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Comparison of Breast- and Formula-Fed Normal Newborns in Time to First Stool and Urine

Abstract

BACKGROUND: Although the time to an infant's first stool is used as a marker for a normal GI tract, there have been no studies to date which have evaluated the contribution of the first 24 hours' feeding type to time to first meconium stool, directly comparing breast feeding vs formula feeding.

OBJECTIVE: To compare breast- and formula-fed healthy infants ≥34 weeks gestation in time to first stool and urine.

STUDY DESIGN: A chart review of 1000 consecutive infants ≥34 weeks of gestational age admitted to the normal newborn nursery of Children's Hospital of Buffalo from June to October 2000. Infants (n=979) were grouped based on feeding type in the first 24 hours: breast-fed (n=211), formula-fed (n=540), and mixed feeding (n=228); n=21 excluded for Neonatal Intensive Care Unit admission. We initially compared the time to first stool and urine between the breast- and formula-fed groups and then examined multiple maternal and infant demographic and clinical factors for their effect on time to first stool using univariate and multivariate analyses.

RESULTS: Breast-fed infants were fed earlier and more frequently than formula-fed but there was no significant difference in time to first stool (7.6 vs 7.9 hours). Breast-fed infants were earlier in time to first urine (p=0.03) (7.3 vs 8.5 hours). In multiple regression analysis, gestational age was the only significant (p=0.000) factor in predicting time to first stool.

CONCLUSION: Type of feeding did not predict time to first stool but gestational age was important even in this near-term and term population of infants ≥34 weeks gestation.

Although the time to an infant's first stool is used as a marker for a normal GI tract, there have been no studies to date which have evaluated the contribution of the first 24 hours' feeding type to time to first meconium stool, directly comparing breast feeding vs formula feeding.

INTRODUCTION

Passage of an infant's first meconium stool is often used as a screen for a normal GI tract.1,2,3,4 Two studies in the American population state that 94%5 and 98.5%6 of term infants had their first meconium stool by 24 hours. These studies were performed some time ago when practices in the normal newborn nursery were likely quite different from current practices. These studies did not examine the type and schedule of feedings. In recent studies in Asia,2,3,4 the contribution of different physiologic factors (sex, gestational age, birth weight, type of delivery, Apgar scores) to time to first meconium stool was evaluated. There have been no studies that have evaluated the contribution of first 24 hours' feeding type to time to first meconium stool, directly comparing breastfeeding vs formula feeding. With an increase in the last two decades in breastfeeding in the US, it is important to document the effect, if any, on the time to first stool and first urine since the content, frequency and volume of feeds are different in breast- and formula-fed infants. It is possible that breast-fed infants may have a delay in time to first stool because there is a lower volume of feeding in the first 24 hours. Alternatively, breast-fed infants are often fed earlier and more frequently, and colostrum is considered by some to be a cathartic, so breast-fed infants may have a shorter time to first stool. Similarly, the type of feeding may affect time to first urine if breast-fed infants receive lower volume of feeding in the first 24 hours. The purpose of our study was to compare breast- and formula-fed healthy infants ≥34 weeks of gestational age in time to first stool and first urine.

METHODS

Most infants of 34 weeks gestation and older are cared for in the newborn nursery. We reviewed retrospectively all maternal and infant charts of 1000 consecutive infants ≥34 weeks gestational age admitted and discharged from the normal newborn nursery of Children's Hospital of Buffalo (June to October 2000). Infants were excluded if they were transferred to the Neonatal Intensive Care Unit (NICU) at any time, even if they were ultimately discharged from normal newborn nursery. Infants were defined in three groups based on feeding type during the first 24 hours of age: breast-fed, formula-fed, and mixed feeding. The infants in the breast-fed group were fed exclusively with breast milk for the first 24 hours (although 25% had dextrose water as their first feeding). The infants in the formula-fed group were fed exclusively with formula for the first 24 hours (although 98% had dextrose water as their first feeding). The infants in the mixed feeding group received both breast milk and formula during the first 24 hours (and 60% had dextrose water, while the remainder had breast milk as their first feeding).

Data collection. Maternal demographic variables collected included maternal age, race, gravity, parity, illicit drug use, intrapartum drug use (narcotics, antibiotics), mode of delivery, meconium stained amniotic fluid, and maternal diabetes. Infant variables collected included gender, Apgar scores, gestational age, birth weight, congenital malformations, age at first feed, number of feeds from 0 to 12 hours of age, 12 to 24 hours of age, 24 to 48 hours of age, age at discharge, age at first and second urine, age at first and second stool, description of first stool; meconium staining of the amniotic fluid was not considered a first stool.

Statistics. Data entry and analysis were performed using STATA (State College, TX). Variables of exclusively breast-fed and exclusively formula-fed infants were compared using t-test, linear regression and χ2 statistic as appropriate with p<0.05 as the significance level. A p-value of 0.000 reported by STATA is equivalent to a p-value <0.00005. All factors were examined by univariate analysis in the total population for the effect on time to first stool and first urine. Variables with a significance level ≤0.1 on univariate analysis were then examined in a multivariate analysis using stepwise linear regression to evaluate independent predictors of time to first stool or first urine, with a final significance level of p<0.05.

RESULTS

Charts for 1000 consecutive infants were reviewed. We excluded 21 infants who were admitted to the NICU, then returned to newborn nursery prior to discharge. Of the remaining 979 infants, 211 were in the breast-fed group, 540 were in the formula-fed group, and 228 were in the mixed feeding group. In our population there were 496 boys and 483 girls. In all, 723 infants (74%) were born vaginally, while 256 (26%) were born by Cesarean section. A total of 55 infants (6%) were born to diabetic mothers (47 with gestational diabetes). There was meconium staining of amniotic fluid in 148 (15%) cases. A total of 31 infants (3%) had congenital anomalies. There was one case each of Trisomy 21 and cleft palate, and six cases of hypospadias. Intrapartum nalbuphine hydrochloride (nubain) was received by 269 (27%) of mothers and 231 (24%) received antibiotics prior to delivery. In total, 97% of our infants had their first stool by 24 hours and 99.8% of them had stool by 36 hours. A total of 74% of our infants had passed urine by 12 hours of age and 97.9% of them by 24 hours. Figure 1a and b provides more detailed information.

Figure 1
figure1

These graphs depict the cumulative percentage of infants who had their first stool (a) or first urine (b) by a certain hour of age. For graph a, 99.7 infants had their first stool by 34 hours; there were an additional three infants who had their first stool later (36, 37, and 61 hours). For graph (b), 99.8 infants had their first urine by 34 hours; there were an additional two infants who had their first stool later (35 and 37 hours).

Comparison of Breast- and Formula-fed Infants

Important maternal and infant demographic and medical information is reported for the exclusively breast-fed and exclusively formula-fed group (first 24 hours) in Table 1. Stooling and urination results by type of feeding are reported in Table 2. Breast-fed infants were initially fed significantly earlier than formula-fed infants. They also had an average of one more feeding in the first 12 and second 12 hours of life compared to formula-fed infants. Breast-fed infants were discharged earlier. The breast-fed infants lost more weight than the formula-fed infants and were 96% of birthweight vs 98% of birthweight, respectively, at the time of the second weight. The average age (h±SD) of the baby at the time of the second weight was 23.8±7.2 and 23.3±7.9 for breast- and bottle-fed infants, respectively. There was no difference in time to first stool between two groups, and no difference in the percent of infants who stooled by 24 hours of age. Breast-fed infants were significantly earlier in time to first urine (p<0.05).

Table 1 Characteristics of breast-fed and formula-fed infants
Table 2 Comparison of breast-fed and formula-fed infants

Factors which Contribute to Time to First Stool

After the direct comparison of breast- and formula-fed infants was performed, all 979 infants were studied by univariate analysis for other factors that may contribute to time to first stool. p-Values for each variable are represented in Table 3. As the presence of meconium in the amniotic fluid would appear to be an indicator of gut peristalsis (as well as fetal stress), it is interesting to note that infants who passed meconium in utero passed stool significantly earlier than non-meconium-stained infants, as described in Table 3. We then performed multivariate analysis including all the variables with a significant level ≤0.1 in univariate analysis. In the multiple regression analysis, only gestational age remained significant (p=0.000) in predicting time to first stool.

Table 3 Univariate Analysis of Maternal and Infant Factors Predicting Time to First Stool

Factors which Contribute to Time to First Urine

A similar analysis was carried out for time to first urine examining all 979 infants by univariate analysis for factors that may contribute to time to first stool. p-Values for each variable are represented in Table 4. We then performed multivariate analysis including all the variables with a significant level ≤0.1 in univariate analysis. In the multiple regression analysis, three factors remained significant in time to first urine: gestational age (p=0.000), mode of delivery (p=0.000), and type of feeding in first 24 hours (p=0.004).

Table 4 Univariate Analysis of Maternal and Infant Factors Predicting Time to First Urine

Gestational age

Since the gestational age was highly significant in predicting time to first stool, we compared preterm and term infants in this population by type of feeding. In term infants, there was no difference in mean (SD) time to first stool which was 7.2 (6.4) hours in breast-fed infants (n=204) and 7.2 (6.9) hours in formula-fed infants (n=481). However, there was a statistical difference in time to first urine which was 7.2 (7.3) hours in breast-fed infants vs 8.7 (7.1) hours in formula-fed infants (p<0.05). In preterm infants, there was no difference in time to first urine which was 6.3 (5.8) hours in breast-fed infants (n=7) and 6.4 (6.7) hours in formula-fed infants (n=59). Although there was a trend in the breast-fed premature infants to have a small delay in time to first stool (16.1 (13) hours vs 12.5 (7.5) in formula-fed infants), since most premature infants were formula-fed the n is small and no conclusions can be drawn. The importance of gestational age in time to first stool in the total population is demonstrated by Figure 2.

Figure 2
figure2

This graph depicts the average time to first stool for each gestational age. Each age plotted represents all infants at that week of gestational age; for example, 36 weeks represents infants 36 0/7 weeks to 36 6/7 weeks inclusive. Standard deviations were omitted from the graph but ranged from 3.8 to 9.9 hours.

DISCUSSION

Time to first stool is not related to the age at first feeding or to type of feeding. In total, 33% of our infants had their first stool prior to any feeding, and despite breast-fed infants feeding twice as early as formula-fed infants, there was no difference in time to first stool between the two groups. Therefore, factors other than timing and type of first feed must be important in triggering gut motility in the healthy newborn infant.

Infants with meconium-stained fluid stooled significantly sooner than infants with no meconium at birth. There was no difference in time to first stool between vaginally and cesarean section born infants as has been reported previously.4

Motor activity is detectable in the small intestine as early as 26 weeks. These random contractions progress to more regular bursts of motor activity by 30 weeks. These are replaced by migrating motor complexes by 33 weeks gestation. Coordinated sucking and swallowing also required for independent feeding is not achieved until 34 to 35 weeks gestation. This gestational age seems to coincide with a significant increase in defecation rate, a surge in concentration of circulating intestinal regulatory polypeptides in response to feeding.7 Although sometimes clinicians associate stooling to feeding, in our study one-third of the infants had stooled prior to any feeding. Feeding may be a more important factor in subsequent stooling.

A total of 10% of our population was premature (34 to 36 6/7 weeks). Overall, premature infants stooled significantly later than full-term infants did (p=0.000), implying that the maturation of the GI tract (especially small gut and colon) continues until late in pregnancy. Only a small number of these premature infants were breast fed; therefore, no conclusions could be drawn regarding type of feeding. Kumar et al.8 have previously suggested that delayed passage of stools is both a function of illness severity and gestational immaturity. Our study shows that this is a valid conclusion even in otherwise healthy preterm infants cared for in the normal newborn nursery. Overall, breast-fed infants passed urine sooner than formula-fed infants; most of these infants were term. No conclusion can be drawn regarding type of feeding and passage of urine in the premature infants. We conclude that despite differences in timing and number of feedings, breast- and formula-fed term infants have no difference in time to first stool, but gestational age remains important even in this near-term and term healthy population (≥34 weeks).

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Correspondence to Rita M Ryan MD.

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Metaj, M., Laroia, N., Lawrence, R. et al. Comparison of Breast- and Formula-Fed Normal Newborns in Time to First Stool and Urine. J Perinatol 23, 624–628 (2003). https://doi.org/10.1038/sj.jp.7210997

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