Your Third Trimester: Growth Scan
Your obstetrician may refer you for a scan in the third trimester if you:
- Are clinically too large or too small for dates
- Have vaginal fluid loss
- Have a low-lying placenta on 19-20 week scan
- Have premature contractions
- Had a small baby in the past
- Have high blood pressure
- Have diabetes or other medical condition
- Have pain in your abdomen
- Have twins / multiple pregnancy
The ultrasound will be used to assess:
- Fetal growth / size
- Amount of amniotic fluid present
- Blood flow patterns in the cord, fetal brain and liver vein
- Fetal well-being
- Placental location
Some parts of the fetus become harder to see as the baby occupies much more space in the uterus, so in general we cannot see the baby as well as on earlier scans. Things like the fingers and toes, hands and feet are often wedged away out of sight. The further into the pregnancy you go, the thicker the skull bones become, which can shadow and obscure the fetal brain detail. However, you will see some things better such as the face and heart. This always depends on what position the baby is in and how cooperative the fetus wants to be for us. If there is enough fluid in front of the fetal face, we may be able to get a 3D or 4D image for you.
Assessing fetal health
Ultrasound is a major tool in the assessment of the current fetal health by Doppler assessment and evaluating the fetal biophysical profile. Other assessments include the clinical history and examination and cardiotocography (CTG) assessment.
The ultrasound biophysical profile is a test that attempts to gauge whether or not the fetus is getting too little oxygen (fetal hypoxia). The biophysical profile examines fetal body movement, breathing movement, tone, and amniotic fluid volume and heart rate variability. This will give an indication of fetal well being. The baby has a number of mechanisms for coping with placental insufficiency (insufficient oxygen in the fetal blood from the placenta). These include diverting blood flow to the brain, away from the kidneys and bowel which in turn reduces the amount of urine and hence the amniotic fluid volume around the baby. The blood flows through the brain at reduced resistance. The main artery in the brain can be assessed for this.
Fetal well-being is also reflected by the presence of fetal movements. When there is a significant problem with placental function there tends to be a decline in fetal movements.
Initially, fetal chest movements or breathing movements disappear first, followed by limb and trunk movements. The placental function can also be assessed by examining the blood flow through the umbilical cord artery. The severity of increasing resistance to flow will be reflected in the cord Doppler readings showing high waveforms to more severe changes of absent or reversed flow in the artery during cardiac relaxation (diastole). Flow in the brain arteries and in the liver veins also helps to give an overall assessment of the current state of fetal well being.