Table 4 Summary of oxytocin levels during labour and birth (converted to pg/mL)

Oxytocin levels varied significantly between the studies. Very low values around 1.7–3.4 pg/mL were reported by: Leake 1981 [19], Amico 1984 [24], and Fuchs 1991 [31]. Very high levels, around 167–340 pg/mL, were reported by Coch 1965 [13], Kumaresan 1974 [14] and 1975 [15], Vasicka 1978 [17], and De Geest 1985 [27]. These high levels were obtained using bioassay or RIA without previous extraction of the plasma samples, which likely explains the higher-than-expected results.

The majority of the articles report the levels as expected, between 17 and 85 pg/mL; Gibbens 1976 [16], Dawood 1979 [18], Otsuki 1983 [20], Goodfellow 1983 [21], Husslein 1983 [22], Fuchs 1983 [23], Takeda 1985 [25], Takagi 1985 [26], Kuwabara 1987 [28], Thornton 1988 [29], Oosterbaan 1989 [30], and Stoche 2001 [32].

Difference in oxytocin levels between pregnancy and labor

A rise of oxytocin levels between pregnancy and labor was observed by Dawood 1979 [18], Husslein 1983 [22], Fuchs 1983 [23], and Fuchs 1991 [31].

No rise was observed by Kumaresan 1974 [14] and 1975 [15], and De Geest 1985 [27], which was most likely due to a lack of sensitivity of the assay used.

Difference in oxytocin levels over time during labor, or between the 1st and the 2nd stage of labor

A rise of oxytocin levels during labor was observed by Coch 1965 [13] and Gibbens 1976 [16].

In addition, Dawood 1979 [18], Otsuki 1983 [20], and Fuchs 1991 [31] found an increasing incidence, frequency, and/or amplitude of oxytocin pulses, from late pregnancy through to the first and second phases of labor. Fuchs 1991 [31] found a maximum pulse frequency of 3 per 10 min in late labor.

Rise of oxytocin levels in connection with birth

A very pronounced rise of oxytocin levels (sometimes as much as a 4-fold increase) was observed in connection with birth of the baby, compared to levels in pregnancy, early labor, or immediately following PLCS: Vasicka 1978 [17], Leake 1981 [19], Goodfellow 1983 [21], Husslein 1983 [22], Kuwabara 1987 [28], Thornton 1988 [29], Oosterbaan 1989 [30].

Elevation of oxytocin levels in connection with expulsion of the placenta or postpartum

In some studies, elevated oxytocin levels were also found during the third stage of labor, likely in connection with expulsion of the placenta (Kumaresan 1975 [15], Husslein 1983 [22], Thornton 1988 [29], Fuchs 1991 [31])