The findings suggest that obstetric and clinical outcomes were better in nulliparous women who received a total fluid volume of ≥ 300 mL/h during labor than in those who received ≤ 300 mL/h. The lower values for duration of the first stage of labor, total duration of labor, duration of oxytocin, and duration of epidural anesthesia were statistically significant. Similarly, the incidence of prolonged labor (> 12 hours) was lower. Clinically relevant differences included a lower incidence of intrapartum fever and cesarean delivery. No woman spent more than 24 hours with ruptured membranes.
As for the newborns, more favorable, clinically relevant outcomes were recorded for those whose mothers received a total volume ≥ 300 mL/h. The most noteworthy findings were lower weight loss at 24 and 48 hours, lower incidence of weight loss > 7% at 48 hours, and greater incidence of breastfeeding. Furthermore, three of four newborns admitted to the neonatal ICU were the children of mothers who received a lower quantity of fluids (< 300 mL/h).
The findings associated with the duration of the first and second stages of labor and, total duration of labor are relevant, considering that the participants in the present study were recruited with a median cervical dilation of 2 cm (latent phase) compared with 4 cm (active phase) in the studies of Fong et al. [37] and Zhang et al. [38]. The median duration of the first stage of the better hydrated women (≥ 300 mL/h) was 420 minutes, which is superior to the results reported by Fong et al. [37] (399.5 minutes for the group receiving 2.5% dextrose in normal saline [250 mL/h]), considering baseline dilation status. This results are consistent with those of Harper et al. [39].
As for the volume administered during labor, was observed a statistically significant association between total volume administered ≥ 300 mL/h and reduced median duration of the first stage (420 min), total duration of labor (526 min), duration of analgesia (403 min), and duration of oxytocin (466 min). In this sense, previous studies found an association between the increase in total volume administered and reduced duration of labor [21, 22]. One clinical trial [21] found that intravenous hydration significantly reduced the duration of active labor, the frequency of prolonged labor, and the administration of oxytocin in nulliparous women. A meta-analysis [22] showed that intravenous administration of fluids reduced the duration of labor.
The number of cesarean deliveries was greater in the lesser hydrated group (< 300 mL/h), with an incidence of 18.7% compared with 14.3% in the better hydrated group (≥ 300 mL/h). The clinical trial (with 195 nulliparous women) by Garite et al. [1] (195 nulliparous women) analyzed the association between increased fluids during labor and progress of labor: the number of cesarean deliveries decreased when women received intravenous solutions at 250 mL/h compared with 125 mL/h1. Other studies were unable to demonstrate a similar effect [18, 22, 37].
The incidence of intrapartum fever in the better hydrated group (≥ 300 mL/h) was 5.5%, which is more favorable than the data reported by Towers et al. [40] (7%) and Braun et al. [41] (9%) and like the percentage reported by Alexander et al. [42] (5%).
The women in the present study had a median choluria value (Armstrong) of 4 in the puerperium, with median osmolality in urine of < 300 mOsm/kg. A clinical trial performed in 201514 and a descriptive study in 2013 [43] found that pregnant women with choluria ≥ 4 probably had a urine osmolality of ≥ 500 mOsm/kg. Therefore, the authors considered that increased fluid consumption could improve general hydration. The findings of the current study enable us to state that the women were generally well hydrated during labor, although optimal needs should be addressed in greater depth.
The percentage of weight loss at 24 and 48 hours and the incidence of weight loss > 7% at 48 hours was lower in the children of the better hydrated mothers (≥ 300 mL/h). In this sense, some studies reported significant positive associations between the intrapartum fluid dose and weight loss [23, 24], whereas others did not report significant associations [25, 28].
With respect to cord blood biochemical parameters, the median (IQR) concentrations were as follows: sodium, 135 (134–137) mmol/L; potassium 5.4 (4.9–6.2) mmol/L; and glucose, 71 (61–77) mg/dL. These values are consistent with international recommendations [44, 45], and the ranges reported are like those found by Kratz et al. [46] and slightly higher than those reported by Bequer et al. [47].
Clinical implications
Hydration regimens improved during labor, with an increase in the volumes administered and a reduction in glucose solutions compared with crystalloid solutions. This decision was taken with the aim of reducing the risk of hypoglycemia in the neonate. Another modification, which was made at the initiation of the study, concerned antibiotic therapy. Antibiotics were administered initially to women whose temperature was ≥ 38ºC, although we found that temperature decreased in women who were better hydrated. Thus, in the case of fever, the first step was to increase the volume administered; antibiotics were only administered when the woman’s temperature did not decrease.
Research implications
To consolidate the results of the cohort study, we proposed a randomized controlled clinical trial that would enable us to continue the research line initiated. The study protocol was published in 2020 with the title Efficacy of “optimal hydration” during labor: HYDRATA study protocol for a randomized clinical trial [48]. The trial was registered at www.clinicaltrials.gov, with number XXXXXXXXXX.
Strengths and limitations
The study design is considered a strong point. The prospective cohort study allows obtaining relevant data on the association between hydration and obstetric and neonatal outcomes. The methodological rigor allowed the control of possible biases.
One limitation was the lack of some data, because they were not collected in routine practice, as was the case with laboratory values. Further analyzes revealed that women whose labor was induced may require a higher volume of fluids per hour to achieve optimal hydration. This hypothesis will be considered in the ongoing clinical trial.