Meconium-stained amniotic fluid across gestation and neonatal acid-base status

OBJECTIVE: To estimate whether the acid-base status of neonates born to women with meconium-stained amniotic fluid varies across gestation. METHODS: We carried out a retrospective cohort study of all pregnancies that were complicated by meconium-stained amniotic fluid in 2004. Cases were identified from a perinatal pathology database that contained data on all pregnancies complicated by meconium-stained amniotic fluid. Data abstracted from the charts included gestational age at delivery, umbilical arterial pH, birth weight, and the presence or absence of labor. Cases were stratified according to gestational age at delivery. The distribution of meconium-stained amniotic fluid across gestation was computed. The mean umbilical arterial pH values (with 95% confidence intervals) across gestation were assessed by analysis of variance. RESULTS: The mean umbilical arterial pH in women with meconium-stained amniotic fluid did not differ across gestation. The overall incidence of meconium-stained amniotic fluid was 12.0% (766 of 6,403 deliveries). The rates of meconium-stained amniotic fluid increased from 1.2% at 32 weeks to 100% at 42 weeks. CONCLUSION: The rising incidence of meconium-stained amniotic fluid with gestational age is consistent with the hypothesis that fetal maturation is a major etiologic factor in meconium passage. Also, the lack of variation of mean umbilical arterial pH across gestation suggests that fetal acidemia is not increased when meconium passage occurs earlier in pregnancy rather than at later gestational ages.     

Does amniotic fluid analysis reflect acid-base balance in fetal blood?

OBJECTIVE: The purpose of the study was to examine whether the acid-base balance in amniotic fluid reflects the acid-base state in fetal blood. STUDY DESIGN: The pH, PCO2, PO2, bicarbonate, and base excess concentrations were measured in umbilical venous, umbilical arterial, and maternal venous blood and in amniotic fluid samples obtained at fetoscopy. This was performed before intraamniotic termination of 16 normal pregnancies (mean gestation 19.4 weeks), as well as in 14 red blood cell isoimmunized pregnancies (mean gestation 28.4 weeks) undergoing fetoscopy for the diagnosis and treatment of fetal anemia. RESULTS: In normal pregnancies, amniotic fluid pH and bicarbonate were significantly lower than umbilical venous and umbilical arterial blood concentrations, and this was due to the high concentration of base deficit in amniotic fluid. Whereas amniotic fluid PCO2 did correlate significantly with fetal umbilical venous PCO2 values, there was no correlation between other acid-base characteristics. CONCLUSION: Simultaneous sampling of human fetal umbilical blood and amniotic fluid under the acute conditions of this study did not show significant correlations in acid-base values.     

Meconium concentration and amniotic fluid index influence the outcome of amnioinfusion

BACKGROUND: To determine the usefulness of amnioinfusion as a function of meconium concentration and amniotic fluid index. METHODS: This was a prospective study of 206 pregnant women in whom amniotic fluid was moderately or heavily stained with meconium, according to subjective evaluation. The women were assigned randomly to receive amnioinfusion (n=103) or no amnioinfusion (control group, n=103). The results were compared in women with =/<15 % or >15 % meconium in the amniotic fluid (measured by centrifugation), and in women in whom the amniotic fluid index calculated 60 min after insertion of the amnioinfusion catheter was <10 or =/>10. RESULTS: In women with >15% meconium, amnioinfusion decreased the rate of cesarian sections motivated by fetal distress (2.5% vs 22.2%), and in women with =/<15% meconium, amnioinfusion decreased the presence of meconium below the vocal cords (6.4% vs 25.9%). Greater benefits after amnioinfusion were seen in women with an amniotic fluid index =/>10: the rate of cesarian sections was lower (1.3% vs 13.3%), as was the frequency of meconium below the vocal cords (10.1% vs 33.3%). CONCLUSIONS: Beneficial effects of amnioinfusion were seen in women with high and low concentrations of meconium, and with high and low amniotic fluid indexes. These criteria should therefore not be used to decide whether amnioinfusion is indicated when the amniotic fluid is moderately or heavily stained with meconium.     

Changes in fetal acid-base chemistries following tromethamine (THAM) or bicarbonate infusions into fetal circulation of amniotic fluid

Direct infusions of equimolar quantities of bicarbonate and tromethamine into rhesus monkey fetal circulation near term resulted in roughly equivalent increases in fetal pH. However, intra-amniotic infusion of tromethamine was not followed by alkaline changes in the fetus and in most instances was followed by a progressive fetal acidosis. This sharply contrasts with previous experimental success in elevating fetal pH via amniotic fluid bicarbonate infusions.     

Evaluation of fetal and neonatal acid-base status

Determination of fetal acid-base status, either at birth, intrapartum, or antepartum, is the gold standard for the diagnosis of fetal asphyxia. Indirect methods such as Apgar scores and fetal heart rate monitoring show at best only minimal correlation. It is evident that intrauterine hypoxia is a rare cause of central nervous system injury, and the finding of a normal acid-base status at birth should lead the clinician to search for other causes of central nervous system injury or birth depression. A combination of techniques including fetal acid-base assessment, electronic fetal heart rate monitoring, and thorough neonatal evaluation probably provides the best predictor of long-term outcome. Newer methods of umbilical cord sampling and acid-base determinations in the intrauterine growth retarded fetus hold some hope for the future.     

Meconium stained amniotic fluid in very low risk pregnancies at term gestation

Objective: To determine the prevalence and clinical significance of meconium stained amniotic fluid (MSAF) in a low risk population at term gestation and to investigate whether MSAF is a predictor for intrapartum and neonatal morbidity. Methods: A very low risk population including 37 085 consecutive deliveries at term composed the study population. A cross-sectional study was conducted and two groups of patients were identified according to the presence (n=6164) or absence (n=30 921) of meconium in the amniotic fluid at delivery and the outcomes of the two groups compared. Results: The prevalence of MSAF was 16.6%. The incidence of cesarean section (5.6% vs 2.3% P<0.01), instrumental deliveries (3.2% vs 1.8% P<0.01), fetal distress (6.5% vs. 2.1% P<0.01), clinical chorioamnionitis (0.2% vs. 0.1% P<0.01), post-partum infection (0.5% vs. 0.2% P<0.01), 1-minute Apgar score <3 (1.9% vs. 1.1% P<0.01), small for gestational age (7.4% vs. 6.4% P<0.01). was significantly higher in the MSAF compared with the clear amniotic fluid group. Intrapartum and neonatal mortality in this low risk population was significantly higher in the MSAF group ( ) compared with women with clear AF ( ). Conclusions: MSAF in a low risk population at term gestation is a predictor for adverse perinatal outcome and peripartum complications.     

Effect of anaemia on fetal acid-base status

Summary. In 79 pregnancies with erythroblastosis fetalis, fetal blood was sampled to assess the level of anaemia. Significant correlations were found between the haemoglobin concentration and umbilical artery pH, bicarbonate concentration and base excess. Compensatory mechanisms help to maintain oxygen supply in anaemia but their failure is associated with hydrops fetalis.     

Effect of anaemia on fetal acid-base status

In 79 pregnancies with erythroblastosis fetalis, fetal blood was sampled to assess the level of anaemia. Significant correlations were found between the haemoglobin concentration and umbilical artery pH, bicarbonate concentration and base excess. Compensatory mechanisms help to maintain oxygen supply in anaemia but their failure is associated with hydrops fetalis.