Birth announcements almost always mention a baby’s birthweight – it forms a considerable point of interest for parents, family and friends alike. Beyond this social curiosity, birthweight is an important health measure, as very small or very large babies may need additional care. A significant component of pregnancy care is aimed at identifying babies who are ‘small for gestational age’.

What is ‘small for gestational age’ (SGA)?

A small for gestational age (SGA) baby is generally considered to be one whose weight is less than the 10th centile (i.e. in the lowest 10% of the normal range of weight). This may be identified at birth using the birthweight, but can also be picked up during pregnancy using an ultrasound to calculate the ‘estimated fetal weight’ (EFW). This estimate is compared to the estimates for many other babies at the same gestation to determine where the baby’s weight falls in this range.

Just as healthy adults demonstrate a wide range of weights, so too do babies. Statistically, we expect around 10% of babies to be less than the 10th centile. Many of these babies are simply destined genetically to be on the smaller side (‘constitutionally small’) but are otherwise completely healthy. From a maternity care perspective, our interest lies in identifying those babies who are smaller than they ‘should’ be, for some reason. These are babies who are at greater risk of problems before and after birth, because they have not met their ‘genetic growth potential’, and are thus considered to be ‘growth-restricted’.

The smaller a baby is, the more likely it is to be ‘growth-restricted’ rather than ‘constitutionally small’.

What causes a baby to be ‘small for gestational age’?

There are many ‘pathological’ reasons for a baby being smaller than it should be. The most common being placental insufficiency, where the placenta does not work as well as it should. This is more common in mothers with pre-existing medical conditions such as high blood pressure or lupus, but is also seen more frequently in twin pregnancies, or can result from the use of certain medications or tobacco smoking. Other causes include infections (such as cytomegalovirus or CMV) and genetic syndromes. Most of the time, a baby is small for reasons out of their mother’s control.

How are SGA babies identified?

This is easy once a baby is born, but a bit harder beforehand! In women with singleton pregnancies, after 24 weeks, the distance from the top of the uterus to the pubic bone (the ‘symphysio-fundal height’) in centimetres roughly approximates the gestational age in weeks. If this measurement is smaller than expected or doesn’t increase appropriately as the weeks pass, it may be a sign of an SGA baby. In these circumstances, an ultrasound is advisable to determine the estimated fetal weight. Ultrasounds are also suggested to women for whom measurements of the symphysio-fundal height are likely to be inaccurate, such as larger women, those with twins or uterine fibroids.

When an ultrasound is performed, key measurements are taken to help estimate the baby’s weight. These generally include the length of the femur (thigh bone), the abdominal circumference, the head circumference, and the biparietal diameter (distance across the skull). These measurements are combined into a formula that produces an estimate of the baby’s weight. And it is just that – an estimate, with a margin for error of at least 10%. That said, ultrasound is the most accurate means available to determine a baby’s size, and given that it is safe and readily available, it remains the standard of care in identifying a small baby.

What happens if my baby is SGA?

If your baby is identified on ultrasound to be small for gestational age, your maternity care provider will discuss with you the potential causes relevant to your circumstances, and outline additional investigations that could be performed to determine the underlying cause. After that, depending on your gestation, regular ultrasounds are arranged to observe the baby’s ongoing growth and assess its wellbeing. Results of these tests will help your care provider give you information to determine the best way and time to have your baby. With monitoring, many small babies may safely be carried to term or near-term, but some will do better if they are born earlier.

Baby doctors (neonatal paediatricians) commonly attend the births of babies who are expected to be small, to provide additional support with breathing should it be required. Small babies may need to be admitted to the neonatal nursery for additional care with temperature regulation and feeding, but many can ‘room in’ with their mothers. How long your baby will have to spend in the hospital will depend on many factors, but primarily the size and gestation at birth.

It is generally recommended that the placenta from SGA pregnancies be sent to the laboratory for analysis, which may identify specific reasons for the baby’s smallness.

What happens if my last baby was SGA?

If your last baby was SGA, it is a good idea to seek obstetric care before a further pregnancy, so that the results of tests performed in the previous pregnancy can be reviewed and a plan developed for the next one. As with many conditions in medicine, having had a previous SGA baby does increase the chance of having another one, although this risk varies depending on the cause of the SGA. There is some evidence to suggest that low dose aspirin from the first trimester can reduce the recurrence of placental insufficiency. Close monitoring of your baby’s growth will be undertaken in future pregnancies, typically with several ultrasound scans.

What about the long term?

Growth restricted babies (those who are smaller than they should be) are at increased risk of long term cardiovascular and metabolic disease, such as diabetes and obesity. These risks can be managed with clinical surveillance and attention to lifestyle factors. However, most babies who are born small go on to have healthy lives as children and adults.

Dr Stefan Kane has recently joined the SVPHM team and is a part of the Melbourne Maternal Fetal Medicine group (Melbourne MFM). In addition to providing care to women with uncomplicated pregnancies, Dr Kane has extensive experience in managing complex, high-risk pregnancies as a Maternal Fetal Medicine Sub Specialist.

View Dr Stefan Kane’s Profile

Melbourne MFM Website