Clapp J F
Am J Obstet Gynecol. 1985 Jul 15;152(6 Pt 1):659-60. doi: 10.1016/s0002-9378(85)80040-5.
Serial morning heart rates were obtained from 10 women runners prior to and during pregnancy. By 8 weeks' menstrual age, maternal heart rate had risen 8 bpm with an overall pregnancy-associated increase of 16 bpm. Potential mechanisms are discussed.