What is the question?

• The question is: What is the relationship between dietary patterns before and during pregnancy and gestational age- and sex-specific birth weight?

What is the answer to the question?

• No conclusion can be drawn on the association between dietary patterns during pregnancy and birth weight outcomes. Although research is available, the ability to draw a conclusion is restricted by 
  • inconsistency in study findings,
  • inadequate adjustment of birth weight for gestational age and sex, and
  • variation in study design, dietary assessment methodology, and adjustment of key confounding factors. 
• Insufficient evidence exists to estimate the association between dietary patterns before pregnancy and birth weight outcomes. There are not enough studies available to answer this question.

Why was this question asked? 

• This important public health question was identified and prioritized as part of the U.S. Department of Agriculture and Department of Health and Human Services Pregnancy and Birth to 24 Months Project.

How was this question answered? 

• A team of staff from the Nutrition Evidence Systematic Review conducted a systematic review in collaboration with a group of experts called a Technical Expert Collaborative

What is the population of interest? 

• Women who are pregnant or able to become pregnant, ages 15-44 years.

What evidence was found? 

• This review includes 21 studies.
• These studies assessed the relationship between dietary patterns before and during pregnancy and birth weight outcomes. 
• Only one-third of studies used both gestational age- and sex -specific cut-off values when defining birth weight outcomes. 
• Findings were highly inconsistent across this body of evidence. 
• Roughly half of studies found no association between dietary patterns and birth weight outcomes. 
• Those that did find an association were inconsistent in the direction of effect and dietary patterns measured.
• There are limitations in the evidence as follows: inconsistencies across different studies in terms of study design, how dietary patterns were assessed and adjustment of key confounders. 
• Additional research is needed to assess the relationship between dietary patterns before and during pregnancy and gestational age- and sex-specific birth weight.

How up-to-date is this systematic review? 

• This review includes literature from 01/1980 to 01/2017.

Background

• The goal of this systematic review was to examine the following question: What is the relationship between dietary patterns before and during pregnancy and gestational age- and sex-specific birth weight?
• Systematic reviews were conducted as part of the U.S. Department of Agriculture and Department of Health and Human Services Pregnancy and Birth to 24 Months Project. 
• Dietary patterns were defined as the quantities, proportions, variety or combinations of different foods and beverages in diets, and the frequency with which they are habitually consumed.

Conclusion Statement and Grade

• No conclusion can be drawn on the association between dietary patterns during pregnancy and birth weight outcomes. Although research is available, the ability to draw a conclusion is restricted by 
  • inconsistency in study findings,
  • inadequate adjustment of birth weight for gestational age and sex, and
  • variation in study design, dietary assessment methodology, and adjustment of key confounding factors. 
    
    Grade: Grade not assignable


• Insufficient evidence exists to estimate the association between dietary patterns before pregnancy and birth weight outcomes. There are not enough studies available to answer this question.
  
  Grade: Grade not assignable

Methods 

• The systematic review was conducted by a team of staff from the Nutrition Evidence Systematic Review in collaboration with a Technical Expert Collaborative.
• Literature searches were conducted using PubMed, Embase, Cochrane, and other databases to identify studies that evaluated the relationship between dietary patterns before and during pregnancy and gestational age- and sex-specific birth weight. A manual search was conducted to identify articles that may not have been included in the electronic databases searched. Articles were screened by two authors independently for inclusion based on pre-determined criteria.
• Data from each included article were extracted, risks of bias were assessed, and both were checked for accuracy. The body of evidence was qualitatively synthesized, a conclusion statement was developed, and the strength of the evidence (grade) was assessed using pre-established criteria including evaluation of the internal validity/risk of bias, adequacy, consistency, impact, and generalizability of available evidence.

Summary of Evidence

• This systematic review includes18 prospective cohort, 1 retrospective cohort and 2 randomized control trials published between 1986 and 2016. 
• The studies used multiple approaches to assess dietary patterns:
  • Nine studies used an index/score to assess dietary patterns.
  • Eight studies used factor/principal component analysis (PCA). 
  • Two randomized controlled trials assigned subjects to one of two experimental diets.
  • One study did not use a formal method to arrive at a dietary pattern.
  • One study used both logistic regression and PCA.
• Many studies did not standardize for gestational age and/or infant sex when assessing birth weight. 
  • Just one-third of studies (n=7) used both gestational age- and sex -specific cut-off values when defining small for gestational age (SGA), large for gestational age (LGA), appropriate for gestational age (AGA), or intrauterine growth restriction (IUGR). 
  • Nine out of 21 studies reported birth weight, alone, without standardizing for gestational age or sex using z-scores. 
• Study findings were highly inconsistent across the body of evidence. About half of studies (n=10) found no association between dietary patterns and birth weight outcomes. Among studies that observed an association, there was limited consistency in direction of effect and the dietary patterns generated. 
• There are serious limitations to the generalizability of this review. Minority, lower-SES, and adolescent populations are underrepresented in the body of evidence. 
• The ability to draw strong conclusions was limited by the following issues: 
  • There was a lack of consistency in study findings.
  • The data were primarily observational in nature, making it difficult to determine causal effect of the dietary patterns.
  • Many studies did not adjust birth weight for gestational age and sex, and there was heterogeneity among the standardized measures that were used.
  • The timing of exposure assessment and the duration of recall periods varied across studies.
  • Key confounding factors were not consistently accounted for.
  • None of the studies assessed effect measure modification between dietary patterns and maternal pre-pregnancy BMI in the context of birth weight outcomes.
  • Many studies were conducted outside of the U.S. 
  • Adolescent, minority, and lower-SES populations were underrepresented.
• Additional research is needed that should: 
  • Include diverse populations from the U.S. and elsewhere with varying age groups (including adolescents) and different racial/ethnic and socioeconomic backgrounds.
  • Assess effect measure modification by pre-pregnancy BMI and gestational weight gain.
  • Use a standardized birth size measure (such as one developed by the INTERGROWTH-21^st project) that would enable valid comparisons between and within countries.
  • Include well-designed and sufficiently powered RCTs.
  • Foster collaborative efforts across different regions and populations so that dietary patterns can be more consistently scored, compared and reproduced across studies.
  • Develop and validate novel epidemiological tools that can accurately capture the complexity of dietary habits.
  • Promote harmonization of research methods across various cohorts and randomized trials, similar to the National Cancer Institute’s Dietary Patterns Methods Project.
  • Adjust for key confounding factors in observational studies, including parity, educational attainment, smoking status, race/ethnicity, maternal age, family poverty income ratio, pre-pregnancy BMI, mean total energy intake and gestational weight gain.