Aim: The knowledge on biological rhythms is rapidly expanding. We aimed to define the longitudinal development of the daily (24-hour) fetal heart rate rhythm in an unrestricted, out-of-hospital setting and to examine the effects of maternal physical activity, season and fetal sex. Methods: We recruited 48 women with low-risk singleton pregnancies. Using a portable monitor for continuous fetal electrocardiography, fetal heart rate recordings were obtained around gestational weeks 24, 28, 32 and 36. Daily rhythms in fetal heart rate and fetal heart rate variation were detected by cosinor analysis; developmental trends were calculated by population-mean cosinor and multilevel analysis. Results: For the fetal heart rate and fetal heart rate variation, a significant daily rhythm was present in 122/123 (99.2%) and 116/121 (95.9%) of the individual recordings respectively. The rhythms were best described by combining cosine waves with periods of 24 and 8 hours. With increasing gestational age, the magnitude of the fetal heart rate rhythm increased, and the peak of the fetal heart rate variation rhythm shifted from a mean of 14:25 (24 weeks) to 20:52 (36 weeks). With advancing gestation, the rhythm-adjusted mean value of the fetal heart rate decreased linearly in females (P <.001) and nonlinearly in males (quadratic function, P =.001). At 32 and 36 weeks, interindividual rhythm diversity was found in male fetuses during higher maternal physical activity and during the summer season. Conclusion: The dynamic development of the daily fetal heart rate rhythm during the second half of pregnancy is modified by fetal sex, maternal physical activity and season.
Bibliographical noteFunding Information:
This longitudinal observational study was approved by the Regional Committee for Medical and Health Research Ethics (REK Vest, 2013/745-5) and was conducted between September 2013 and March 2017 in Bergen, Norway. Women scheduled for second trimester routine ultrasound scan received an invitation to the study (N ≈ 5000). Fifty-two women wished to participate, four of them did not meet the inclusion criteria; only healthy pregnant women with an uncomplicated obstetric history and a current uneventful singleton pregnancy were finally recruited to the study after written informed consent. Exclusion criteria were as follows: multiple pregnancies, congenital malformations or previous pregnancy complications. We did not exclude two complications occurring after inclusion (N = 1 with premature rupture of the membranes and N = 1 with gestational diabetes), as the study population should reflect a clinically relevant low-risk population. An ordinary power calculation was difficult. The number of participants was based on the experience from other studies using cosinor analysis and accounting for data loss due to poor fECG signal detection.9,10
© 2018 Scandinavian Physiological Society. Published by John Wiley & Sons Ltd
- daily rhythm
- fetal development
- fetal heart rate