Learn more about the Fetal Growth Scan an it's role in assessing the growth of the fetus.

Summary

  • The growth scan takes place when the mother is between 23 and 40 weeks pregnant.
  • It checks how well the baby is growing and the position in the uterus (womb).
  • The scan assesses the baby’s wellbeing through its movements and growth
  • Certain parameters such as isolating the source of bleeding or checking whether pregnancy dates are right is difficult later on in the pregnancy.
  • Some babies may experience restricted growth. This is called Fetal Growth Restriction or Intrauterine Growth Restriction.
  • Signs that may indicate FGR are changes in the umbilical cord blood flow and reduced amniotic fluid volume.

Growth Scan

The growth scan, sometimes called the wellbeing scan or positioning scan, takes place when the mother is between 23 and 40 weeks pregnant. It checks how well the baby is growing and the position in the uterus (womb).

    During the scan the doctor:

  • Measures the baby's head, abdomen and thigh bone
  • Assesses the amount of amniotic fluid around the baby
  • Observes the baby's activity
  • Measures the blood flow in the umbilical cord using doppler ultrasound
  • Records the position of the placenta.

The measurements of the head, abdomen and leg allow the doctor to estimate fetal weight. All the measurements are plotted on a chart against the normal range and to assess the fetal growth. Because babies grow at different rates from week to week, a series of scans can be more helpful than just one.

Why is a growth scan required?

If the baby has an average head size, but a big abdomen, he/she may just be getting a good food supply from the placenta. If the mother has diabetes in pregnancy (gestational diabetes) she may have a big baby (macrosomia). This happens when the mother's blood sugar levels have risen too high. This can make the baby measure larger than expected for all parameters.

If the baby has an average head size and a small abdomen, this may simply indicate a small, healthy baby. Occasionally, though, this can be a sign that the baby is not growing properly. The scan may also show that the amount of amniotic fluid is low. The two sometimes occur together.

If the baby is smaller than expected, he/she may have a low birth weight. To find out why the baby is small, the doctor might ask for a Doppler scan.

Scans are usually accurate for assessing the baby's size in the first half of thepregnancy. By the time the mother is in the later stages of pregnancy, scans continue to be accurate, as long as the baby is small or of average size. The closer it gets to the due date, and the bigger the baby is, the harder it will be to record measurements.

The baby's head may be too low in the pelvis in late pregnancy to get a measurement. Even if the baby's abdomen can be measured, it's very difficult to take other factors into account, such as how long the baby is.

Why is a scan even needed in the third trimester?

The most common reason for a scan in the third trimester is to check that the baby is growing normally. The mother will be offered a growth and fetal wellbeing scan between 28 weeks and 32 weeks of pregnancy. This will show the doctor how the baby is growing.

The mother will get another growth scan and colour Doppler studies closer to thedue date (if required), between 36 and 40 weeks to:

  • Check the position of umbilical cord
  • Measure the amount of amniotic fluid
  • Check the placental position and maturity
  • Know the baby's position and weight
  • Check the baby's wellbeing and circulation
  • Assess how a previous caesarean scar appears

    The mother might get additional scans in thethird trimester if:

  • The baby isn't moving as well or as often as he should be
  • The baby is in the breech, oblique or transverse position
  • The mother is carrying twins or more
  • The amount of amniotic fluid is more or less than it should be
  • The baby feels smaller or larger than expected for her gestational age

What can third trimester scans detect?

    The third trimester scans will look at the following: 

    Thebaby's wellbeing 

    This is measured by his biophysical profile. A healthy baby:

  • Stretches and flexes
  • Moves his/her arms and legs frequently
  • Opens and closes his/her hands
  • Makes breathing movements

The doctor will observe the baby doing all these activities on the scan.

The doctor will observe the baby doing all these activities on the scan.

  • Whether the dates are right: After 20 weeks, babies become more individual in size and shape. If the baby is smaller or bigger than average at, say 34 weeks, it doesn't mean the baby is younger or older. The due date needs to be established by 20 weeks.
  • Where bleeding is coming from: Bleeding in the third trimester may be coming from the cervix or further inside the womb. A scan can reassure that the baby is not affected by the bleed but the scan can rarely see the cause of it. The scan can detect the cause only if it is due to a low-lying placenta.
  • How much the baby weighs: The bigger the baby and the nearer to term, the harder it becomes to assess the weight.

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 the patient is 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
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An estimate of fetal weight (EFW) can be calculated by combining the above measurements. The EFW is 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

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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 as expected. 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).

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.

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:

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  • 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,the doctor may recommend 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 thebaby has a chromosomal abnormality
a maternal blood test to check for infection.

The pregnancy will be monitored closely with regular ultrasounds to measure:

  • On-going 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.

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.

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