The aim of this study was to investigate changes in heart rate, namely the intraindividual variation, in normal embryos/fetuses between 6 and 13 weeks' gestation. A prospective study was performed of 41 singleton pregnancies undergoing routine ultrasound examination during the first trimester of pregnancy. For each case, the embryonic/fetal heart rate was measured on three consecutive occasions with a mean interval of 4 min. The difference in heart rate values between the serial measurements varied from 0 to 18 beats/min and was significantly associated with gestational age (Spearman r = 0.26, p < 0.01). In the group of 15 fetuses (examined before 10 weeks' gestation), the difference between measurements (median = 0 beats/min, range 0-8 beats/min) was significantly lower (z = 2.48, p < 0.05) than in fetuses assessed after 10 weeks of gestation (median = 3, range 0-18 beats/min). Reliable and reproducible information concerning embryonic/fetal heart rate may be obtained from a single measurement, especially when pregnancies are examined before 10 weeks of gestation.