Minimum oxytocin dose requirement after cesarean delivery for labor arrest

OBJECTIVE: To estimate the minimum effective intravenous dose of oxytocin required for adequate uterine contraction after cesarean delivery for labor arrest. METHODS: A randomized single-blinded study was undertaken in 30 parturients undergoing cesarean deliveries under epidural anesthesia for labor arrest despite intravenous oxytocin augmentation. Oxytocin was administered as a slow intravenous bolus immediately after delivery of the infant, according to a biased coin up-down sequential allocation scheme. After assisted spontaneous delivery of the placenta, the obstetrician, blinded to the oxytocin dose, assessed uterine contraction as either satisfactory or unsatisfactory. Additional boluses of oxytocin were administered as required, followed by a maintenance infusion. Data were interpreted and analyzed by a logistic regression model at 95% confidence intervals. RESULTS: All patients received oxytocin infusions at a mean +/- standard deviation of 9.8 +/- 6.3 hours before cesarean delivery (maximum infusion dose 10.3 +/- 8.2 mU/min). The minimum effective dose of oxytocin required to produce adequate uterine response in 90% of women (ED90) was estimated to be 2.99 IU (95% confidence interval 2.32-3.67). The estimated blood loss was 1,178 +/- 716 mL. CONCLUSION: Women requiring cesarean delivery for labor arrest after oxytocin augmentation require approximately 3 IU rapid intravenous infusion of oxytocin to achieve effective uterine contraction after delivery. This dose is 9 times more than previously reported after elective cesarean delivery in nonlaboring women at term, suggesting oxytocin receptor desensitization from exogenous oxytocin administration during labor. Therefore, alternative uterotonic agents, rather than additional oxytocin, may achieve superior uterine contraction and control of blood loss during cesarean delivery for labor arrest. LEVEL OF EVIDENCE: I.     

Pulsatile oxytocin for induction of labor: a randomized prospective controlled study

In a prospective randomized study, 20 patients with term pregnancies underwent induction of labor with either continuous or pulsed (every 8 minutes) intravenous oxytocin infusion. There were no significant differences with respect to induction-labor interval, induction-delivery interval, cesarean section rates, need for pain relief and Apgar scores. Sixty percent of patients receiving continuous oxytocin infusion developed uterine hyperstimulation but only 10% receiving pulsed oxytocin did so. However, the difference was not significant. The mean +/- SEM total amount of oxytocin given by continuous infusion was 4237 +/- 1066 mU which was 70% more than by pulsatile infusion (2454 +/- 808 mU). The highest rate of oxytocin infused was significantly lower by pulsatile administration (5.2 +/- 0.8 mU/min) than by continuous infusion (9.2 +/- 1.8 mU/min, p = less than 0.05). Our study demonstrates that pulsed administration of oxytocin every 8 minutes is as effective and safe as continuous intravenous infusion of oxytocin for induction of labor, requires less oxytocin with therefore, a wider margin of safety and is consistent with the pulsatile release of oxytocin during normal labor.     

A prospective comparison of hourly and quarter-hourly oxytocin dose increase intervals for the induction of labor at term

Fifty-two women undergoing labor induction and vaginal delivery at term were randomized between two oxytocin infusion protocols, involving hourly versus quarter-hourly increases in dose. Potential differences were sought of duration of labor, amount of uterine activity generated, and amount of oxytocin required. Starting at 0.5 mU/minute, oxytocin infusion was increased regularly in small increments every hour or every 15 minutes, according to group assignment. No differences were observed in potentially confounding clinical and demographic factors between the groups, including time to ruptured membranes. There were no clinically or statistically significant differences found for the duration of any phase or stage of labor, quantitative assessment of uterine activity, incidence of hyperstimulation, or neonatal outcome. The average dose of oxytocin used was lower in the hourly than in the quarter-hourly, protocol (4.4 versus 6.7 mU/minute; P less than .005). Significantly fewer patients on the hourly protocol required a maximum infusion rate exceeding 8 mU/minute (P less than .05). More patients on the hourly protocol either had oxytocin discontinued completely or were maintained at 4 mU/minute or less during the active phase of labor (P less than .05 and P less than .001, respectively). We conclude that a slower rate of increase in oxytocin administration via continuous infusion results in no prolongation of any phase of induced labor, while permitting lower infusion rates of the drug.     

A randomized trial of pulsatile vs continuous oxytocin infusion for labor induction

In a prospective randomized study, 560 pregnant women were subjected to labor induction with continuous or pulsed intravenous oxytocin infusion. There were no significant differences with respect to maternal history, Bishop score and perinatal morbidity. The mean induction to delivery interval was shorter in the pulsed infusion group than in the continuous infusion group (325 +/- 63 vs 433 +/- 67 min in primiparous, p < 0.001 and 204 +/- 52 vs 236 +/- 87 min in multiparous women, p < 0.01). The mean amount of oxytocin administered in the pulsed infusion group was also significantly lower than in the continuous infusion group (4.7 +/- 0.6 mU/min versus 9.6 +/- 3.4 mU/min in primiparous, p < 0.001 and 2.1 +/- 0.4 mU/min versus 5.2 +/- 2.3 mU/min in multiparous women, p < 0.001). Our study demonstrates that pulsatile administration of oxytocin is as safe as continuous intravenous infusion, requires less oxytocin and is more effective as it reduces labor duration.     

Evolving concepts of oxytocin for induction of labor

Based on findings of maternal and fetal circulating oxytocin levels during spontaneous labor and the available information on oxytocin secretion patterns, the dosage and mode of administration of oxytocin for induction of labor is discussed. It is recommended that intravenous infusion rates should not exceed 2 to 8 mU/min. Oxytocin administered in a pulsatile fashion beginning with 1 mU/min every 8 minutes with doubling the pulse dose every 24 minutes is equally as efficient as continuous infusion. Because both the peak levels and the total dose of oxytocin needed for induction are lower with pulsatile than continuous administration, the risk of adverse effects for the mother and the fetus is significantly reduced.     

Pulsatile administration of oxytocin for augmentation of labor

In a randomized study, 94 patients with term pregnancies underwent augmentation of labor with either continuous or pulsed (every 8 minutes) intravenous oxytocin infusion. There were no significant differences with respect to the maternal characteristics, cervical dilatation and effacement, induction-to-labor interval, induction-to-delivery interval, cesarean section rate, analgesia for labor, or low Apgar scores. No hyperstimulation was noted in either group. In each group, 20% of the patients had dysfunctional labor patterns, with coupling and tripling of the uterine contractions. The mean +/- SEM oxytocin administered in the pulsed-infusion group was significantly lower than that in the continuous-infusion group (2.1 +/- 0.4 versus 4.1 +/- 0.4 mU/minute; P less than .001). The mean +/- SEM total amount of oxytocin administered was 1300 +/- 332 mU for the pulsed group and 1803 +/- 302 mU for the continuous group, indicating that lower amounts of oxytocin were required for pulsed administration. Our study demonstrates that pulsatile administration of oxytocin is similar in efficacy to our standard continuous oxytocin infusion and requires a lower total amount and rate of oxytocin administered, which may afford a greater margin of safety.     

Pulsatile versus continuous oxytocin infusion for the oxytocin challenge test

In a prospective study, 140 patients had an oxytocin challenge test with either a continuous or a pulsed infusion (one minute of infusion in every five minutes). Both infusion regimens had similar success rates in terms of uterine contractions (97.1 vs 98.6%). The potency ratio (pulsed versus continuous infusion) was significant at 2.7 (1.27 to 5.2), which means that more uterine activity was induced with each mU of oxytocin with pulsatile than with continuous administration. The total amount of oxytocin required to obtain three good contractions in 10 minutes was about 40% less with pulsed administration than with continuous infusion, but the test took 40 minutes longer with the pulsed than with the continuous infusion (P<0.01).     

Rate of increase in oxytocin dose on the outcome of labor induction.

OBJECTIVE: To compare the efficacy and safety of arithmetic and geometric increases in oxytocin infusion dosage during induction of labor. METHODS: A total of 120 pregnant women requiring induction of labor at term were randomly assigned to receive oxytocin at dosages increasing arithmetically or geometrically. Maternal demographics, labor delivery data, and newborn outcomes were compared. The setting was the maternity unit of the Obafemi Awolowo University Teaching Hospitals Complex, Ile-Ife, Nigeria. RESULTS: The mean maximum rates of oxytocin delivery needed to achieve adequate uterine contractions were similar in the 2 groups (24.66+/-8.34 mU/min vs. 26.38+/-8.77 mU/min, P=0.24). Labor duration was significantly shorter in the geometric progression group (496.33+/-54.77 min vs. 421.34+/-63.91 min, P<0.001). There were no differences in the rates of cesarean sections, vaginal deliveries, or uterine hyperstimulation, or in neonatal outcomes. CONCLUSION: A geometric rise in the rate of oxytocin infusion delivery reduced the duration of labor without affecting the rates of cesarean sections and uterine hyperstimulation, or newborn outcomes.     

Oxytocin and the initiation of human parturition. IV. Plasma concentrations of oxytocin and 13,14-dihydro-15-keto-prostaglandin F2 alpha during induction of labor by artificial rupture of the membranes

The influence of artificial rupture of the membranes on plasma levels of 13,14-dihydro-15-keto-prostaglandin F2 alpha (PGFM) and oxytocin was examined in 23 pregnant women at term. Serial blood samples were collected before and 15 minutes, 2 hours, 5 hours, and 8 hours after artificial rupture of the membranes. A significant rise in the concentration of plasma PGFM was observed at 15 minutes in the majority of women (20 of 23), but the magnitude of this early rise or the lack thereof was not related to the subsequent course of labor. The concentration of plasma PGFM at 2 hours was, on the other hand, significantly correlated with the induction-delivery interval. Amniotomy, by itself, induced labor and delivery when the increased PGFM levels were maintained from 2 to 5 hours after the procedure (n = 16). In those cases where Pitocin stimulation was required for adequate uterine contractions, it was found that plasma PGFM levels had declined to initial values at 2 hours. Pitocin infusions then partially reversed this decline. In one patient, the cervix failed to dilate in spite of prolonged Pitocin infusion which did not induce significant uterine contractions, and the infusion did not reverse the marked fall in plasma PGFM after the early but transient rise. Mean plasma oxytocin levels did not rise significantly during labor induced by artificial rupture of the membranes and were, on the average, similar to the levels observed during the first stage of spontaneous or oxytocin-induced labor. Considering the previously demonstrated maximal levels of uterine oxytocin receptors in early labor, the absence of a rise in the plasma oxytocin levels does not negate a role for oxytocin in working synergistically with prostaglandins in the mechanism of labor.     

Early postpartum hemorrhage after induction of labor

M.M., a 25‐year‐old Hispanic primigravida at 41 weeks and 6 days gestation, presented to the labor and delivery unit for postdates induction of labor, accompanied by the father of the baby. She reported normal fetal movement and denied any loss of fluid or vaginal bleeding. Her prenatal course was uncomplicated. The fetal heart rate was reassuring. Occasional contractions were recorded by the tocometer, but not felt by M.M. A pelvic exam found her cervix to be 1 cm dilated, 80% effaced, soft and posterior, with the vertex at ?2 station with membranes intact. Based on a Bishop score of 7, the decision was made to proceed with induction of labor with intravenous oxytocin per hospital protocol of 1 mU every 15 minutes to a maximum of 5 contractions in 10 minutes lasting no more than 60 to 90 seconds or a maximum dose of 20 mU/min. Three hours after oxytocin was initiated, M.M.'s cervical exam was 2 cm dilated, 80% effaced, with the vertex at ?2 station. The fetal heart rate baseline was 135 beats per minute with moderate variability and no decelerations. Six hours after admission, M.M. requested pain relief. Her cervical exam was 4 cm dilated, 90% effaced, with the vertex at ?1 station. The oxytocin was infusing at 16 mU/min and the fetal heart rate was reassuring. Pain management options were discussed with the client and, after being counseled on the risks and benefits, she opted for epidural anesthesia. Eight hours after admission, M.M.'s uterine contractions became difficult to detect with the external tocometer. At examination, her cervix was found to be 7 cm dilated, 100% effaced, with the vertex at 0 station. The oxytocin was infusing at 20 mU/min. The midwife counseled the client on the need to monitor uterine contractions when receiving oxytocin and the decision was made to rupture M.M's bag of waters and place an intrauterine pressure catheter. M.M.'s membranes were ruptured and it was noted that she had a moderate amount of clear amniotic fluid. The intrauterine pressure catheter was placed without complications and recorded adequate uterine contractions of 230 MvU. Eleven hours after admission, M.M. reported rectal pressure and a desire to push. Her cervix was examined and found to be fully dilated with the vertex at +2 station, and again the fetal heart rate was reassuring. Pushing efforts were commenced. One and a half hours later—12 and half hours after the initiation of induction—M.M. gave birth to a vigorous baby girl weighing 3000 g, with Apgar scores of 9 and 9 at 1 and 5 minutes, respectively. The oxytocin infusion was discontinued immediately after delivery. Twenty‐five minutes later, the placenta was expelled spontaneously and appeared intact. An infusion of 20 units of oxytocin in 1000 cc of lactated ringers was given intravenously at 125 ml/hr. Fundal massage was started immediately after delivery of the placenta, because the uterine fundus was found to be boggy, and vaginal bleeding continued to increase. Bimanual uterine compression was added to control the hypotonic uterine bleeding. The urinary bladder was catheterized, producing 100 ml of urine. Ten units of oxytocin were administered intramuscularly in the thigh, again with poor response. Methylergonovine maleate 0.2 mg was then given intramuscularly. The obstetrician on call was notified of the client's uncontrolled immediate postpartum hemorrhage related to atony. Four minutes after Methergine administration, the fundus was firm and bleeding minimal. Twelve hours after delivery, M.M.'s hemoglobin (Hgb) and hematocrit (Hct) were noted to have decreased 23% from her levels at admission. Her Hgb was 7.8 g/dL, and she had an Hct of 23.9%. Her vital signs remained stable throughout, urine output satisfactory, and the client was able to tolerate walking without assistance.     

Management of spontaneous rupture of the membranes in the absence of labor in primigravid women at term

One hundred and thirty-five healthy primigravid women at or near term with spontaneous rupture of the membranes without uterine contractions were submitted to a prospective trial of management. Rupture of the membranes was diagnosed by speculum examination. If labor did not commence, induction was performed by oxytocin infusion starting at 9 AM following admission. One hundred and five women went into labor spontaneously before induction became necessary. Sixty-three of these women required augmentation with oxytocin. Twenty-seven percent of the induced group required cesarean section delivery compared to 10% of those in spontaneous labor augmented by oxytocin and to none of those who did not require oxytocin (p less than 0.01). Ninety-four percent of those in spontaneous labor were delivered vaginally compared to 73% of the induced group (p less than 0.01). Forty-one percent of the augmented group were delivered by forceps. Awaiting the spontaneous onset of labor for 24 hours or less did not result in clinical maternal or neonatal infection. We would therefore advocate awaiting the spontaneous onset of labor after spontaneous rupture of membranes without contractions at or near term in healthy primigravid women for up to 24 hours because it seems to confer significant advantages without producing any additional hazard.     

Induction of labor by using PGE2 and oxytocin in high risk pregnancies]

The study group consisted of 82 primigravid and 55 multiparous women with post term pregnancy, preeclampsia, intrauterine growth retardation, insufficiency of placenta and diabetes mellitus have induced labor. Prepidil (Upjohn) in dosage 0.5 mg was given into uterine cervix of 46 patients (PG group) and oxytocin was infused to 42 patients in dosage ranged from 5 mU/min to 30 mU/min (Ox group). Induction of labor has been considered as successful, if after 12 hours of drug administration, regular contractions of uterus and dilation of cervix more than 3 cm were obtained. Significant improvement of cervix state, measured by Bishop score has been observed only in PG group, even if the induction of labor failed. Similar rates of caesarean sections and the same occurrences of late and variable decelerations have been observed in both study groups. Results obtained in both these groups suggest that induction of labor in such pregnancies after prostaglandins administration is more effective than oxytocin infusion.     

Misoprostol versus low-dose oxytocin for cervical ripening: a prospective,randomized, double-masked trial

OBJECTIVE: A variety of cervical ripening agents exist, yet none is ideal. We performed a prospective, randomized, double-masked comparison of low-dose minimal-escalation oxytocin to misoprostol in a predominantly high-risk population. STUDY DESIGN: Patients were allocated prospectively in a double-masked, randomized, stratified basis by an investigational pharmacist between December 1996 and December 2000 to receive either active intravenous oxytocin and placebo intravaginal misoprostol or intravenous placebo oxytocin and 50 microg of active intravaginal misoprostol. The infusion rate of oxytocin was increased from 1 to 4 mU/min; misoprostol (25 microg) was repeated at 4 hourly intervals if there were <3 uterine contractions per 10-minute interval. RESULTS: Demographic characteristics did not differ between study groups nor did the indications for induction. Overall, the interval to delivery was less in the misoprostol group; however, vaginal delivery occurred in 61% versus 66% (not significant) of patients in the misoprostol versus oxytocin group. Indication for cesarean delivery in the misoprostol group was fetal intolerance to labor in 27% compared with 8% in the oxytocin groups (P <.05), whereas labor abnormalities were more commonly the cause in the oxytocin group versus misoprostol (26% vs 10%, P <.05). The proportion of patients was similar in each group overall and when evaluated on the basis of parity and when delivery was compared at 12, 24, and 36 hours after the initiation of cervical priming. CONCLUSION: Our data indicate that misoprostol and low-dose minimal-escalation oxytocin appear to be equally effective for cervical priming. Low-dose oxytocin may have a preferential role in the high-risk parturient whose fetus is at increased risk for fetal intolerance to labor     

The effect of oxytocin infusion on uterine activity levels in slow labour

Summary. Uterine activity was studied in 31 women who were progressing slowly in spontaneous labour. In 75%, levels of uterine activity were below the tenth centile for normal spontaneous labour (mean uterine activity integral, UAI, 593 kPas/15 min; SD 296). Following oxytocin infusion, there was a significant increase in uterine activity to a mean of 1124 kPas/15 min (SD 276), which was t h e same as in normal spontaneous labour. The response t o oxytocin was dependent upon the pre-existing level of uterine activity, and sensitivity to oxytocin, rather than the dose rate; 84% responded t o infusion rates of < 8 mU/min. The response t o oxytocin was best expressed in terms of active contraction area (uterine activity integral, UAI) or Montevideo units, rather than the frequency or active pressure of contractions. The rate of cervical dilatation following oxytocin augmentation could not be predicted either by t h e increase in uterine activity or by the actual level of activity achieved.     

The effect of oxytocin infusion on uterine activity levels in slow labour

Uterine activity was studied in 31 women who were progressing slowly in spontaneous labour. In 75%, levels of uterine activity were below the tenth centile for normal spontaneous labour (mean uterine activity integral, UAI, 593 kPas/15 min; SD 296). Following oxytocin infusion, there was a significant increase in uterine activity to a mean of 1124 kPas/15 min (SD 276), which was the same as in normal spontaneous labour. The response to oxytocin was dependent upon the pre-existing level of uterine activity, and sensitivity to oxytocin, rather than the dose rate; 84% responded to infusion rates of less than 8 mU/min. The response to oxytocin was best expressed in terms of active contraction area (uterine activity integral, UAI) or Montevideo units, rather than the frequency or active pressure of contractions. The rate of cervical dilatation following oxytocin augmentation could not be predicted either by the increase in uterine activity or by the actual level of activity achieved.     

Spontaneous labor and elective induction--a prospective randomized study. I. Effects on mother and fetus.

In a prospective randomized study spontaneous and oxytocin induced labor "for convenience" have been compared with respect to uterine activity, duration of labor, the condition of the fetus and the newborn infant. The study consists of 84 normal patients, of whom 43 were induced at full term by amniotomy and oxytocin infusion using the Cardiff Infusion System Mark II; 41 patients served as controls. No difference in maternal age, number of previous pregnancies and pelvic score one week before the day of delivery were found between the groups. The following parameters were calculated: duration of labor, uterine activity, amount of bleeding in the third stage of labor, number of early and late decelerations as well as number of episodes of bradycardia in the CTG-recordings, birth weight, Apgar score one and five minutes post-delivery and blood gases in mother and child 60 seconds after delivery. No significant differences between the two groups were found. It is concluded that there are no increased risks to mother or fetus compared to normal labor provided that there is cephalic presentation and normal pregnancy, careful selection with respect to the length of pregnancy and the condition of the cervix and that the Cardiff infusion system is used with intrauterine pressure recording and continuous fetal heart monitoring.     

Intrauterine pressure studies in multigravid patients in spontaneous labour: effect of oxytocin augmentation in delayed first stage

Uterine activity was assessed by measuring the uterine activity integral (UAI) in 116 black multigravid patients in spontaneous active labour at term. Although individual patients showed a wide range of mean UAI levels, normal labour (group I, n = 54) was associated with a mean UAI level of 1640 kPas/15 min which was higher than the mean level observed in patients in whom delay in cervical dilatation occurred in active labour. The patients who failed to progress in labour were treated with oxytocin infusion and 32 of them made good progress in labour and achieved vaginal delivery (group IIa: mean UAI pre-oxytocin treatment 1040 (SD 424) kPas/15 min, post-oxytocin 1890 (SD 559) kPas/15 min). The other 23 patients required operative delivery (group IIb) despite correction of uterine activity after oxytocin treatment (pre-oxytocin mean UAI 1230 (SD 570) kPas/15 min, post-oxytocin 1815 (SD 650) kPas/15 min). The rate of oxytocin infusion varied between patients from 2 to 16 mU/min but in 75% uterine activity was corrected to normal levels by a dose of less than or equal to 8 mU/min. It is concluded that delay in progress in labour is associated with lower intrauterine pressures than are present in normal labour, and that management of patients presenting with inefficient uterine action may benefit from the use of uterine activity integral measurements.     

Intrauterine pressure studies in multigravid patients in spontaneous labour: effect of oxytocin augmentation in delayed first stage

Summary. Uterine activity was assessed by measuring the uterine activity integral (UAI) in 116 black multigravid patients in spontaneous active labour at term. Although individual patients showed a wide range of mean UAI levels, normal labour (group I, n = 54) was associated with a mean UAI level of 1640 kPas/15 min which was higher than the mean level observed in patients in whom delay in cervical dilatation occurred in active labour. The patients who failed to progress in labour were treated with oxytocin infusion and 32 of them made good progress in labour and achieved vaginal delivery (group IIa: mean UAI pre-oxytocin treatment 1040 (SD424) kPas/15min, post-oxytocin 1890 (SD 559) kPas/15min). The other 23 patients required operative delivery (group IIb) despite correction of uterine activity after oxytocin treatment (pre-oxytocin mean UAI 1230 (SD570) kPas/15min, post-oxytocin 1815 (SD650) kPas/15min). The rate of oxytocin infusion varied between patients from 2 to 16 mU/min but in 75% uterine activity was corrected to normal levels by a dose of ≤8 mU/min. It is concluded that delay in progress in labour is associated with lower intrauterine pressures than are present in normal labour, and that management of patients presenting with inefficient uterine action may benefit from the use of uterine activity integral measurements.     

Oxytocin receptor blockade: a new principle in the treatment of preterm labor?

The concentration of myometrial and decidual oxytocin receptors increases dramatically in normal women in late pregnancy, causing enhanced uterine sensitivity to physiologic levels of oxytocin. Similar increase in myometrial oxytocin receptors has been found in women in preterm labor, indicating a role for oxytocin also in idiopathic preterm labor. A newly synthesized oxytocin analogue, 1-deamino-2-D-Tyr-(OEt)-4-Thr-8-Orn-oxytocin, has been found to be a competitive inhibitor of oxytocin. The present study was conducted to test its efficacy in suppressing uterine contractions during preterm labor in women. Twelve patients with established, uncomplicated preterm labor between 27 and 33 weeks of gestational age were given intravenous infusions of the analogue for 1.5 to 13 hours during continuous external cardiotocographic monitoring. In nine patients inhibition of uterine contractions was achieved and further progression in cervical scores was arrested. In three patients, all at 27 weeks of gestational age, no significant tocolytic effect was observed during a 1.5-hour infusion of the analogue and the patients were then given ritodrine intravenously. No side effects were observed in any of the patients. These preliminary findings support the concept that an increased concentration of uterine oxytocin receptors is an important etiologic factor in uncomplicated preterm labor and therefore oxytocin receptor blockade may be a therapeutic alternative for this condition.     

Intrauterine fetal transfusion in the treatment of erythroblastosis fetalis

Assay of serum oxytocinase levels by the chemical method, using L-cystine-di-β-naphthylamide as substrate and the dynamic oxytocin test with Syntocinon were made in 136 hospitalized pregnant women before delivery. A statistically significant relationship was found between prenatal levels of serum oxytocinase and the following clinical data: body surface of the fetus, state of neonate, placental weight, perinatal blood loss, and duration of labor. No differences were found in the level of the enzyme when the reactivity of the pregnant uterus to oxytocin was in the range of 10 to 60 mU. of Syntocinon injected intravenously. However, the serum oxytocinase level in women requiring administration of oxytocin for induction of labor was significantly lower than in other parturient women, regardless of whether delivery was spontaneous or operative. The observed facts were explained in the light of a biochemical theory concerning pregnancy and labor.