When intrauterine growth restriction (IUGR) is suspected during the third trimester, the majority of IUGR fetuses will either be constitutionally small, or have growth restriction due to placental insufficiency. Other less common causes are aneuploidy, other genetic disorders, fetal malformations, and fetal infection. However, in most cases, the typical question is whether the fetus is constitutionally small or growth restricted from placental insufficiency.
To differentiate a constitutionally small fetus from a growth-restricted fetus, we use several tests. The most simple approach is analyzing the parents’ size and ethnicity. Smaller parents are more likely to have smaller babies. Additionally, the morphology of the fetus may differentiate growth restriction (small abdomen and normal head) from a constitutionally small fetus (similar sized abdomen and head, small long bones).
Other signs of placental insufficiency, such as pre-eclampsia, oligohydramnios, and an abnormal biophysical profile or non-stress testing would also indicate that a small fetus is more likely to be growth restricted, as these tests indicate placental insufficiency. Finally, the two most valuable tests are interval growth over time and umbilical artery Doppler velocitometry.
A fetus that is small but growing is less likely to be growth restricted, and is more likely to benefit from remaining in utero, especially in light of other reassuring testing. In the setting of reassuring testing, it is not recommended to deliver a preterm small fetus with adequate interval growth.
Doppler velocitometry is helpful in assessing small fetuses, and has been associated with a decreased mortality in small fetuses. Normal umbilical artery Doppler findings are very reassuring and imply a normally functioning placenta, making growth restriction from placental insufficiency less likely or less severe. While Doppler assessment would likely be normal in a fetus that is small due to a genetic abnormality or early infection, in such cases, early delivery would be very unlikely to improve outcomes, and iatrogenic prematurity would likely worsen outcomes for such fetuses.
Carnegie Imaging for Women blogs are intended for educational purposes only and do not replace certified professional care. Medical conditions vary and change frequently. Please ask your doctor any questions you may have regarding your condition to receive a proper diagnosis or risk analysis. Thank you!