title: Sex differences in the fetal heart rate variability indices of twins.
abstract: OBJECTIVE
  To evaluate the differences in linear and complex heart rate dynamics in twin pairs according to fetal sex combination [male-female (MF), male-male (MM), and female-female (FF)].
  
  
  METHODS
  Fourteen twin pairs (6 MF, 3 MM, and 5 FF) were monitored between 31 and 36.4 weeks of gestation. Twenty-six fetal heart rate (FHR) recordings of both twins were simultaneously acquired and analyzed with a system for computerized analysis of cardiotocograms. Linear and nonlinear FHR indices were calculated.
  
  
  RESULTS
  Overall, MM twins presented higher intrapair average in linear indices than the other pairs, whereas FF twins showed higher sympathetic-vagal balance. MF twins exhibited higher intrapair average in entropy indices and MM twins presented lower entropy values than FF twins considering the (automatically selected) threshold rLu. MM twin pairs showed higher intrapair differences in linear heart rate indices than MF and FF twins, whereas FF twins exhibited lower intrapair differences in entropy indices.
  
  
  CONCLUSIONS
  The results of this exploratory study suggest that twins have sex-specific differences in linear and nonlinear indices of FHR. MM twins expressed signs of a more active autonomic nervous system and MF twins showed the most active complexity control system. These results suggest that fetal sex combination should be taken into consideration when performing detailed evaluation of the FHR in twins.
authors.forename: Iva
authors.initials: I
authors.lastname: Tendais
authors.forename: Bárbara
authors.initials: B
authors.lastname: Figueiredo
authors.forename: Hernâni
authors.initials: H
authors.lastname: Gonçalves
authors.forename: João
authors.initials: J
authors.lastname: Bernardes
authors.forename: Diogo
authors.initials: D
authors.lastname: Ayres-de-Campos
authors.forename: Nuno
authors.initials: N
authors.lastname: Montenegro
mesh_terms: D000328:Adult; D005260:Female; D006340:Heart Rate, Fetal; D006801:Humans; D008297:Male; D011247:Pregnancy; D059285:Pregnancy, Twin; D012727:Sex Characteristics; D014427:Twins; D055815:Young Adult