What is the question?

• The question is: What is the relationship between dietary patterns before and during pregnancy and gestational age at birth?

What is the answer to the question?

• Limited but consistent evidence suggests that certain dietary patterns during pregnancy are associated with a lower risk of preterm birth and spontaneous preterm birth. These protective dietary patterns are:
  • higher in vegetables; fruits; whole grains; nuts, legumes and seeds; and seafood (preterm birth, only), and
  • lower in red and processed meats and fried foods. 

Most of the research was conducted in healthy, Caucasian women with access to health care.

• Evidence is insufficient to estimate the association between dietary patterns before pregnancy and gestational age at birth as well as the risk of preterm birth.

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 11 studies published between 2005 and 2016.
• These studies assessed the relationship between dietary patterns before and during pregnancy and gestational age at birth, including premature/preterm birth and gestational age at delivery. 
• Eight studies considered dietary patterns during pregnancy and preterm birth. Five of these studies found a statistically significant association. A sixth study found an association between dietary patterns during pregnancy and early preterm birth, but not preterm birth. 
• Additionally, five studies considered dietary patterns during pregnancy and spontaneous preterm birth. Four of these studies found a statistically significant association. The fifth study showed different associations for women depending on whether or not it was their first pregnancy.  
• There is not enough evidence to estimate the association between dietary patterns during pregnancy and gestational age at birth when measured in days.
• The body of evidence is limited in several ways. For example, minority women and those of lower SES are underrepresented in this body of evidence.

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 at birth?
• 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

• Limited but consistent evidence suggests that certain dietary patterns during pregnancy are associated with a lower risk of preterm birth and spontaneous preterm birth. These protective dietary patterns are:
• higher in vegetables; fruits; whole grains; nuts, legumes and seeds; and seafood (preterm birth, only), and
• lower in red and processed meats and fried foods. 
  
  Most of the research was conducted in healthy, Caucasian women with access to health care.
  
  Grade: Limited
• Evidence is insufficient to estimate the association between dietary patterns before pregnancy and gestational age at birth as well as the risk of preterm birth.
  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 includes 10 prospective cohort studies and 1 randomized controlled trial (RCT) published between 2005 and 2016.
• The studies used multiple approaches to assess dietary patterns: 
  • Four studies used indices/scores to assess dietary patterns.
  • Four studies used factor or principal component analysis (PCA).
  • One study used both indices/scores and PCA.
  • One RCT assigned subjects to one of two experimental diets.
  • One study did not use a formal method to arrive at a dietary pattern.
• Despite this variability, 5 of the 8 studies that assessed the relationship between dietary patterns during pregnancy and preterm birth found a statistically significant association. A sixth study found an association between dietary patterns during pregnancy and early preterm birth, but not preterm birth. 
  • Highest adherence to a protective dietary pattern during pregnancy was associated with a preterm birth risk reduction of 9% to 90%. 
  • Highest adherence to a detrimental dietary pattern during pregnancy was associated with an increase in preterm birth risk of 53% to 55%.
• Additionally, 4 of the 5 studies that assessed the relationship between dietary patterns during pregnancy and spontaneous preterm birth found a statistically significant association. The fifth study showed an effect measure modification by parity.  
  • Highest adherence to a protective dietary pattern during pregnancy was associated with a spontaneous preterm birth risk reduction of 15% to 45%. 
  • Highest adherence to a detrimental dietary pattern during pregnancy was associated with an increase in spontaneous preterm birth risk of 18% to 92%.
• There is insufficient evidence to estimate the association between dietary patterns during pregnancy and gestational age at birth when measured in days.
• Generalizability of the included studies is limited to healthy Caucasian women who have access to health care. Minority women and those of lower SES are underrepresented in this body of evidence.
• The ability to draw strong conclusions was limited by the following issues: 
  • The data were primarily observational in nature, limiting the ability to determine causal effect of the dietary patterns
  • There was heterogeneity in terms of when dietary data were assessed
  • There was a lack of uniformity in outcome assessment, and some studies used less robust methods than others
  • Key confounding factors were not consistently controlled across studies
  • Only two studies were conducted in the U.S, one of which was primarily conducted in adolescent girls
  • Minority and lower SES populations were underrepresented
• There was a lack of diversity based on BMI, parity, age at conception and smoking status