope and may show signs of fatigue earlier in labour. Signs that a baby is not coping with labour include an abnormal heart trace and meconium stained liquor (liquor is the water that surrounds the baby and meconium staining happens when the baby opens its bowels). Because of this, women with a baby thought to be growth restricted should be cared for by a consultant obstetrician and give birth in a maternity unit with appropriate neonatal care facilities.

Babies that are growth restricted may have difficulties transitioning to life outside the uterus; they may be less rigorous at birth and may easily become cold and develop low blood glucose levels. Therefore, it’s recommended and important to dry and wrap baby well following birth. Some small babies may need to be cared for on a neonatal unit until they can maintain their temperature and glucose levels by themselves.

What are the risk factors?

Intrauterine growth restriction (IUGR) occurs when the baby does not receive enough nutrition and oxygen through the placenta. Some risk factors can increase the chance of having a growth-restricted baby, these include maternal high blood pressure, kidney disease, diabetes (long standing), cardiovascular disease, infection, smoking and drugs use (including cocaine and heroin) and poor diet, multiple pregnancy (twins/triplets), some birth defects and some chromosomal abnormalities.

The risk factors and conditions above can interfere with the development of the placenta. This may lead to reduced placental blood flow, which decreases the amount of nutrients and oxygen available to the baby. Some birth defects including cardiac defects and genetic disorders can interfere with growth even when the placenta is functioning properly.

Intrauterine growth restriction is linked to some genetic disorders including Trisomy 13 (Patau syndrome), Trisomy 18 (Edward’s syndrome) and Trisomy 21 (Down syndrome), Triploidy and Turner’s syndrome. However, these conditions are rare and most growth restriction is due to poor functioning of the placenta and an inadequate nutrition and oxygen supply.

 Symmetrical and asymmetrical IUGR

There are two types of growth restriction: asymmetrical, in which a baby’s head and brain develop as normal but the body is smaller. This type of growth restriction shows that the baby is trying to grow normally and is protecting brain growth, which is why the head is of average size. If the cause of the growth restriction is prolonged, the baby may be unable to maintain brain growth and symmetrical growth restriction will occur, where a baby’s head and body are similarly small.

What can help baby’s growth?

There may be no obvious reason for a baby being small. However, some factors may increase the risks of IUGR, such as cigarette smoking and poor maternal nutrition. The risk of having a small baby (or developing a health condition like high blood pressure that may lead to growth restriction) can be reduced by being in the best possible shape before birth. Briefly that would mean being a healthy weight, eating a healthy diet and not gaining an excessive amount of weight in pregnancy, not smoking or drinking alcohol and being moderately active. Avoiding harmful behaviours such as smoking and drinking alcohol, eating a healthy diet, and attending antenatal care visits may help decrease the risks for IUGR. Do contact your midwife or maternity unit if you are worried about your baby’s growth or movements, as this can help in the early detection of growth restriction. There is no treatment for growth restriction apart from birth of the baby, which would be likely if growth had completely stopped. However, it is a balance between early birth and the ill-effects of prematurity. If your doctor or midwife is concerned they will discuss care options with you.

Are there any longer-term complications for SGA babies?

Because restricted growth is often linked to medical conditions such as high blood pressure, it is difficult to untangle what affects longer-term health; is it being small on its own, being born to a mother who has hypertension, or is it both? Although SGA babies seem to grow quickly and may catch up with average-sized babies in the first years of life, there may be some cognitive (intellectual and thinking ability) and motor developmental (physical ability) effects that last into adulthood. SGA babies may also be more at risk of developing certain ill-health conditions including obesity and cardiovascular disease. Therefore, it is important that babies born small grow up eating a healthy diet and are physically active.

Main Points

• Small for gestational age (SGA) is a term that describes a baby who is smaller than most other babies of the same gestational age.
• The physical and neurodevelopment of an SGA baby may progress as normal, so if born at term a small baby will be just as mature as a normal-sized full-term baby, but they are smaller in size.
• An SGA baby is usually detected by your doctor or midwife at your antenatal appointments, where they will examine your abdomen to determine where the top of your uterus is in relation to your pelvis and ribcage, giving them an indication of your baby’s size.
• Measurements of a baby’s bone length, head and abdominal circumference are taken and compared to the measurements of the ‘average’ baby (of the same gestational age).
• If your baby’s measurement falls on or below the 10th percentile, meaning they are smaller than 90% of babies at the same stage of development, your baby will be considered small for gestational age.
• Intrauterine growth restriction (IUGR) occurs when the baby does not receive enough nutrition and oxygen through the placenta. Your doctor can therefore use a doppler to determine how well your placenta is functioning if you’re suspected of having an SGA baby.
• Risk factors of IUGR include maternal high blood pressure, kidney disease, diabetes (long-standing), cardiovascular disease, infection, smoking and drugs use, and poor diet, multiple pregnancy (twins/triplets), some birth defects, and some chromosomal abnormalities.
• Growth-restricted babies do not have the same ‘reserves’ as babies of average size and so may struggle to cope with labour. Women with growth-restricted babies should give birth in a maternity unit with appropriate neonatal care facilities.
• Babies that are growth restricted may have difficulties transitioning to life outside the uterus; they may be less rigorous at birth and may easily become cold and develop low blood glucose levels. Some small babies may need to be cared for on a neonatal unit until they can maintain their temperature and glucose levels by themselves.
• There are two types of growth restriction: asymmetrical, where a baby’s head and brain are of normal size but their body is smaller, and symmetrical, where all body parts including the head and brain are proportionately small.
• Although rare, intrauterine growth restriction is sometimes linked to genetic disorders including Trisomy 13 (Patau syndrome), Trisomy 18 (Edward’s syndrome) and Trisomy 21 (Down syndrome), Triploidy and Turner’s syndrome.
• The risk of having a small baby (or developing a health condition like high blood pressure that may lead to growth restriction) can be reduced by being in the best possible shape before birth. Always contact your midwife or maternity unit if you are worried about your baby’s growth or movements.
• SGA babies may also be more at risk of developing certain ill-health conditions including obesity and cardiovascular disease, so it is essential that babies born small grow up eating a healthy diet and are physically active.