Fetal Growth Restriction: An Update
/Two years ago we did a podcast on fetal growth restriction with Dr. Chris Nau based on ACOG PB 204. Little did we know that soon there would be even more changes to fetal growth restriction management! We offer up today’s podcast as an overview of those changes.
More reading on these changes in the form of SMFM Consult Series #52 and the new ACOG PB #227.
Terminology to Know
Estimated fetal weight - is… an estimated fetal weight! In utero
Fetal growth restriction - means “fetal” growth restriction. Again… in utero!
SGA = small for gestational age - refers to the baby when it is born. So:
A fetus cannot be SGA, but can be FGR;
A baby can be SGA, but not FGR.
Etiologies of FGR
Unchanged… review our prior episode!
Realize that it can result from multiple maternal, fetal, and placental disorders
Why do we care?
Fetal growth restriction occurs in up to 10% of pregnancies and is a cause of infant morbidity and mortality around the world
Fetuses <10th%ile at any gestational age have a risk of stillbirth of 1.5%, which is 2x the rate of fetuses with normal growth
Infants with birthweights <10th%ile have increase risk of acidosis at birth, low 5 min Apgar scores, and need for NICU admission, as well as 2-5x rates of perinatal death
Who is considered fetally growth restricted, and how do we figure that out?
The SMFM Consult Series #52 recommended the definition to be:
ultrasonographic EFW < 10%, OR
AC <10% for gestational age
You need to do an ultrasound - but prior to that, you probably need to have suspicion.
This can be done with fundal heights at appointments.
Make sure you have appropriate dating!
US uses population-based fetal growth references (such as Hadlock) in determining fetal weight percentiles:
Hadlock was generated from a study of 392 pregnancies in predominantly white, middle-class women at a single institution in TX
An NICHD study previously developed racial/ethnic standards for fetal growth
It was found that Hadlock still was better at predicting SGA and composite neonatal morbidity at birth, and had a lower ultrasound-to-birthweight percentile discrepancy than the NICHD growth standard.
Hadlock is usually calculated by using BPD, HC, AC, and Femur length
Classification of fetal growth restriction
Early vs. late fetal growth restriction
Again, per SMFM consult series defined as onset <32 weeks (early) or late (at or after 32 weeks)
Early FGR tends to be more severe, tends to follow an established Doppler pattern of fetal deterioration, and can show more severe placental dysfunction than late-onset FGR.
Also, early FGR can be associated with genetic abnormalities
Therefore, in early FGR, should get detailed ultrasound.
The SMFM consult series also recommends chromosomal microarray analysis if there is also fetal malformation or polyhydramnios is noted.
Severity
EFW <3% has been associated with an increased risk of adverse perinatal outcome irrespective of UA or MCA Dopplers.
Management of Fetal Growth Restrictions
Remember: The reason we care about fetal growth restriction is its association with stillbirth and perinatal mortality/morbidity. To prevent that, we try and look for signs that the baby/placenta is not doing well. We can do this with umbilical artery dopplers and antenatal testing (ie. BPPs, modified BPPs … see our recent episode!)
What are umbilical artery dopplers?
Assessment of blood flow toward the placenta in the umbilical arteries of the fetus
In systole, the blood is being pumped forward, and in diastole, the blood should still move forward, but may be slower than in systole.
We look at the S:D ratios, or the speed of the blood flow toward the placenta in systole compared to diastole
With increasing placental dysfunction comes placental resistance. Therefore, this can start to affect forward flow from the umbilical arteries.
In systole, the blood should always flow forward.
However, in diastole, without the heart as a pump, that blood flow can slow down. This is where we can begin to see elevated S:D ratios! Generally, elevated is >95%ile.
If there is even more resistance, blood flow during diastole stops. This is when you have “absent end diastolic flow”
In very severe cases, the resistance in the placenta is so high that the blood flows backward toward the fetus. This is called “reverse end diastolic flow”
Why do we use them?
As a way to assess placental dysfunction
Absent and reverse end diastolic flow are associated with high rates of perinatal mortality. ‘
One study shows an odds ratio for fetal death of 3.59 and 7.27 for AEDV and REDV, respectively!
What do we do with UAs?
Once fetal growth restriction is diagnosed, UAs should be serially assessed, usually 1-2 weeks depending on your institution
If elevated, they should be assessed more frequently
The SMFM series also recommends assessment of dopplers 2-3x/week when UAs become AEDF to assess for REDF
Management and Delivery Planning
We should mention that this can vary to some degree based on your institution! Generally speaking:
If FGR but >/=3rd%ile with normal UA dopplers:
Serial growth scans (every 3-4 weeks)
Weekly or every 2 week UA dopplers
Weekly or 2x/week antenatal testing
Delivery by 39th week
If FGR but <3rd%ile with normal UA dopplers
Same as above, but delivery at 37 weeks
If Elevated S:D ratios (meaning decrease end diastolic flow)
Continue weekly dopplers (some institutions will do 2x/week)
Growth scans q2-4 weeks
2x/week antenatal testing
Delivery at 37 weeks
If absent end diastolic flow
Increase to 2-3x/week dopplers
Discuss corticosteroids for fetal lung maturity
Antenatal testing 2x/week
Consider q2 week growth scans
Deliver at 33-34 weeks (per SMFM). Can consider cesarean delivery.
If reversed end diastolic flow (highest risk for stillbirth)
Inpatient admission
Corticosteroids for FLM
1-2x/day antenatal testing
Consider q2 week growth scans
Deliver at 30-32 weeks. Can consider cesarean delivery.