Biparietal diameter at 11-13 weeks' gestation in fetuses with open spina bifida

Abstract

Objective: To ascertain the reported association between reduced biparietal diameter (BPD) at 11-13 weeks' gestation and open spina bifida and to investigate its predictive value in a single-center study.

Methods: This was a retrospective study of fetuses in which BPD was measured at 11-13 weeks' gestation, including 27 fetuses with isolated open spina bifida subsequently diagnosed at 16-24 weeks and 7775 unaffected controls. BPD values were converted into multiples of the expected median (MoM) after adjustment for crown-rump length and maternal characteristics. Multivariable logistic regression analysis was used to determine the maternal characteristics significantly associated with spina bifida. The performance of screening was determined by receiver-operating characteristics curve analysis. BPD values at 11-13 weeks' gestation were compared with those measured in the second trimester using Z-scores.

Results: BPD values at 11-13 weeks' gestation were below the 5(th) centile in 44.4% of cases of open spina bifida. In these fetuses, the median BPD MoM value was significantly smaller than that in the control group (0.930 vs 0.998 MoM; P < 0.0001). Multivariable logistic regression analysis showed a significant contribution from maternal age (P = 0.008) and BMI (P = 0.028) to the association between BPD MoM and spina bifida. The detection rate using BPD measurements in the first trimester was 55.6% with a false-positive rate of 11.6%. In fetuses with open spina bifida, the BPD Z-scores were significantly lower at 16-24 weeks compared to those recorded at 11-13 weeks (median, -1.71 (range, -3.98 to -0.20) vs -1.30 (-3.75 to 2.61); P = 0.006).

Conclusion: Fetuses with open spina bifida have a smaller BPD in the first trimester. This observation may be useful in early screening. It is likely that a combination of maternal characteristics such as age and BMI, fetal BPD and maternal serum alpha-fetoprotein measured in the first trimester would provide a clinically useful screening test for open spina bifida.

Keywords: biparietal diameter; first trimester; prenatal diagnosis; screening; spina bifida; ultrasound.