The association between second trimester estimated fetal weight and small for gestational age at birth

Objective: To analyze diagnostic accuracy of second trimester ultrasound fetal growth parameters as predictors of small for gestational age (SGA) birth weight.Methods: We reviewed the fetal biometry from 714 consecutive patients with second trimester ultrasounds. The estimated fetal weight (EFW) and abdominal circumference (AC) percentiles were tested as predictors of SGA at birth (<10).Results: 87 (12.2%) patients had an SGA baby. Patients with a second trimester EFW25 parts per thousand were significantly more likely to have SGA at birth (24.2% versus 10.3%, p<0.001). Similar results were seen for women with second trimester AC25 parts per thousand (likelihood of SGA 21.9% versus 11.2%, p=0.013). A second trimester EFW25 parts per thousand was a better predictor of SGA at birth than a second trimester EFW10 parts per thousand (Positive likelihood ratio 2.30 versus 2.09). In the second trimester, only 9 (1.3%) patients had an EFW 0-10 parts per thousand, only 43 (6%) patients had an EFW 11-20 parts per thousand, and only 46 (6.4%) patients had an EFW 91-99 parts per thousand. Each other EFW centile had more than 10% of the patients.Conclusions: The incidence of second trimester EFW or AC10 parts per thousand is less common than expected from standard tables. An EFW25 parts per thousand and an AC25 parts per thousand should be considered the second trimester marker for risk of SGA at birth. However, due to the low likelihood ratio of, it is not clear if second trimester ultrasound should be used as a predictor of SGA at birth.