Table 2 Characteristics of included studies

Coch 1965 [13]

Blood samples from the jugular and a peripheral vein at various stages of labour and postpartum, in 18 women.

Bioassay^a

Bioassay based on milk ejection in rabbits calculating “oxytocin equivalent activity.” Very high levels compared to modern assays using RIA.

Kumaresan 1974 [14]

Single blood samples between weeks 4 and 40 during pregnancy in 280 women.

79 random samples during active labour in 5 women.

Serial samples were obtained before and after oxytocin infusion of 100 mU per min to women with mid-pregnancy terminations.

RIA^b

OT determinations performed with RIA without prior extraction, explaining why high levels. OT levels were already very high at term, but did not rise further during labour, maybe due to an insensitivity of this assay at high levels of OT.

Kumaresan 1975 [15]

Single maternal blood samples were collected from 29 women 10 min before birth, just after birth and daily for 4 days postpartum.

RIA^b

OT determinations performed with RIA without prior extraction, giving higher levels. Lack of a significant peak at birth suggests possible insensitivity of the assay at high levels.

Gibbens 1976 [16]

8 serial blood samples from 97 women during spontaneous labour (1st stage 33, 2nd stage 14, 3rd 10). Single samples from a further 30 women during the 3rd stage of labour. The serial samples were collected over a period of 4 to 8 min.

RIA^c

Pulsatile release of OT was detected. Very low basal levels of OT may be due to insensitivity of the RIA used, or to a loss of OT during the extraction procedure.

Vasicka 1978 [17]

Blood samples were collected in a longitudinal study (during pregnancy, onset of labour and birth) in 15 women. Samples at one- to two-week intervals during pregnancy and one minute to one hour intervals throughout labour and birth.

RIA^b

OT determinations performed with RIA without prior extraction, explaining high levels.

Dawood 1979 [18]

362 blood samples were collected from normal pregnant women.

Serial blood samples of blood were obtained from 10 pregnant women through gestation until labour onset.

Serial samples taken at 1 min intervals over a 10 min period in 7 pregnancies and 3 in 1st stage of labour.

RIA^c

OT levels recorded within the range normally observed with established RIA after prior extraction of the samples.

Leake 1981 [19]

Plasma oxytocin levels in 102 non-pregnant women, 20 women receiving oral contraceptive medications and 59 pregnant women from 15 to 42 w of pregnancy. Repeated samples were collected in 38 healthy women during spontaneous labour.

RIA^c

Very low OT levels. RIA did not pick up differences between pregnant and non-pregnant women; the rise of oxytocin occurring during pregnancy; or the rise of OT observed during labour, suggesting that the RIA used was very insensitive. Significant rise of OT level observed was in connection with birth only.

Otsuki 1983 [20]

Individual samples from 38 normal pregnant women between the 15th and the 41 st w of pregnancy. Serial samples from 2 women in the mid trimester, from 4 women at term without labour contractions and from 6 women in the 1st stage of normal spontaneous labour, at 10 s intervals over a period of 2–3 min.

RIA^c

Relevant OT levels within the range normally observed with established RIAs.

Goodfellow 1983 [21]

20 primigravidae with normal labour, after 37 to 41 weeks of pregnancy.

Epidural analgesia was chosen by half of the women.

Blood samples at the beginning and end of 2nd phase of labour.

RIA^c

Relevant OT levels within the range normally observed with established RIA.

Husslein 1983

[22]

20 women with spontaneous labour. Blood samples were collected just before the 2nd phase of labour, 5 min postpartum, 30 min after expulsion of placenta and 2 h after birth. 7 women received oxytocin drip in a low dosage at the end of the first phase of labour. Directly after birth 10 women received oxytocin drip in a high dosage (100-150mIU/min).

RIA^c

Relevant OT levels giving values within the range normally observed with established RIA.

Fuchs 1983 [23]

17 women in early spontaneous labour and 15 women at term who were given oxytocin induction.

Five blood samples. In one group at admission to the labour ward and subsequent samples were taken at 1–3 h intervals. In another group a blood sample just before infusion of oxytocin was started and thereafter just before the infusion rate was increased. Infusion was begun at the rate of 1–2 mU/min and increased stepwise every 15 min until contractions occurred with about 3 min intervals. Thereafter the infusion rate was kept constant.

10 of these 17 women contributed a blood sample 1–2 w before labour.

The control group consisted of 4 pregnant women at term but not in labour. Four serial samples were taken at 2 h intervals from each woman.

RIA^c

Relevant OT levels within the range of values normally observed with established RIA.

Amico 1984 [24]

Eleven women with “hypocontractile labour” received oxytocin infusion in a dose of 1 mU/min and it was increased by one 1 mU every 40 min until adequate contractions were observed.

Blood samples were drawn before start of infusion and at every 20 min, during infusion and for 60 min after the end of infusion.

RIA^c

Very low basal levels of OT maybe due to insensitivity of the RIA used or to loss of OT during the extraction procedure.

Takeda 1985 [25]

42 participants were included in the study, 4 healthy males, 15 non-pregnant women, and 23 pregnant women (11 before and 12 in labour).

Blood and cerebrospinal fluid (CSF) samples were collected simultaneously from all the participants.

RIA^c

Relevant OT levels within the range of values normally observed with established RIA.

Takagi 1985 [26]

36 women were included in the study (7 non pregnant women, 11 pregnant women having an emergency CS and 18 pregnant women having an elective CS.

1 blood and cerebrospinal fluid (CSF) samples were collected simultaneously from all the women

RIA^c

Relevant oxytocin levels within the range of values normally observed with established RIA.

De Geest 1985 [27]

10 pregnant women and 15 women during normal labour, 5 of which received EDA. Four blood samples during pregnancy and during the 1st and 2nd stage of labour. Blood samples were also collected from umbilical arterial and venous blood vessels after birth

RIA^b

OT determinations performed with RIA without prior extraction, which explains high OT levels. A rise of OT level observed during pregnancy, but not during labour, suggesting that the RIA used was insensitive at higher levels than those observed during pregnancy.

Kuwabara 1987 [28]

Repeated blood samples in 6 normal pregnant women and blood samples were taken in 7 normal pregnant women every 2 days for at least 14 days before the onset of labour. Simultaneous blood samples were collected from maternal venous blood, umbilical arterial and venous vessels in 10 normal deliveries, 15 elective caesarean sections, and 5 emergency caesarean sections. Amniotic fluid samples were collected during pregnancy and in elective caesarean sections. Blood samples were also collected from 10 non-pregnant women.

RIA^c

Relevant OT levels within the range of values normally observed with established RIA.

Thornton 1988 [29]

25 women having spontaneous labour, of these 10 women received synthetic OT i.m. when the anterior shoulder was delivered. Blood samples were collected every 30th second (for 15 min) after crowning of the head.

RIA^c

Low, but relevant OT levels within the range of values observed with established RIA.

Oosterbaan 1989 [30]

Maternal venous blood was collected at the time of amniocentesis (n = 17), elective CS (n = 18) and/or immediately after normal birth of the baby (n = 44). Mixed cord blood or blood from the umbilical artery or vein was collected at the time of elective CS or after spontaneous birth of the baby.

RIA^c

Relevant OT levels within the range of values normally observed with established RIA.

Fuchs 1991 [31]

50 pregnant women (38 to 42 weeks of gestation)

Samples were collected with 1 min intervals for 30 min from the following groups:

1) Women at term who were scheduled for elective CS, with a closed cervix and not in labour (n = 11).

2) Women in the 1st stage of spontaneous labour with < 6 cm cervical dilation (n = 13)

3) Women in the 2nd stage of spontaneous labour with full dilatation of the cervix (n = 8). Five of these women delivered during the 30 min long sampling period and therefore samples were collected also after birth until delivery of the placenta.

4) 18 women who were not in labour were give bolus injections of OT iv, 2, 4, 8 or 16 mU.

8 blood samples before and 30 s, 1, 2, 3, 4, 5 and 10 min after injection (n = 18).

RIA^d

Very low basal levels of OT perhaps due to insensitivity of the RIA used or a loss of OT during the extraction procedure. Frequent sampling of blood, together with low basal levels of OT, allowed recording and quantitative analysis of individual OT pulses.

Stocche 2001 [32]

30 women included in a randomized open label. Each women was in spontaneous labour at > 5 cm cervix dilatation.

Patients received either intrathecal sufentanil 10 microgram or epidural plain bupivacaine 0.25%

Serial blood samples were collected before analgesia and 15, 30, 60 and 90 min after the induction of the analgesia.

RIA^c

Low, but relevant OT levels within the range of values observed with established RIA.