Changes in spectral power of fetal heart rate variability in small-for-gestational-age fetuses are associated with fetal sex

Background: Little is known about the influences of fetal weight and sex on spectral analysis of fetal heart rate (FHR) variability. Aim: The study aims to assess whether there are differences in spectral power of FHR variability according to fetal weight and sex during labor. Study design: Case-control study. A total of 414 singleton term deliveries without fetal acidemia were divided into small-for-gestational-age (SGA) (n = 29) and non-SGA (n = 385) groups. Analyses were performed separately according to fetal sex. Subjects: FHR recordings obtained with cardiotocography during the last 2. h of labor preceding delivery. Outcome measures: Our outcome measures include spectral power of FHR variability. Results: For the male group, SGA fetuses had significantly lower values for low, movement, high, and total frequencies of spectral power compared with non-SGA fetuses (all P < 0.005). Normalized low frequency (LFn) was significantly higher, and normalized high frequency (HFn) was significantly lower in SGA fetuses compared with non-SGA fetuses (all P < 0.005). In contrast, for the female group, there were no significant differences in any of the indices of spectral power between the SGA and non-SGA fetuses. In addition, SGA males had significantly higher LFn spectral power and lower HFn spectral power compared to SGA females (P= 0.016, and 0.041, respectively). Conclusions: SGA males have decreased spectral power of FHR variability compared with non-SGA males during labor. However, there are no differences between SGA and non-SGA female fetuses. It is important in the clinical setting to take fetal weight and sex into account during FHR monitoring using spectral analysis.