title: Sex-related differences in the development of fetal heart rate dynamics.
abstract: BACKGROUND
  Despite previous efforts to explain the general advantages of female fetuses over males regarding health, sex-related differences in the dynamics or complexity of fetal heart rate (FHR) variability and FHR maturation patterns have not yet been identified.
  
  
  OBJECTIVE
  To make linear and nonlinear comparisons of antepartum FHR indices, dynamics, complexity, and reactivity to the non-stress test (NST) and vibroacoustic-stimulation test (VAST) in male and female fetuses.
  
  
  METHODS
  A total of 3835 singleton term deliveries without maternal and fetal complications were divided into female (n=1849) and male (n=1986) groups, and subjected to comparison and analyses.
  
  
  METHODS
  Linear FHR indices, approximate entropy (ApEn), sample entropy (SampEn), short-term/long-term exponents (α1/α2), correlation dimension (CD), NST and VAST criteria, and modified nonlinear reactive criteria (MNRC) were used to evaluate outcomes.
  
  
  RESULTS
  ApEn was consistently higher in female fetuses than in male ones. ApEn in female fetuses was maximal at 29-30 gestational weeks, while the increase in ApEn was delayed in male fetuses but more rapid, reaching its peak at 31-32 gestational weeks. In both sexes, CD increased up to term, and α2 rapidly decreased up to 31-32weeks in an analogous manner. The two sexes differed significantly in response to VAST at <31 gestational weeks and there was a structural difference in reactive patterns under MNRC.
  
  
  CONCLUSIONS
  Female fetuses exhibit greater heart rate dynamics in early gestational periods, suggesting that their cardiovascular system matures earlier than that of males. Male fetuses undergo a compensatory period of rapid change to catch up with females at term.
authors.affiliation: Department of Anesthesia and Pain Medicine, College of Medicine, Hanyang University, Seoul, Republic of Korea.
authors.forename: Kyu Nam
authors.initials: KN
authors.lastname: Kim
authors.affiliation: Department of Mathematics and Research Institute for Natural Sciences, Hanyang University, Seoul, Republic of Korea.
authors.forename: Young-Sun
authors.initials: YS
authors.lastname: Park
authors.affiliation: Department of Obstetrics and Gynecology, College of Medicine, Hanyang University, Seoul, Republic of Korea. Electronic address: hohjk@hanyang.ac.kr.
authors.forename: Jeong-Kyu
authors.initials: JK
authors.lastname: Hoh
mesh_terms: D000328:Adult; D005260:Female; D005318:Fetal Heart / Q000254:growth & development / Q000502:physiology*; D006339:Heart Rate; D006801:Humans; D008297:Male; D011247:Pregnancy; D012737:Sex Factors
keywords: Approximate entropy; Computerized fetal monitoring; Correlation dimension; Female; Fetal heart rate; Male; Modified nonlinear reactive criteria; Non-stress test; Vibroacoustic stimulation test