Fetal Weight Assessment at 18 Weeks Gestation
A fetal weight of 300 grams at 18 weeks gestation is significantly above normal and warrants immediate detailed evaluation for macrosomia, gestational diabetes, or dating error. 1, 2
Normal Fetal Weight Parameters at 18 Weeks
The expected fetal weight at 18 weeks (mid-second trimester) is approximately 200-250 grams, based on standard population references used for fetal growth assessment. 1, 3 A weight of 300 grams represents a substantial deviation from this norm—approximately 20-50% above expected values.
Clinical Interpretation
This finding requires urgent investigation for three primary possibilities:
Dating error: First-trimester crown-rump length measurement is the most reliable method for pregnancy dating, and any discrepancy should prompt review of early ultrasound findings to confirm gestational age. 1 If dating is incorrect and the pregnancy is actually more advanced (closer to 20-21 weeks), the weight would be more appropriate.
Fetal macrosomia: If dating is accurate, this represents a fetus tracking above the 90th percentile for gestational age, which requires screening for maternal gestational diabetes mellitus and evaluation for other causes of excessive fetal growth. 4, 5
Measurement error: Ultrasonographic estimated fetal weight is generated using regression equations combining biparietal diameter, head circumference, abdominal circumference, and femur length—any technical error in these measurements could produce an inaccurate weight estimate. 1
Recommended Management Algorithm
Immediate steps:
Verify pregnancy dating by reviewing first-trimester crown-rump length measurements, as accurate dating is the essential prerequisite for interpreting fetal weight. 1 If no first-trimester ultrasound was performed, consider that gestational age may be underestimated.
Perform detailed anatomic survey to evaluate for structural anomalies that may be associated with macrosomia or to confirm accurate biometric measurements. 1, 6
Screen for gestational diabetes if not already completed, as maternal hyperglycemia is a primary driver of excessive fetal growth in the second trimester. 4, 5
If dating is confirmed accurate and weight remains elevated:
Serial growth ultrasounds every 3-4 weeks to track growth velocity and determine if the fetus is maintaining an accelerated growth trajectory. 1
Maternal evaluation for conditions associated with fetal macrosomia, including pre-existing diabetes, excessive gestational weight gain, and obesity. 4, 5
Critical Context
The Society for Maternal-Fetal Medicine and American College of Obstetricians and Gynecologists define abnormal fetal growth using the 10th and 90th percentiles as cutoffs. 1, 6 While most guidelines focus on fetal growth restriction (below 10th percentile), a fetus measuring significantly above expected norms also represents a deviation requiring evaluation. 1
Common pitfall: Assuming all large measurements represent normal variation without verifying dating accuracy or screening for pathologic causes of macrosomia. 1 The difference between 250 grams (normal) and 300 grams (20% excess) at this early gestational age is clinically meaningful and should not be dismissed.