Understanding Late Onset Foetal Growth Restrictions

MedPiper Technologies and JournoMed conducted a World Pre-eclampsia Week Webinar Series along with National IRIA Preventive Radiology and Samrakshan Committees where the discussion was about Late-Onset Foetal Growth Restriction (FGR) in Pregnancy.

Dr Rijo Mathew and Dr Subhash Tailor spoke about how Samrakshan has included protocols for detecting and treating late-onset foetal growth restriction which is still a major cause for stillbirths and perinatal mortalities.

Dr Neelam Jain spoke about the key differences between early-onset and late-onset foetal growth restrictions. Early-onset FGR is easy to spot and understand in the scan as dopplers are easily available. In late-onset FGR, it is difficult to observe the features as they are hidden. The radiologist should have keen observation skills and be conscious of the doppler and graph while detecting and confirming late-onset FGR. Late-onset foetal growth restriction is very difficult to understand.

What is Foetal growth restriction (FGR)?

Foetal growth restriction refers to a condition in which the foetus is unable to achieve its genetically determined size as a consequence of placental insufficiency.

Any foetus that is less than 10th centile is growth restricted. However, a small for gestational age foetus is not a foetal growth restriction. When small for gestational age foetuses is less than 3rd centile, then it is foetal growth restriction.

What is late-onset foetal growth restriction?

In late-onset foetal growth restriction, the estimated foetal weight is less than 3rd centile after 32 weeks. Late-onset FGR constitutes two categories: small for gestational age and appropriate for gestational age.

The small for gestational age babies are have normal doppler readings and their estimated foetal weight is less than 3rd centile. Appropriate for gestational age babies are not traditionally small for gestational age. They will be more than 10th centile in the estimated foetal weight but they might have doppler abnormalities and show a decline in growth velocity.

Stage I Late-onset FGR (Severe smallness or mild placental insufficiency)

When stage I FGR is observed, the baby is monitored weekly and delivered at 37 weeks. The estimated foetal weight is less than the 3rd centile and the cerebral placenta ratio is less than 5th centile.

Stage II Late Onset FGR (Severe placental insufficiency)

In Stage II, there is severe placental insufficiency with the changes in the foetal umbilical artery. The foetus is monitored biweekly and delivered at 34 weeks.

Recognising the foetus in the growth charts

Foetus condition in the mother’s womb can be predicted by the values that are present in the growth charts. Foetal weight determines the condition of the foetus i.e. whether it is a healthy foetus or if it has any sort of abnormalities. The foetus is represented with multiple dots from which growth velocity and percentile can be determined. In scanning the foetus, a decline in centile values provides more information regarding abnormalities.

Data on stillbirth

The statistical data in India is differentiated into pre-term babies and stillbirths. Most of the pre-term stillbirths are small foetuses for gestational age and most of the term stillbirths (at term) are not small foetuses for gestational age. While smallness is a predominant feature in early-onset FGR, it is not the only criterion for diagnosis. In the late-onset foetal growth restriction, the foetal smallness may or may not be an important factor.

Why does late-onset foetal growth restriction behave differently?

In early-onset FGR, the placental damage is profound leading to visible abnormalities. In late-onset FGR, the placental damage is less than 30%. Earlier on in the first trimester, the foetus receives enough nutrition from the placenta when the placental damage is very less. During the consequent trimesters, placental insufficiency manifests and growth restriction takes place.

Newer approaches investigate an estimated foetal weight less than 10th centile. Some of the foetuses lie between 10-50th centile and may go on to have of late-onset FGR. Hence, radiologists must be very careful during evaluation.

Utero placental vessels and their Significance

Utero placental circulation

Each of the blood vessels responsible for utero placental circulation have their own significance. The uterine artery reflects the status of uteroplacental circulation in a non-gravid uterus. In a non-gravid uterus, the uterine artery shows more resistance and has low diastolic flow. When pregnancy occurs, the diastolic flow starts increasing from second trimester to third trimester and the wave patterns of this flow need to be monitored carefully. The uterine artery represents the maternal compartment of foetal maternal exchange

Foetal growth restriction + Abnormal uterine artery PI

Earlier, the uterine artery was not considered in research in regards to foetal development. However, recently, the uterine artery PI value has gained significance when it comes to FGR. In such a foetus, there is an increased risk of abnormal cerebral doppler at term and indicates poor perinatal outcome. If the uterine artery resistance is high, it increases the risk of maternal pre-eclampsia and hypertensive disorders.

Foetal-placental circulation

The umbilical artery represents the foetal compartment of the foetal maternal exchange and is a crucial vessel in diagnosing early-onset FGR. Any variations in the umbilical artery is directly related to placental damage. If placental damage is less than 30%, placental tissue is still available for umbilical artery thus, pumping the blood to the foetus. Hence, the umbilical artery does not show a lot of resistance.

Use of umbilical artery doppler in foetal growth restriction

If the umbilical artery doppler in late-onset FGR is normal, then it does not cause any placental disease.

The umbilical artery doppler plotted on charts gives information regarding placental insufficiency. An increasing trend of PI alerts the possibility of late-onset FGR which could even lead to cardiac dysfunction in extreme cases.

Normal MCA Doppler

The middle cerebral artery (MCA) is the star performer in the late-onset FGR and is the best indicator of growth restriction.

A normal MCA doppler reflects the foetal brain oxygenation. It has a high resistance flow and low diastolic flow. In late-onset FGR, MCA doppler changes i.e. cerebral changes are indicated. There is a decrease in resistance in the MCA and foetal hypoxia conditions are observed. During hypoxia, the foetus tries to save its crucial organs especially brain. MCA Abnormality in late-onset FGR leads to decline in the MCA PI value and trend which could indicate brain sparing.

Cerebral placental ratio – The best bet in late-onset FGR

The cerebral-placenta ratio (CPR) is the ratio of the Middle cerebral artery PI to the Uterine artery PI. It is more sensitive than the middle cerebral artery doppler. CPR abnormality precedes MCA abnormalities in most cases. While the MCA and the umbilical artery alone may look fine, an altered cerebral placental ratio is the first clue regarding foetal growth restriction.

Cerebral placental ratio in the late-onset FGR predicts foetal hypoxia. This ratio can be predicted by observing the mammogram. At any gestational age, if CPR is less than the 5th percentile, it is termed critical and it marks hypoxia in the foetus.

Umbilical vein doppler- An ongoing research

The umbilical vein is a promising marker and a better reflector of placental function. The umbilical vein flow might be low in during the first or the second trimester in late-onset FGR.

A 2019 study states that it is very difficult to identify foetal growth restrictions using the umbilical vein doppler. The umbilical doppler is a technique that has been not studied enough. Doppler calculations are complex and automated which might lead to huge errors. Hence further research is required to understand the significance of the umbilical vein doppler.

Growth rate, a marker for foetal growth restriction

The Delphi consensus describes about the growth rate. The values must meet at least two out of three criteria to categorize late-onset FGR and these include:

1. If the growth declines by two quartiles,
2. If the absolute weight of AC/estimated foetal weight is less than 10th centile.
3. If the cerebroplacental ratio is less than 5th centile or uterine artery PI is greater than 95th centile.

After plotting the values on the growth chart, if there is a dip in the growth values, this indicates late-onset FGR.

Most obstetricians prefer a growth scan in the third trimester which shows a decline in growth velocity as there would be a chance of missing the late-onset FGR. The placental damage is mild in late-onset foetal growth restriction.

Conclusion

Late-onset foetal growth restriction is the biggest imposter in growth restriction hence, the growth charts should be monitored carefully. Always include the doppler scans in the third trimester and at least two scans need to be performed in the late third trimester so as predict foetal maternal exchange, foetal abnormalities and other perinatal outcomes.