The hypotheses were, for the most part, supported and there were additional notable findings. There were positive relationships among the variables: maternal IV fluids, neonatal output, and newborn weight loss, although the relationship between maternal IV fluids was not evident until 60 hours postpartum. We are not certain if this result is because it was the nadir, therefore the greatest effect size, or because it took time for fluid and weight to settle.
The neonatal output results were the most intriguing and unexpected. The hypotheses partially held. The positive correlation between maternal IV fluids and neonatal output were limited in both type and timing. Specifically, only the two-hour pre-birth maternal fluid amount was statistically significant for the first 24 hours. The relationship between neonatal output and newborn weight loss indicates each of the three days has a different correlation, as the correlation moves from positive to no relationship to a negative relationship. Only the first 24 hours of neonatal output and newborn weight loss were positively correlated. We interpret these results to suggest the newborn experiences diuresis, but only in the first 24 hours. Our interpretation is substantiated by the finding maternal IV fluid is only related to neonatal output on the first day.
There is little information in the literature about the relationship between IV fluids and neonatal weight loss in the first week postpartum. In a recent study, Lamp and Macke found no relationship between intrapartum maternal fluids and neonatal weight loss . There are three main differences between their study and this study: (a) data were collected for 48 hours versus 72 hours and 14 days, respectively; (b) the amounts of fluids from admission to birth were quite different (2522.5 to 5013.75 mls. versus 0 to 7200 mls. for our study); (c) all fluids in Lamp and Macke's study were measured from admission to birth (i.e., they did not collect data about IV fluids specifically within two hours of birth) . With our study, correlations between fluids and weight loss appear at 60 hours and 72 hours. The wider range of fluid amounts and longer data collection period may account for positive findings in this study. More recently, Chantry et al. found an association between excess weight loss and maternal intrapartum fluid balance . The neonates in their study were weighed at three days, so it appears they captured the timing when the correlation appears.
Neonatal output in our study was significantly related to weight change. These findings corroborate results from three studies. Lamp and Macke  observed that the number of wet diapers was predictive of weight loss and Mulder et al.  reported that total voids were a predictor of excessive weight loss (> 7%). Chantry et al.  compared number of neonatal voids in the first four hours with categories of maternal fluids and determined a positive relationship. Their methods differ from our design, as we report the days separately and output as total weight of diapers.
It appears lactogenesis II affects output on Day 3 when the relationship between output and weight loss became negative. A negative relationship indicates one of two possibilities: the neonate who has increased output has increased weight gain or the newborn with decreased output is losing weight.
Overall, this convenience sample is comparable to Ontario, the provincial population of origin. In hospital, 27% of study participants supplemented their babies, and the provincial rate of hospital supplementation is 28% . In 2007-08, 43% of women who gave birth in Ontario were first-time mothers . Likewise, 42.2% of the study participants are primiparous. The study participants also have similar rates of caesarean sections and epidural use when compared to Ontario provincial rates (25% versus 28.4% and 64% versus 62%, respectively) .
Comparisons of neonatal weight loss are difficult because some authors count birth as Day 1 and others treat the first post-birth day as Day 1 (i.e., birth = Day 0), and it is often not clear which was used. The Day 1 weight for this study is birth weight (0-24 hours) and Day 2 was the weight taken at 24 hrs. Day 3 is 48 to 72 hours. Newborns in the study seem to experience weight losses comparable to reports in the literature [8, 9]. The percentage of weight lost peaked at 60 hours (i.e., 3rd day) with a mean 6.57% loss (SD 2.51, range 1.83 to 13.06, n = 96).
In a systematic review of early weight loss patterns, 11 studies demonstrated a mean loss of about 6% with a standard deviation of about 2 (median was also about 6%); the nadir (point of lowest weight) was the third day . Martens and Romphf determined exclusively breastfed babies lost a mean of 5.49% and supplemented breastfed babies lost an average of 5.52% in hospital . With weight measures only in hospital, the nadir of weight loss may not have been reached . MacDonald et al. completed a prospective study and found breastfed babies lost a median of 6.6% of birth weight (95 centile = 11.8%) within a median time of 2.7 days . Crossland et al. developed a centile chart (n = 111) capturing weight loss in the first two weeks postpartum . They also showed that breastfed neonates average a loss of 6.4% of birth weight with the majority reaching the point of maximum weight loss on the third day .
Strengths and limitations
Strengths include: (a) data collected prospectively; we collected data about the three key variables which were not available in the medical records; (b) participants used the same scale to ensure internal consistency; (c) weight measurements every 12 hours for the first 72 hours permitted detection of the nadir of weight loss; and (d) measurements post discharge added valuable information.
The limitations are attributable to data collection issues. Although our main concern was fluid shifts, we could not collect voids separate from stools. The late decision to collect additional data about fluids in the final two hours before birth meant a small sample for this analysis. The first weeks following birth are an intense time for parents, and data were frequently missed. We did not attempt to input missing data regarding weights and fluids, because we could not be certain of the direction (e.g., should weight go up, down, or stay the same). Babies managed to void and stool when their diapers were off. Parents were asked to document missed output, and we estimated to account for the loss.
Reconsider birth weight as baseline
Clinicians debate the limits of acceptable neonatal weight loss in the first days. Current clinical practice guidelines recommend interventions, including extra assessments or supplementation with formula, when weight loss exceeds 7% [4–7]. Some authors identify a loss of ≥10% as a sign of breastfeeding inadequacy [35, 37]. Weight loss, in this case, is the percentage of weight lost from the first weight measured (i.e., birth weight). Birth weight as a baseline against which to assess weight loss is a universal choice, but it lacks sufficient empirical evidence.
It appears that the neonates in our study experienced diuresis in the first 24 hours as evidenced by the positive correlation of the first 24-hour output to both the maternal two-hour prebirth IV fluids and the weight loss at 24 hours. With birth weight as baseline, newborns may have an artificially high reference point for weight loss. Resetting baseline to a point after the diuresis has occurred (i.e., the newborn's weight has stabilized) would be a better gauge for assessment. This premise is supported by van Dommelen et al. who determined a 10% rule of thumb produces false positives (i.e., not a good indicator to detect hypernatremic dehydration) .
We ran frequency analyses of percentage weight loss to contrast two possible baselines (see Additional file 1). With a 24-hour baseline, 2.3% lost between 7 and 10%, and none lost in excess of 10%. In contrast, one third of the newborns lost between 7 and 10% of their birth weight and 7.3% lost more than 10%. With a 24-hour baseline, 90% regained their baseline weight by Day 9 (n = 88). Whereas, 64% regained birth weight by Day 9 (n = 97). By Day 14, 12% had not regained their birth weight (n = 102), but 99% had regained their 24 hour weight measurement (n = 93).
Intuitively, clinicians and parents want to see the neonate return to birth weight. If it is an inflated measurement, then the expectations for a return to birth weight in the first days are questionable. In the dialysis literature, the term "dry weight" is used to describe a patient's weight without additional fluid, and this measurement is the patient's post-dialysis goal weight . The neonates in this study appear to reach their dry weight around 24 hours, although the timing of this iatrogenic weight loss might depend on birth practices.
Further research is needed to understand the effects of iatrogenic factors such as maternal fluids during parturition. Evidence is needed to confirm why and how timing of maternal fluid is a factor; especially IV fluids administered in the last hours before birth. Tracking maternal output might provide insight into the phenomena. Researchers should note the strong correlation between maternal IV fluids and epidurals and birth types, as these latter two factors may be not be the source of weight loss.
A study with diaper weights grouped by 12 hours might determine the peak of diuresis. It is possible that diuresis continues to 36 hours, but we could not analyze output and weight loss at 36 hours because the diapers were weighed in 24 hour segments.
The findings about delayed lactogenesis II should be investigated further. The positive correlation between maternal fluids and onset of lactogenesis II was a serendipitous finding. The finding that delayed lactogenesis II was related to newborn weight loss was not unexpected, but that the effect was not evident in the regression model at 60 hours, only showing up at 72 hours, was unanticipated. In the literature, the frequency of delayed onset of lactogenesis II ranges widely from 22% to 44% [35, 44]. The modifiable factors that affect onset of lactogenesis II and the effects of delayed onset need to be better understood.
Research is needed to correlate morbidity and mortality to newborn weight loss. Using a percentage (i.e., 10%) as a red flag does not appear to have a connection to morbidity. The criterion seems reversed. For example, Manganaro et al. divided their sample based on 10% then completed blood tests in the > 10% loss group, instead of determining the relationship between morbidity and percentage of weight loss .
Researchers who plan studies about neonatal weight loss need to be careful to use hours and not days for their protocols. For example, diapers for Day 1 can be interpreted many ways (e.g., participants could restart the count the following morning). Stipulating from birth to 12 or 24 hours is clearer. Additionally, conditions for daily weight measurements could be specified for consistency (e.g., weigh before feeds).