A total of 2,500 fetuses from carefully selected pregnant women with physiological course of pregnancy at the gestational term of 32 to 40 weeks were studied in Stage 2. Single-time recording of external ECG or external cardiotachogram (CTG) was conducted during 20–25 min. To determine the intrauterine condition of the fetus, functional tests were carried out 10–12 min following the commencement of the background recording. Breath-hold at expiration, physical exertion, sound stimulation, and non-stress test (NST) were used as functional loads. Breath-hold at expiration was carried out during 20–25 sec. For "sound load", a telephone handset was placed anteriorly on the abdominal wall in the fetal head projection area and a permanent sound was delivered at the frequency of 500–1000 Hz for 60 sec at an intensity of 105 decibel. The mothers wore ear-plugs during the stimulation. Physical exertion was given in the form of bending and straightening of the trunk 10 times for 45–50 sec. Functional state of the fetus was evaluated based on the analysis of a one-time CTG (focus being placed on the HR fluctuations, baseline rhythm, intra-minute oscillation, fetal reactivity to functional loads, and number of variable accelerations and decelerations). The trials were dynamically conducted during both "active" (9 a.m. to 2 p.m.) and "quiet" (2 p.m. to 7 p.m.) periods. The electrographic findings of the intrauterine fetus were correlated with evaluations by Apgar scale as well as with the data regarding the course of postnatal period. The time of the recordings was strictly fixed for each observation of the fetus. Differences between the parameters under study were considered reliable at p < 0.05.

The methodical approach utilized in our study of daily rhythms of external ECG allowed us to perform a digital analysis of hourly values of maternal and fetal heart rate during 24-hours of uninterrupted recording. We were able to evaluate changes in the functional levels of two processes in the fetus: heart action and locomotor activity. The uninterrupted recordings of fetal ECGs allowed us to use a new self-designed technique for analysis of daily cardiotachograms (CTG) of the fetus and the pregnant woman. The analysis of heart rate changes in healthy pregnant women showed that function enhances from 7 a.m. (66.5 bpm) to 12 p.m. (79.5 bpm), slightly reduces, and then continues to increase until 5 p.m. (76.5 bpm). Minimal heart rate is observed at 2 a.m. (61 bpm). Thus, in healthy pregnant women, the daily rhythms of HR fluctuations seem to be preserved (Table 1).

The study of 24-hour FHR fluctuations demonstrated that the HR magnitude at 7 a.m. was 133.0 ± 1.9 bpm; at 10 p.m. it was 136.5 ± 2.0 bpm, and at 2 a.m. the value approximated that at 7 a.m. (133.5 ± 1.6 bpm). Statistical processing of fetal HR magnitude during the 24-hour period did not show any significant difference between day and night measurements (Fig. 1, Fig. 2). Consequently, analysis of daily FHR fluctuations using the method of frequency changes computing did not permit to judge on the presence of daily periodicity of the fetal heart rhythm.

Karr et al 16 and Hellbrugge 13 investigated 24-hour heart rate rhythms of the mother and the fetus using single auscultations of maternal heart rate and pulse with 2-hour intervals throughout 24 hours. N.N. Konstantinova 10 and Hoppenbrowers 14 directed their investigations mainly toward the study of fetal heart rate changes during the mother's sleep. Our findings agree with Hellbrugge's 13 results indicating the occurrence of typical 24-hour HR rhythms in the pregnant woman. At the same time, FHR is more or less identical during the day time and at night, averaging 133.0 ± 5.0 bpm. Hoppenbrouwers 14 found that fetal HR did not undergo substantial changes during the sleep and waking periods of the mother. N.N. Konstantinova 10 reported a dependence of fetal HR on maternal sleep-wakefulness cycles.

We have undertaken the task of investigating the principles of conformity between maternal and fetal rhythms with the result of taking notice of heterogeneity in fetal cardiotachograms, which allowed us to classify four types of oscillations occurring on an hourly cardiotachogram in different combinations, and sequences, and continuously replacing one another. Depending on the differences in form, amplitude and duration, each of the oscilation types was designated by a proper term: "peak-like", "rounded", "flat" and "mixed" (Fig. 3). Type I ("peak-like") is characterized by rapid fluctuations of FHR during 5–10 sec with the intra-minute fluctuations of ± 12–30 bpm. Type II ("rounded") is characterized by a gradual intensification of fetal HR by ± 18–34 bpm and further gradual return to the initial values. Type III ("flat") is characterized by low intra-minute fluctuation of ± 1–4 bpm. Type IV ("mixed") is characterized by the baseline rhythm of 120–150 bpm and intra-minute fluctuation of ± 7–15 bpm.

Having separated the 24-hour cardiotachograms into hourly parts, we carried out a careful visual and digital analysis of the findings and discovered certain irregularities in the distribution of one or another type of oscillations across a daily rhythmogram. Further, having computed the amount of time necessary for each type of cardiotachogram to repeat within one hour of the observation, we paid attention to the findings that the period when the background cardiotachogram is presented by "flat (III) type" oscillation (51% and 55 % of the recording time) was observed to occur twice during 24 hours, predominating over the other three types (Fig. 4, Fig. 5). The periods from 4 a.m. to 9 a.m. and from 2 p.m. to 7 p.m. were termed "quiet" hours. During the rest of the day, i.e. from 9 a.m. to 2 p.m., and from 7 p.m. to 4 a.m., the background cardiotachogram was represented by "mixed" oscillations with the prevalence of types I, II and IV (87% and 89% of the recording time). These periods were termed "active" hours (Table 2, Table 3, Table 4, Fig. 6, Fig. 7).

Issues regarding the onset and formation of the fetal intrauterine rhythm were studied on 28 pregnant women at the gestational term of 16–28 weeks (at 16–20 weeks – 8 pregnant participants, at 21–24 weeks – 12, at 26–28 weeks – 8). The self-designed methodological approach previously employed in our investigations was entirely preserved. Analyses of daily rhythmograms were performed using the original method described above 71719. The results showed regular rhythms of daily fluctuations of HR in all 28 pregnant women. Sleep duration in this group was 8 hrs 32 min. The general structural characteristics and each phase of sleep were typical of those of a healthy individual.

The analysis of 24-hour CTGs of the fetus demonstrated a clear-cut daily rhythm of heart rate and locomotor activity in 21 fetuses (16–20 weeks gestation – 6 cases; 21 – 24 weeks gestation – 8 cases; 26–28 weeks gestation – 7 cases).

Just as in a full-term fetus at 16–28 weeks gestation, the fetus' background cardiotachogram for "active" hours, from 9 a.m. to 2 p.m. and from 7 p.m. to 4 a.m., was represented by mixed (type IV) oscillations with markedly expressed "peak-like" and "rounded" types. During "quiet" periods (4 a.m. to 9 a.m. and 2 p.m to 7 p.m.), the background rhythmogram was represented by "flat" (type III) oscillations. The locomotor activity in "active" hours was 11 (in a full-term fetus 6–7) times as intensive as compared to that in "quiet" periods of the day. Consequently, as early as at 16–20 weeks pregnancy the fetus clearly expressed 24-hour rhythms of the heartbeat and locomotor activity. On the other hand, in 7 cases (out of 28) the analysis of daily cardiotachograms showed mostly "mixed" oscillations during both "active" and "quiet" periods with "peak-like, "rounded" and "flat" types (types I, II, III) being observed with different intensity against the background cardiotachograms. In these 7 cases (at 16–20 weeks' gestation – 2 cases; 21–24 weeks' gestation – 4 cases; 26–28 weeks' gestation – 1 case), the concentration of "flat" (type) oscillations in "active" hours was equal to 17 ± 0.6 min, i.e. 3.03% of the total recording time from 9 a.m. to 2 p.m. and from 7 p.m. to 4 a.m. In "quiet" hours the concentration of flat oscillations was 14 ± 1.03 min, corresponding to 28% of the total recording time from 4 a.m. to 9 a.m. and from 2 p.m. to 7 p.m. The locomotor activity of these 7 fetuses was 3.5 fold higher in "active" hours than that in "quiet" periods. Comparative analysis of these findings for 21 fetuses at 16–28 weeks' gestation with expressed fetal rhythms showed that the concentration of "flat" (type III) oscillations in "active" hours made up 49 ± 0.6 min, i.e. 8.7% of the total recording time. During "quiet" hours, the concentration of "flat" oscillations was 56.2% of the total recording time. Fetal movements in "active" periods were observed to be 11 times more than in "quiet" hours.

The study allows assuming that, at the gestational age of 16–20 weeks, the hypothalamo-hypophyseal system of a healthy fetus reaches the degree of maturation which is sufficient to provide well-developed capacities for adaptation to the environment. A healthy fetus at the gestational age of 16–20 weeks has pronounced daily rhythms of the heartbeat and locomotor activity. The fetuses in which we failed identify any distinctly expressed rhythms of heart beat and locomotor act