Why do I need a scan in the third trimester?
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 18-20 week scan.
- Have premature contractions.
- Had a small baby in the past.
- Have high blood pressure.
- Have diabetes or other medical conditions.
- Have pain in your abdomen.
- Have twins/multiple pregnancy
This scan aims to determine the growth and health of the fetus by:
- Measurement of the size of the fetal head, abdomen and thigh bone and calculation of an estimate of fetal weight.
- Examination of the movement of the fetus.
- Evaluation of the placental position and appearance.
- Measurement of the amount of amniotic fluid.
- Assessment of blood flow to the placenta and fetus by colour Doppler ultrasound.
Can I see baby in the same detail as the 18-20 week scan?
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 much of 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 we go the thicker the skull bones become which can shadow and obscure the fetal brain detail. We do however see some things better such as the face and heart. This always depends on what position the baby is in and how co-operative 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.
How is the fetal well-being assessed?
Ultrasound forms a major tool in the assessment of the current fetal health by Doppler assessment. Other assessments include the clinical history and examination and cardiotocography (CTG) assessment. 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 help give an overall assessment of the current state of fetal well being.