Small biparietal diameter in fetuses with spina bifida on 11-13-week and mid-gestation ultrasound.

Abstract

To assess whether, at 11-13-week and mid-trimester ultrasound examinations, the biparietal diameter (BPD) in fetuses with open spina bifida is smaller than the reference range. In a multicenter retrospective analysis of data from 23 fetuses with open spina bifida diagnosed at 16-24 weeks, BPD at diagnosis was compared with that measured at 11-13 weeks, before diagnosis. Z-scores were calculated for comparison of BPD deviations from the reference range within the groups at 11-13 weeks and 16-24 weeks, and between these two time intervals. BPD Z-scores at 11-13 weeks and 16-24 weeks were significantly lower (P < 0.0001) in fetuses with open spina bifida when compared to the reference range, with mean values of - 1.29 and - 2.14, respectively, corresponding to the 10(th) and 1.5(th) centiles, respectively. At 11-13 weeks only six of the 23 (26%) fetuses with open spina bifida had BPD below the 5(th) centile, in comparison to 16 (69%) at mid-gestation. This was confirmed by paired t-test comparing the first-trimester with the second-trimester BPD measurements, which showed significantly slower growth of the BPD in fetuses with spina bifida. In fetuses with open spina bifida, BPD is smaller not only in the second trimester, as previously reported, but also in the first trimester. This is most likely due to loss of cerebrospinal fluid. We suggest that a BPD value < 10(th) centile in an otherwise normally grown fetus at 11-13 weeks' gestation should be considered as a potential subtle early marker for open spina bifida.