Your maternity care provider has referred you for an ultrasound scan to assess the baby’s size and the amniotic fluid volume. This type of ultrasound scan is referred to as a fetal growth scan.
Why is a fetal growth scan needed?
Common reasons to have a fetal growth scan in pregnancy include the following:
- Your abdomen measures smaller or larger than expected.
- Previous pregnancy complications.
- Maternal diabetes or high blood pressure.
What happens during a fetal growth scan?
During the fetal growth scan, various measurements are taken of the fetus. The measurements are plotted on a growth chart, according to the number of weeks pregnant that you are at the time of the scan (gestational age).The main fetal measurements taken for a growth scan include:
Biparietal diameter (BPD) measures across the head
Head Circumference (HC) – measures around the head
Abdominal Circumference (AC) – measures around the abdomen
Femur Length (FL) – measures the length of the thigh bone
An estimate of fetal weight (EFW) can be calculated by combining the above measurements. The EFW can be plotted on a graph to help determine whether the fetus is average, larger or smaller in size for its gestational age. If the fetal weight estimate is below the bottom 10 per cent line on the graph, it is considered to be small for gestational age (SGA). If the fetal weight is above the top 10 per cent line on the graph, it is considered to be large for gestational age (LGA).
It is important to note that repeated ultrasound measurements of the same fetus can vary and the estimated fetal weight may be incorrect by as much as 20 per cent.
A fetal growth scan does not routinely check the baby for abnormities. An ultrasound scan to look for major fetal abnormalities is routinely performed earlier in pregnancy between 18 and 20 weeks gestation (see the brochure Ultrasound—your 18 to 20 week scan)
Small for gestational age (SGA)
Most fetuses that are shown to be small for gestational age are healthy but some may require further ultrasound assessment to ensure that they are growing adequately. Other ultrasound tests can also be performed such as checking the amount of amniotic fluid around the fetus and measuring the blood flow in the umbilical cord (umbilical artery Doppler).
What is fetal growth restriction (FGR)?
Fetal growth restriction (FGR) is a term used to describe a fetus that is not growing adequately before birth. It is also called intrauterine growth restriction (IUGR).
FGR may only become obvious after two or more ultrasounds have been performed. Due to the variation of ultrasound measurements, a minimum of two weeks is required between fetal growth scans. Other signs that may indicate FGR are changes in the umbilical cord blood flow and reduced amniotic fluid volume.
What causes fetal growth restriction?
Some of the underlying causes of FGR include the following:
- Placental insufficiency—when the placenta fails to provide adequate nutrition to the developing fetus.
- Fetal abnormality—some fetal abnormalities are associated with delayed growth.
- Multiple pregnancy—can affect one or more of the fetuses.
- Infection of the mother e.g. cytomegalovirus (CMV).
- Poor nutrition.
- Smoking, alcohol, illicit drugs and some medications.
- Medical conditions affecting the mother such as high blood pressure and diabetes.
How is fetal growth restriction managed?
If fetal growth restriction is suspected your health carer may recommend some tests to identify the underlying cause. Tests that may be offered include:
- an extended ultrasound assessment to check for major structural abnormalities
- an amniocentesis to identify if your baby has a chromosomal abnormality (see Mater’s brochure: Chorionic villus sampling (CVS) and amniocentesis)
- a maternal blood test to check for infection.
Your pregnancy will be monitored closely with regular ultrasounds to measure:
- ongoing fetal growth (usually every two weeks)
- the umbilical artery blood flow using Doppler ultrasound
- other blood flow Doppler studies as indicated
- the amniotic fluid volume.
If growth restriction is suspected, hospital admission may be required so that regular fetal surveillance can be undertaken. Fetal heart rate monitoring by cardiotocograph (CTG) may be performed. If the fetal condition is considered poor and continuation of the pregnancy is not considered safe, then delivery is considered.
If delivery is recommended before 37 weeks gestation, your maternity care provider will consider maternal steroid injections and magnesium sulphate (if less than 30 weeks) to reduce the risk to the newborn of complications associated with prematurity.
What can I do?
It is important that you give your baby the best possible chance by:
- maintaining a healthy diet
- not smoking, drinking alcohol or taking illicit drugs.
What happens after birth?
A newborn baby that has had trouble growing in the uterus may need to be cared for in the Neonatal Critical Care Unit depending on their prematurity, birth weight and how well they adapt to life outside the uterus.
Large for gestational age (LGA)
Most fetuses that are shown to be large for gestational age on ultrasound are well nourished and healthy at birth. In some cases there is an underlying cause for the fetus to be large such as diabetes or a genetic syndrome.
Diabetes in pregnancy
Some babies are larger due to maternal diabetes during pregnancy and in particular if maternal blood glucose levels have been difficult to control. Larger babies born to mothers with diabetes are at increased risk of birth trauma including shoulder dystocia. Shoulder dystocia occurs when the baby’s head is delivered but the shoulders become stuck requiring specific manoeuvres to be performed in order for the baby to be born. When the fetus is suspected to be excessively large, induction of labour or caesarean section may be considered.
When the mother has diabetes in pregnancy, their babies are also at risk of low blood sugar (hypoglycaemia) after birth as they no longer have access to the high sugar levels that they have become used to while in the uterus. This may require monitoring and treatment in the special care nursery.
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