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Ultrasound/Imaging

Poster Session 2

(522) Femur Length Correlates With Growth Restriction, Low Birthweight In Smokers: Secondary Analysis Of STOP RCT.

Tuesday, February 13, 2024
3:30 PM – 5:00 PM  
Location: Prince George's Exhibit Hall CDE, Lower Atrium Level
Objective: Cigarette smoking is associated with a 3.5 fold increased risk of low birthweight. Smoking induces a late-onset pattern of fetal growth restriction (FGR). This study compares fetal biometry at two timepoints in the 3rd trimester with birthweight in pregnancies complicated by smoking.

Study Design:

This is a secondary analysis of Smoking Cessation Through Optimization of clinical care in Pregnancy (STOP) RCT, which tested a novel antenatal clinic for pregnant smokers. Singleton pregnancies smoking ≥1 cigarette daily were recruited prior to 20 weeks gestation at an urban maternity hospital. Ultrasound screening for FGR was performed at 32 and 36 weeks.

Results:

436 women participated in the STOP RCT, with 285 women undergoing ultrasound screening for FGR. 261 and 228 scans were performed at 32 weeks and 36 weeks, respectively. For normally grown infants the estimated fetal weight (EFW) at 32 weeks correlated poorly with birthweight. However, when biometry of < 10th centile was analyzed, each parameter significantly correlated with low birthweight.

To further characterize the relationship between FL and FGR in smokers, a correlation matrix was generated between biometry at 32 and 36 weeks. The mean FL Z-score at 32 weeks was poorly correlated with EFW at 36 weeks. However, FL < 10th centile at 32 weeks was predictive of EFW < 10th centile at 36 weeks (p=0.003), and birthweight < 10th centile (p=0.001).
A logistic regression analysis provided an odds ratio for birthweight < 10th centile, dependent on a FL < 10th centile at 32 weeks, of 0.98 (95%CI: 0.97 - 0.99; p< 0.001). The odds ratio for EFW < 10th centile at 36 weeks, dependent on a FL < 10th centile at 32 weeks, was 4.9 (95% CI: 1.6 - 15.7; p=0.007).

Conclusion:

FL is easy to obtain, with high accuracy and reproducibility. Short fetal FL is a common abnormal finding in smokers. Although there were strong associations between short FL and birthweight < 10th centile, the positive predictive values were not strong. However, the negative predictive values were > 90%, implying that a FL > 10th centile is a reassuring indicator of later normal fetal growth