Fetal Growth Restriction

On today’s episode, we tackle the new ACOG PB 204 on fetal growth restriction. There’s quite a bit to cover, so you’ll see today’s episode is a bit longer. We’ve enlisted the help of Chris Nau, MD, an MFM fellow at Brown, to help us through all the recommendations.

FGR, as Chris explains, results from a process where there is mismatch between fetal demands and placental supply of oxygen and nutrients. Symmetric growth restriction arises earlier, and asymmetric growth restriction arises later, with the differentiating factor being asymmetric growth restriction resulting in a larger head circumference : abdominal circumference ratio. The PB 204 goes into many of the common causes, but the list is long!

ACOG PB 204

ACOG and SMFM state that a sonographic estimated fetal weight less than the 10th percentile is the measurement definition of fetal growth restriction, though as Chris explains, there are a number of limitations to this definition.

Screening is performed using a fundal height at prenatal visits, and if the height is < 2cm discrepant from the gestational age, an ultrasound should be performed. Alternatively ultrasounds can be used primarily, especially with known maternal conditions that might predispose someone to FGR, or if fundal height assessments are difficult.

Once a growth-restricted infant is identified, you should check the due date calculation and make sure it is right! (Check out our previous episode on dating!) Next, re-review the mother’s medical history and pregnancy history, including aneuploidy screening, to date. A level 2 ultrasound may help identify anatomic abnormalities that point to an etiology. And identifying modifiable risk factors, including optimizing medical conditions or smoking cessation, may be worthwhile.

With respect to management, there are variable institutional protocols with respect to monitoring. Umbilical artery velocimetry reduces risk of perinatal death when added to other antenatal testing (i.e., modified BPP). Normal or elevated systolic : diastolic flow ratio does not carry increased risk to the fetus; however absent or reversed end-diastolic flow increases risk for perinatal mortality. Chris reviews our protocol at Brown in the podcast.

Timing of delivery is a tricky one — there is not great evidence, and the newest guidance from PB 204 states that reassuring fetal testing should deliver between 38w0d - 39w6d. If there is FGR plus concerning maternal or fetal findings, delivery should be considered between 32w0d to 37w6d.

When counseling about future pregnancies, there is about a 20% risk of recurrence. At this time, ACOG/SMFM do not recommend baby aspirin for prevention of FGR in the absence of other risk factors for preeclampsia.

Further reading from the OBGProject:
Fetal Growth Restriction: Diagnosis, Evaluation, and Management
Aspirin Treatment - ACOG and USPSTF Recommendations