Intrauterine Growth Restriction (IUGR), also known as Fetal Growth Restriction (FGR), describes a condition where a fetus does not achieve its genetically determined growth potential. This means the baby is smaller than expected for its gestational age, distinguishing it from a constitutionally small baby. IUGR is typically caused by a pathological process, most commonly placental insufficiency, which deprives the fetus of adequate nutrients and oxygen. Detecting this condition, which affects 5% to 10% of pregnancies, allows healthcare providers to implement timely monitoring and necessary interventions, significantly improving the chances of a favorable outcome.
Initial Suspicion: Routine Prenatal Screening
The first indication of restricted growth often occurs during routine prenatal check-ups after the 20th week. Healthcare providers use fundal height, a simple, non-invasive measurement, to screen for potential growth issues. This involves measuring the distance in centimeters from the pubic bone to the top of the uterus (fundus). After 20 weeks, this measurement should generally correspond to the number of weeks of gestation.
A measurement that lags significantly behind gestational age, typically by 3 to 4 centimeters or more, raises suspicion of IUGR and warrants further investigation. Fundal height is a useful screening tool but is not definitive, as its accuracy can be affected by factors like maternal obesity or placental position. Accurate dating of the pregnancy, confirmed by earlier ultrasounds, is a foundational requirement. If the initial dating is inaccurate, any later size discrepancy may simply be a miscalculation rather than a true growth problem.
Confirmation: Diagnostic Imaging and Criteria
The definitive diagnosis of IUGR relies on specialized ultrasound imaging and specific biometric criteria. This diagnostic stage focuses on calculating the Estimated Fetal Weight (EFW), derived from multiple fetal body measurements.
The primary measurements used include the head circumference (HC), the abdominal circumference (AC), and the femur length (FL). The EFW is then plotted against established growth curves for the corresponding gestational age.
A fetus is formally diagnosed with IUGR when its EFW falls below the 10th percentile for gestational age. A more severe diagnosis is considered if the EFW is below the 3rd percentile. The abdominal circumference is the most sensitive measurement for detecting growth issues, as fetal liver and fat stores are often restricted first.
The diagnosis is strengthened by Doppler velocimetry, a specialized ultrasound that assesses blood flow in the fetal vessels. Doppler studies focus on the umbilical artery (flow to the placenta) and the middle cerebral artery (supplying the brain). Abnormal Doppler flow patterns, such as increased resistance in the umbilical artery or blood flow redistribution to the brain (brain-sparing), provide evidence of placental dysfunction. The combination of a low EFW and abnormal Doppler findings confirms pathological growth restriction, distinguishing it from a healthy, small fetus.
Classification Based on Gestational Age
The timing of the IUGR diagnosis determines its classification, influencing the underlying causes and severity. The two main categories are Early Onset and Late Onset IUGR, with the cut-off point set at 32 weeks of gestation.
Early Onset IUGR is diagnosed before 32 weeks, accounting for 20% to 30% of cases. This type is strongly associated with severe placental insufficiency, significant Doppler abnormalities, and a higher risk of adverse outcomes because the fetus is compromised for a longer period.
Late Onset IUGR is diagnosed at or after 32 weeks, representing 70% to 80% of cases. The progression is often slower, and clinical signs, including Doppler abnormalities, may be less pronounced or absent. Late-onset cases generally carry a lower immediate risk of severe complications compared to their early-onset counterparts.
Immediate Steps Following Diagnosis
Once IUGR is confirmed, management shifts to intensive fetal surveillance to monitor the baby’s well-being and determine the safest time for delivery. Monitoring frequency is significantly increased, often requiring serial ultrasounds and Doppler assessments every two to four weeks to track growth and blood flow patterns. More frequent monitoring may be necessary if the restriction is severe or Doppler results are concerning.
Fetal health assessment includes non-stress tests (NSTs) and biophysical profiles (BPPs). The NST monitors the baby’s heart rate in response to movement. The BPP combines the NST with an ultrasound assessment of fetal movement, breathing, muscle tone, and amniotic fluid volume. The goal of management is to safely prolong the pregnancy to allow for continued fetal development while mitigating risks. Delivery decisions are based on the overall picture of fetal health, with severe Doppler abnormalities often necessitating earlier delivery.