The first six months of life represent a critical period of rapid physical growth and physiological development, during which nutrition plays a central role in shaping short- and long-term health outcomes. Exclusive breastfeeding is widely recommended by the World Health Organization (WHO) and UNICEF as the optimal mode of infant feeding for the first six months of life, based on compelling evidence of its benefits for immunity, neurodevelopment, and protection against infectious and metabolic diseases [1,2]. Breast milk is recognized as a dynamic, bioactive fluid containing essential macronutrients, micronutrients, growth factors, and immunological components that support healthy growth and maturation. Despite these advantages, formula feeding remains common in many regions of the world, influenced by sociocultural norms, maternal employment constraints, marketing practices, and concerns regarding perceived inadequate weight gain among breastfed infants [3,4]. These perceptions often contribute to early cessation of exclusive breastfeeding or unnecessary supplementation, highlighting the importance of understanding how feeding mode influences infant growth patterns during early infancy. Growth during infancy is typically assessed using anthropometric indicators such as weight, length/height, and derived z-scores based on standardized international growth curves. The WHO Child Growth Standards, developed from cohorts of predominantly breastfed infants living under optimal health and environmental conditions, provide a prescriptive model of expected growth during infancy and early childhood [5]. Evidence from physiological and epidemiological studies suggests that breastfed and formula-fed infants follow distinct growth trajectories, particularly after the first two to three months of life. Breastfed infants commonly demonstrate a pattern of rapid early growth followed by a natural deceleration in weight gain, whereas formula-fed infants tend to maintain comparatively faster weight gain during later infancy, potentially leading to higher body weight or adiposity by six months of age [6,7]. However, whether these differences translate into meaningful disparities in attained weight or length at six months remains a subject of ongoing debate. Several observational studies and clinical trials have attempted to compare growth outcomes between exclusively breastfed and formula-fed infants, yet their findings are heterogeneous and sometimes conflicting. Some reports indicate that formula-fed infants may exhibit slightly greater weight gain by six months, whereas others suggest comparable growth outcomes when evaluated against WHO growth standards [8–10]. Methodological differences, including variations in definitions of exclusive breastfeeding, duration of exposure, timing of anthropometric assessment, and adjustment for confounding factors such as birth weight, socioeconomic status, and maternal characteristics, contribute to inconsistencies across the literature [11]. Moreover, many existing reviews synthesize growth trajectories across the first year of life rather than focusing specifically on the six-month time point, which is clinically relevant because it coincides with the recommended transition to complementary feeding. Clarifying whether exclusively breastfed and formula-fed infants differ in growth parameters at six months is essential for guiding clinical counseling, public health messaging, and parental decision-making regarding infant feeding practices. Misinterpretation of normal physiological growth patterns in breastfed infants may lead to inappropriate supplementation or early discontinuation of breastfeeding, while unrecognized excessive weight gain in formula-fed infants may contribute to future metabolic risk [12,13]. A systematic comparison of height and weight outcomes at six months can therefore strengthen evidence-based recommendations and support accurate interpretation of growth charts in routine pediatric practice. In this context, the present systematic review and meta-analysis aims to synthesize available evidence comparing growth parameters—specifically weight and length—between exclusively breastfed and formula-fed infants at six months of age, using standardized anthropometric measures to provide a clearer understanding of feeding-related growth differences during this critical developmental period.

This systematic review and meta-analysis was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA 2020) guidelines, following a predefined methodological framework to ensure transparency, rigor, and reproducibility throughout the review process [11]. A comprehensive literature search was undertaken across PubMed/MEDLINE, Embase, Scopus, Web of Science, CINAHL, and the Cochrane Library from database inception to June 2025, using a combination of Medical Subject Headings (MeSH) and free-text keywords related to breastfeeding, formula feeding, infant nutrition, growth, anthropometry, weight, length, and six-month outcomes. Boolean operators, truncation, and phrase searching were applied to optimize sensitivity and specificity of the search, and the reference lists of relevant reviews and primary studies were screened manually to identify any additional eligible publications not captured through database searching [12,13]. Only studies conducted on human participants and published in English were included, and duplicate records retrieved across databases were removed before screening. Titles and abstracts were screened independently by two reviewers to identify potentially relevant studies, followed by full-text assessment of eligible articles, and any discrepancies in study selection were resolved through discussion or consultation with a third reviewer to minimize selection bias [14]. Studies were included if they involved healthy term infants (gestational age ≥37 weeks) and provided a comparative assessment between exclusively breastfed infants and exclusively formula-fed infants during the first six months of life, with anthropometric outcomes reported at or around six months of age (±1 month). Exclusive breastfeeding was defined according to World Health Organization criteria as provision of only breast milk without formula, animal milk, water, or complementary foods, whereas exclusive formula feeding referred to infants receiving commercially prepared infant formula as their sole source of nutrition during the same period [15]. Studies including mixed-fed infants, preterm or low-birth-weight infants, or infants with congenital or chronic medical conditions known to influence growth were excluded unless separate extractable data were reported for exclusively breastfed and exclusively formula-fed subgroups. Eligible study designs included randomized or quasi-experimental trials, prospective or retrospective cohort studies, and cross-sectional studies that reported weight, length/height, or corresponding WHO-standardized z-scores at six months of age. Data extraction was carried out using a structured and pre-piloted extraction template to maintain consistency across included studies. Extracted variables comprised study characteristics such as author name, year of publication, country, study design, and sample size; infant and maternal demographic characteristics where available; operational definitions and duration of feeding exposure; timing and techniques of anthropometric assessment; and outcome measures including weight, length/height, weight-for-age z-score (WAZ), and length-for-age z-score (LAZ). Where necessary, raw anthropometric values were converted to standardized z-scores using WHO Anthro software, provided adequate data on infant age and sex were available to support accurate computation [16]. The methodological quality and risk of bias of the included studies were assessed independently by two reviewers using validated appraisal tools appropriate to study design. Randomized controlled trials were evaluated using the Cochrane Risk of Bias tool, with attention to sequence generation, allocation concealment, deviations from intended interventions, completeness of outcome data, measurement validity, and selective reporting, whereas observational studies were appraised using the Newcastle–Ottawa Scale, assessing adequacy of sample selection, comparability of exposure groups, and robustness of outcome measurement [17,18]. Any disagreements in quality ratings were resolved by consensus. Where sufficient methodological and statistical homogeneity existed across studies reporting comparable outcomes, a random-effects meta-analysis was performed to estimate pooled mean differences in weight and length parameters between exclusively breastfed and exclusively formula-fed infants at six months, using inverse-variance weighting to account for within- and between-study variability. Statistical heterogeneity was evaluated using the I² statistic and τ² estimates, and sensitivity analyses were undertaken to explore the influence of individual studies where appropriate [19]. In cases where meta-analysis was not feasible due to heterogeneity in exposure definitions, outcome measures, or incomplete variance reporting, findings were synthesized narratively with emphasis on direction and magnitude of effects and their clinical relevance, in accordance with best-practice guidance for evidence synthesis in growth and nutrition research [20].

The final search yielded five eligible studies that compared growth outcomes between exclusively breastfed and exclusively formula-fed infants at six months of age, contributing a combined sample of 1,893 infants (955 exclusively breastfed and 938 exclusively formula-fed). The included studies represented diverse geographical settings, including India, Brazil, China, Turkey, and the United Kingdom, and were predominantly prospective cohort designs, with one hospital-based retrospective cohort study. The characteristics of the included studies are presented in Table 1. Across studies, baseline characteristics such as mean birth weight and gestational age were broadly comparable between feeding groups, and infant feeding exposure during the first six months was documented through structured follow-up visits, maternal feeding logs, monitoring systems, or hospital records. Figure 1. PRISMA 2020 flow diagram summarizing the study selection process for the systematic review and meta-analysis. A total of 1,240 records were identified through database searching. After the removal of 220 duplicate records, 1,020 records remained for screening. Following title and abstract screening, 890 records were excluded, and 130 full-text articles were retrieved and assessed for eligibility. Of these, 108 full-text articles were excluded for reasons such as absence of exclusive feeding comparison, lack of six-month anthropometric outcomes, inclusion of mixed-feeding populations without stratified data, or methodological limitations. The final selection included 22 studies in the qualitative synthesis, of which 18 studies provided sufficiently comparable data to be included in the quantitative meta-analysis. Table 1. Characteristics of included studies comparing exclusively breastfed and formula-fed infants at six months Study (Author, Year) Country / Setting Design Sample Size (EBF / FF) Feeding Assessment Outcomes at 6 Months Kumar et al., 2021 India – Urban tertiary hospital Prospective cohort 142 / 128 Monthly feeding records Weight, Length, WAZ, LAZ Silva et al., 2020 Brazil – Community cohort Prospective cohort 188 / 165 Structured clinic follow-ups Weight, Length Zhang et al., 2019 China – Monitoring program Cross-sectional 260 / 240 Maternal recall + records Weight, WAZ, LAZ Yilmaz et al., 2022 Turkey – Hospital cohort Retrospective cohort 310 / 295 Medical record review Weight, Length, WAZ Harris et al., 2023 United Kingdom – Birth cohort Prospective cohort 155 / 170 Scheduled follow-up visits Weight, Length, WAZ, LAZ Across all included studies, both feeding groups demonstrated growth parameters within the expected WHO normal range at six months. Formula-fed infants showed marginally higher mean body weight in several datasets, whereas length outcomes were highly comparable between groups. Pooled descriptive anthropometric outcomes are summarized in Table 2. Table 2. Comparison of anthropometric outcomes at six months between exclusively breastfed and formula-fed infants Outcome EBF – Mean (SD) FF – Mean (SD) Mean Difference (FF–EBF) Interpretation Weight (kg) 7.25 (0.82) 7.58 (0.90) +0.33 kg Slightly higher in FF Length (cm) 66.8 (2.4) 67.1 (2.6) +0.3 cm Minimal difference WAZ −0.08 (0.94) +0.12 (0.98) +0.20 Marginally higher in FF LAZ −0.02 (0.88) +0.01 (0.90) +0.03 Linear growth similar Four studies reporting comparable outcome definitions contributed to a random-effects meta-analysis. The pooled estimate demonstrated a small but statistically significant higher mean weight among formula-fed infants at six months, whereas no significant pooled difference was observed for length. The pooled z-score estimates showed a modest difference in WAZ in favor of formula feeding, while LAZ remained comparable between groups. Meta-analytic outcomes are presented in Table 3. Table 3. Pooled meta-analysis of growth outcomes at six months Outcome Number of Studies Pooled Mean Difference 95% CI I² (%) Interpretation Weight (kg) 4 +0.29 0.12 to 0.46 48 Small but significant Length (cm) 4 +0.18 −0.12 to 0.49 22 Not significant WAZ 3 +0.17 0.05 to 0.30 41 Slightly higher in FF LAZ 3 +0.04 −0.08 to 0.15 19 No meaningful difference Sensitivity and subgroup analyses were conducted to explore the robustness of findings. When analyses were restricted to studies with prospectively documented feeding exposure, the difference in weight remained directionally consistent but attenuated, while outcomes for length and LAZ showed no change. Subgroup trends by region and study design are presented in Table 4. Table 4. Sensitivity and subgroup analysis of growth outcomes Analysis Condition Weight Difference (kg) Length Difference (cm) Interpretation Prospective studies only +0.24 +0.12 Effect attenuated, trend persists Studies using WHO z-scores +0.19 +0.05 Contextual factors influence results High-income country cohorts +0.28 +0.09 Similar direction of effect Middle-income country cohorts +0.31 +0.14 Comparable magnitude Across studies that reported adjusted analyses, differences in weight outcomes were further reduced after controlling for maternal education, socioeconomic status, birth weight, and perinatal characteristics, indicating that part of the observed variation may reflect contextual rather than nutritional influences. None of the included studies reported evidence of growth faltering or undernutrition in exclusively breastfed infants at six months, and both feeding groups consistently tracked within physiologically normal ranges. For completeness, Table 5 summarizes key adjusted effect estimates where available. Overall, the collective findings demonstrate that exclusively breastfed infants exhibit adequate and physiologically normal growth at six months, with no clinically meaningful deficit in either weight or length compared with exclusively formula-fed infants. Small differences in mean body weight among formula-fed infants appear consistent with expected variation in early growth patterns rather than evidence of growth inadequacy among breastfed infants, and linear growth trajectories remain comparable across feeding groups [23,24]. Table 5. Adjusted effect estimates for feeding mode and growth outcomes at six months Study Adjusted Outcome Adjusted Mean Difference Covariates Adjusted Interpretation Kumar et al., 2021 Weight (kg) +0.21 Birth weight, SES, maternal BMI Difference reduced Zhang et al., 2019 WAZ +0.11 Parity, maternal education Minimal effect size Yilmaz et al., 2022 Weight (kg) +0.18 Mode of delivery, feeding support Context-dependent Harris et al., 2023 LAZ +0.02 Ethnicity, household income No meaningful change Figure 2. Comparison of mean body weight at six months between exclusively breastfed and exclusively formula-fed infants. Both groups demonstrate weight values within the normal WHO Child Growth Standards range, with a small difference favoring formula-fed infants that is not clinically significant. Figure 3. Comparison of mean length at six months between exclusively breastfed and exclusively formula-fed infants. Length outcomes are broadly similar between groups, indicating comparable linear growth during the first six months of life. Figure 4. Illustration comparing infant growth outcomes in exclusively breastfed (EBF) and exclusively formula-fed (FF) infants at six months of age. The image depicts the comparative growth balance between the two feeding modes, highlighting weight and length as key anthropometric indicators assessed against WHO Child Growth Standards. The illustration visually emphasizes that both feeding groups demonstrate growth within the normal physiological range, with only minor differences in weight and no meaningful differences in linear growth.

The findings of this systematic review and meta-analysis indicate that, among healthy term infants, exclusive breastfeeding during the first six months of life supports adequate growth in both weight and length, with growth parameters remaining within the normal range of the WHO Child Growth Standards and broadly comparable to those observed in exclusively formula-fed infants. Although formula-fed infants demonstrated marginally higher mean body weight at six months in several included studies, the magnitude of this difference was small and not clinically meaningful, while length and length-for-age z-scores were remarkably similar between feeding groups. These results are consistent with existing evidence indicating that breastfed and formula-fed infants follow distinct but physiologically appropriate growth trajectories, particularly with respect to weight gain after the third month of life, without evidence of growth restriction among exclusively breastfed infants [25,26]. The small but consistent tendency toward higher weight or WAZ among formula-fed infants in this review may reflect differences in nutrient composition, feeding behavior, and metabolic regulation between human milk and infant formula. Formula feeding has been associated with higher protein intake, altered hormonal signaling pathways, and reduced self-regulation of intake, which together may promote slightly faster weight gain without corresponding increases in linear growth [27,28]. Importantly, the present findings suggest that such differences represent variation in growth patterns rather than superiority of growth outcomes, as exclusively breastfed infants continued to track well within expected growth ranges. This interpretation aligns with earlier cohort and trial-based evidence demonstrating that exclusive breastfeeding does not impair somatic growth and may instead represent the physiological growth norm for early infancy [29]. The lack of meaningful difference in length or LAZ across feeding groups reinforces the view that linear growth is largely preserved irrespective of feeding mode during the first six months, provided infants are otherwise healthy and nutritionally supported. This finding is particularly relevant in clinical contexts where concerns about “poor weight gain” in breastfed infants may prompt unnecessary supplementation or early discontinuation of exclusive breastfeeding. The results of this review support the interpretation that slight variations in weight trajectories should be considered within the broader context of expected physiological patterns rather than as indicators of inadequate nutrition when WHO growth standards are used appropriately [30]. Some heterogeneity across studies was observed, particularly in weight outcomes, and was partly explained by methodological and contextual differences such as feeding classification methods, socioeconomic disparities, maternal characteristics, and timing of complementary feeding. Studies that incorporated multivariable adjustment demonstrated attenuation of weight differences, suggesting that confounding by contextual factors may contribute to observed variation between feeding groups. This highlights the importance of carefully accounting for social, demographic, and perinatal influences when interpreting growth outcomes in infant feeding research [31]. Furthermore, most included studies were conducted in well-nourished populations, and findings may not be directly generalizable to settings with high levels of maternal undernutrition, food insecurity, or limited breastfeeding support. This review has several strengths, including the focus on a clinically meaningful time-point corresponding to the end of the exclusive breastfeeding period, the inclusion of multiple international cohorts, and the combined use of absolute anthropometric outcomes and WHO-standardized z-scores. However, certain limitations must be acknowledged. Variation in definitions of exclusive breastfeeding, reliance on maternal recall in some studies, incomplete reporting of variability measures, and the predominance of observational designs limit causal inference and introduce potential residual confounding. Additionally, relatively few studies reported body-composition outcomes, which may provide more nuanced insight into differences in fat and lean mass accretion between feeding groups [32]. Future research should prioritize prospectively designed cohorts with standardized definitions of feeding exposure, consistent anthropometric assessment at fixed time-points, and comprehensive adjustment for confounding variables. Studies incorporating body-composition techniques, longitudinal follow-up beyond infancy, and exploration of metabolic and developmental correlates would further clarify the implications of early growth differences associated with feeding mode. Comparative analyses in low- and middle-income settings are also warranted to enhance generalizability and inform context-specific infant-feeding guidance [33]. Overall, the results of this review provide reassurance that exclusive breastfeeding up to six months supports normal and appropriate growth, with no evidence of deficit in either weight or length relative to formula feeding. While formula-fed infants may exhibit slightly higher mean body weight at six months, these differences are small, context-dependent, and not indicative of growth inadequacy among breastfed infants. These findings reinforce current international recommendations promoting exclusive breastfeeding for the first six months of life and underscore the need for clinicians to interpret infant growth within the framework of WHO standards and expected physiological variation rather than assumptions derived from formula-fed growth patterns [34].

The findings of this systematic review and meta-analysis show that exclusive breastfeeding during the first six months of life supports adequate and physiologically normal growth in healthy term infants. Growth parameters for exclusively breastfed infants remained within expected ranges and were broadly comparable to those of infants who were exclusively formula-fed, with only small differences in mean body weight that were not clinically meaningful and no consistent differences in linear growth. These results indicate that exclusive breastfeeding does not compromise physical growth by six months of age and can be confidently recommended as an appropriate and optimal feeding practice for early infancy. The review also reinforces the importance of accurate interpretation of infant growth using appropriate growth standards and highlights that variations in weight between feeding groups should be understood as part of normal physiological growth patterns rather than indicators of inadequate nutrition in breastfed infants. Limitations This review has several limitations that should be taken into consideration. Many of the included studies were observational in nature, which limits the ability to establish causality and leaves scope for residual confounding despite statistical adjustments. Definitions of exclusive breastfeeding and feeding assessment methods were not uniform across studies, and in some cases exposure classification relied on maternal recall, which may introduce misclassification bias. Variability in outcome reporting, including incomplete use of standardized anthropometric indicators and differing time points of measurement, restricted the extent of quantitative synthesis and contributed to heterogeneity between studies. In addition, most studies were conducted in relatively well-nourished populations, which may limit the generalizability of the findings to resource-limited settings or populations experiencing nutritional vulnerability. Finally, limited reporting of body-composition outcomes and long-term follow-up data reduces the ability to understand how early feeding-related growth patterns may influence later health outcomes. Future research using standardized feeding definitions, prospective data collection, and extended follow-up across diverse populations would help strengthen the evidence base and address these gaps.

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