Statistics Women were recruited to permit the inclusion of at least 30 women in each of four groups composed according to duration of labour and obstetric outcome. In addition, 30 women delivered by planned caesarean section were included as controls. A mean decrease of the sodium concentration of 5 mmol/l during labour would be considered significant. The sample size of at least 26 women in each group was required for a 90% power to detect a significant difference with a two-sided alpha error of 0.05 (Table 1). Mothers or infants with missing data were excluded from the corresponding analysis only. Group differences were analysed using nonparametric tests for continuous parameters (Mann–Whitney U test when comparing two groups and Kruskal–Wallis analysis of variance when more than two groups were compared). Spearman's rank correlation was used for association between two variables. Categorical variables were compared with chi-square test if the number of subjects was appropriate, otherwise Fisher's exact test was used. Initial analysis of the results indicated that major clarity of the presentation could be achieved by reallocating the study participants to three new groups composed according to total fluid administration during labour (Table 2). Table 1 Baseline values and observed changes related to fluids given until birth. Group differences analysed using Kruskal–Wallis nonparametric ANOVA and Mann–Whitney U test Control group elective caesarean (n = 26) Total fluids given until birth Kruskal–Wallis P value Mann–Whitney P value Fluid group 1 <1000 ml (n = 113) Fluid group 2 1000–2500 ml (n = 87) Fluid group 3 >2500 ml (n = 61) Median Q1–Q3 Median Q1–Q3 Median Q1–Q3 Median Q1–Q3 Mother Age 32 27–35 31 28–34 30 27–33 30 26–33 0.117 Weight before pregnancy (kg) 67 58–78 66 59–73 67 60–77 66 60–77 0.750 Body mass index before pregnancy 26.2 22.6–28.1 23.7 21.0–25.5 24.1 21.5–27.9 23.9 22.1–27.1 0.066 Weight at term (kg) 83 72–89 79 71–89 80 74–90 83 74–91 0.399 Weight increase 12 10–18 13 11–16 14 10–17 15 11–19 0.466 Gestational weeks 38.5 38.3–38.7 39.7 39.0–40.6 40.0 39.1–40.9 40.3 39.6–41.0 0.087 Cervical dilatation at admission (cm) — — 5.0 4.0–7.0 4.0 3.0–4.0 3.0 2.0–4.0 <0.001 <0.001 Duration of labour (hours) — — 1.6 0.9–3.0 5.9 4.3–8.9 12.2 9.8–14.7 <0.001 <0.001 Duration second stage (hours) — — 0.2 0.1–0.5 0.5 0.2–1.0 0.6 0.4–1.0 <0.001 <0.001 Oral fluids (ml/hour) 0 0–0 185 79–316 196 136–305 196 154–263 0.429 Intravenous fluid (ml/hour) Not measured Not measured 0 0–0 52 3–102 94 53–144 <0.001 <0.001 Total fluid (ml/hour) Not measured Not measured 194 100–325 291 201–362 314 260–360 <0.001 <0.001 Total fluid (ml) Not measured Not measured 400 200–575 1680 1355–2050 3570 2925–4355 — Oxytocin (ml) — — 0 0–0 50 0–166 200 100–400 <0.001 <0.001 Oxytocin (units) — — 0.0 0.0–0.0 1.0 0.0–3.3 4.0 2.0–8.0 <0.001 <0.001 Oxytocin (maximum rate mU/minute)* — — 10 5–20 20 10–30 40 20–60 <0.001 <0.001 Duration epidural (hours) — — 1.7 1.7–1.7 4.3 2.9–6.1 8.8 6.8–10.6 — Osmolality at baseline (mOsm/kg) 279 277–281 280 277–282 280 276–282 279 277–281 0.501 Osmolality postpartum (mOsm/kg) 280 277–282 282 279–284 277 276–281 274 272–277 <0.001 <0.001** Na at baseline (mmol/l) 137 136–138 137 136–137 136 135–137 136 135–138 0.238 Na postpartum (mmol/l) 137 137–138 136 135–137 135 133–136 133 130–135 <0.001 <0.001** Na change (mmol/l) 0.0 −1.0 to 1.0 0.0 −1.0 to 1.0 −2.0 −3.0 to −1.0 −3.0 −5.0 to −1.0 <0.001 <0.001** Glucose at baseline (mmol/l) 5.0 4.5–5.5 5.1 4.7–5.8 5.2 4.9–5.9 5.3 4.8–6.2 0.133 Glucose postpartum (mmol/l) 4.8 4.2–5.1 6.7 5.9–7.9 7.7 6.5–9.4 7.8 6.9–9.4 <0.001 <0.001** Umbilical cord Arterial BE −2.0 −3.0 to −1.0 −4.0 −6.0 to −3.0 −5.0 −7.0 to −3.0 −6.0 −9.0 to −3.0 <0.001 0.006** Arterial pH 7.27 7.24–7.29 7.26 7.22–7.31 7.24 7.19–7.29 7.24 7.17–7.29 0.042 0.014*** Arterial Na (mmol/l) 139 138–140 139 137–140 137 136–139 136 134–138 <0.001 <0.001** Arterial–venous Na difference (mmol/l) 1.0 0.0–1.0 1.0 0.0–2.0 1.0 0.0–2.0 1.0 0.0–2.0 0.736 Arterial glucose (mmol/l) 3.4 3.0–3.7 4.2 3.6–5.0 5.2 4.2–6.1 5.4 4.7–6.6 <0.001 <0.001** Infant Birthweight (g) 3415 3120–3790 3535 3305–3850 3670 3360–4160 3805 3580–4040 0.001 0.001** All group differences are compared with Kruskal–Wallis nonparametric analysis of variance (ANOVA). When significant differences are found, Mann–Whitney U test is performed to analyse differences between fluid groups 1 and 3. * Including only subjects who received oxytocin. Maximum rate mU/minute: maximum rate of oxytocin infusion during first stage of labour. ** Differences between control group elective caesarean and fluid group 3: P < 0.001 (Mann–Whitney) (additional significant test). *** Difference between control group elective caesarean and fluid group 3: P = 0.14 (Mann–Whitney) (additional significant test). Table 2 Baseline values and total fluids (oral plus intravenous) given until birth Control group elective caesarean (n = 26) n(%) Total fluids given until birth Chi-square P value Fischer P value Fluid group 1 <1000 ml (n = 113) n(%) Fluid group 2 1000–2500 ml (n = 87) n(%) Fluid group 3 >2500 ml (n = 61) n(%) Initial groups* Vaginal <4 hours — 88 (78) 20 (23) 1 (2) Vaginal >4 hours, no epidural — 13 (12) 34 (39) 6 (10) Vaginal >4 hours with epidural — 0 (0) 30 (34) 40 (66) Emergency caesarean section — 12 (11) 3 (3) 14 (23) Elective caesarean section 26 (100) 0 (0) 0 (0) 0 (0) Parity Nullipara 12 (46) 23 (20) 44 (51) 46 (75) Multipara 14 (54) 90 (80) 43 (49) 15 (25) <0.001 <0.001 Complications Gestational diabetes 0 (0) 0 (0) 3 (3) 0 (0) Pre-eclampsia 0 (0) 3 (3) 0 (0) 5 (8) Hypertension 0 (0) 5 (4) 1 (1) 1 (2) Onset of labour None 26 (100) 5 (4) 0 (0) 0 (0) Induction — 8 (7) 7 (8) 8 (13) Spontaneous — 100 (88) 80 (92) 53 (87) 0.455** Delivery Vaginal, spontaneous — 98 (87) 71 (82) 37 (61) 0.001*** 0.001 Vaginal, instrumental*** — 3 (3) 13 (15) 10 (16) Caesarean section 26 (100) 12 (11) 3 (3) 14 (23) Indication for caesarean section Breech 17 (65) 6 (5) 0 (0) 0 (0) Previous uterine operation 7 (27) 0 (0) 0 (0) 0 (0) Fetal asphyxia — 1 (1) 1 (1) 1 (2) Failure to progress — 2 (2) 2 (2) 12 (20) Others 2 (8) 3 (3) 0 (0) 1 (2) Analgesia Epidural**** — 1 (1) 36 (41) 52 (85) Spinal — 1 (1) 3 (3) 0 (0) Oxytocin Yes — 14 (12) 57 (66) 59 (97) <0.001 <0.001 ≥5 units — 1 (1) 12 (14) 27 (44) 0.012***** Maternal p-Na ≤130 mmol/l 0 (0) 1 (1) 4 (5) 16 (26) <0.001 0.001 Maternal p-glucose >12 mmol/l 0 (0) 2 (2) 2 (2) 7 (11) 0.013 0.009 Infant Apgar <7 at 1 minute 0 (0) 4 (4) 5 (6) 5 (8) 0.219 Apgar <7 at 5 minutes 0 (0) 1 (1) 0 (0) 0 (0) Weight loss >10% 1 (4) 2 (2) 3 (3) 5 (8) 0.050 Hypoglycaemia 2 (8) 4 (4) 3 (3) 2 (3) 1.00 Respiratory problem 1 (4) 5 (4) 7 (8) 8 (13) 0.065 Group differences are compared with chi-square test. When significant differences are found or when cases are few, Fischer's exact test is performed to analyse differences between fluid groups 1 and 3. * Vaginal <4 hours or <4 hours: Vaginal delivery, labour of shorter or longer duration than 4 hours. Epidural: epidural or spinal analgesia. ** Induction versus spontaneous onset of labour. *** Vaginal spontaneous delivery versus instrumental vaginal delivery plus delivery by emergency caesarean section. **** One woman received combined spinal-epidural analgesia during labour. ***** Calculation only includes women who received oxytocin for augmentation of labour. Univariate and multivariate logistic regressions were performed to study the relationship between maternal hyponatraemia and parity, age, and body mass index as well as fluid volumes administered, epidural analgesia, and oxytocin during labour. All tests were two tailed, and a P value of <0.05 was considered statistically significant. The software used was Statistica release 7.1 (Statistica; StatSoft®, Tulsa, OK, USA)