Fetal growth and Well-Being Assessment
What is a fetal growth assessment ultrasound?
A fetal growth assessment is typically performed in the late second and the third trimester of pregnancy. The aim of this examination is to assess:
- The fetal heart activity and rhythm
- The presentation of the fetus (ie head, breech, transverse)
- The fetal size and growth pattern
- The amount of amniotic fluid around the fetus
- The position of the placenta
- The blood flow through the umbilical artery when indicated (and blood flow within the fetal brain or liver if necessary)
- The activity and behaviour of the fetus.
- The length of the cervix
- The presence and position of any uterine fibroids
How can ultrasound tell if my baby is growing appropriately?
As with children and adults, there is a range of size for the fetus at any time in pregnancy and this variation increases greatly as the pregnancy advances. Assessment of the fetal size is ascertained using four basic measurements:
- Biparietal diameter (BPD)
- Head circumference (HC)
- Abdominal circumference (AC)
- Femur length (FL)
The abdominal circumference (measurement around the waist of the fetus) is the single most sensitive indicator of fetal growth.
A single series of measurements at one point in time can diagnose a small or a large fetus, but the overall growth pattern can only be assessed over at least 2 separate scans, at least 14 days apart. With two complete measurement sets we can develop a “growth pattern” for your baby, as every fetus has its own growth rate. The measurements will be plotted onto a fetal growth chart, which is a graphical representation of your baby’s individual growth pattern.
Normal Fetal Growth Rate
Symmetrically Small Fetus
Symmetrically Large Fetus
Failure of interval growth
Asymmetrical growth restriction
Asymmetrical growth advancement
What is a fetoplacental Doppler study?
Doppler ultrasound measures the blood flow through blood vessels (typically arteries but also veins). The speed of blood flow, the direction of blood flow and the down-stream resistance to blood flow may be assessed with the use of Doppler ultrasound.
In obstetrics Doppler ultrasound is used for three major purposes:
- To assess the functional capacity of the placenta in cases where the fetal growth measurements are not normal. In this circumstance assessment of the resistance to blood flow in the umbilical artery would be assessed with Doppler ultrasound
- To assess the fetal response to situations where the placenta is not working appropriately as determined by Doppler of the umbilical artery and the fetus has been shown to be smaller than expected. In this case we would assess blood flow to the fetal brain by measuring blood flow in the Middle Cerebral Artery and the ductus venosus (a vessel linking the umbilical vein with the Inferior Vena Cava in the fetal liver)
- To assess the fetus at risk for anaemia by assessing the velocity of blood flow in the Middle Cerebral Artery. Anaemia most typically occurs in women with preformed red cell antibodies (usually in Rhesus negative women).
What is an Amniotic Fluid Index?
An Amniotic Fluid Index (AFI) is a semi-quantitative assessment of the amniotic fluid volume in the uterus. Too much or too little amniotic fluid can be associated with increased pregnancy risks.
The AFI is measures in four quadrants using the maximal vertical distance in the amniotic cavity in an arbitrary division of the uterus into 4 equal sections, centering on the maternal umbilicus.
The AFI varies with gestation, tending to decrease in the third trimester, especially after 40 weeks gestation.
Will I be able to see my baby at a fetal growth assessment?
The images of the fetus at 28-40 weeks are quite different from those obtained at the Fetal Anatomy Survey. As fetal growth during pregnancy is exponential, and amniotic fluid tends to reduce as gestation advances, the images obtained are quite different. Only one organ system at a time can be seen on the screen and many parents find ultrasound at this time confusing and sometimes emotionally disappointing, depending upon the fetal position. We will endeavour to demonstrate an image of your baby’s face at this time, but frequently this is not possible due to fetal position. Often we can produce beautiful images, but this is not always possible.