Maternal "traditional" or "prudent" diets rich in vegetables, fruit, whole grains, and fish were linked to lower risk for preterm delivery, supporting advice to eat a balanced diet, according to findings from a large prospective cohort study published online March 4 in BMJ.

Preterm delivery, defined as delivery at gestational age less than 37 weeks, accounts for nearly three quarters of newborn deaths and has many adverse effects in the short and long term.

"Maternal dietary habits can directly affect the growing fetus, and awareness has increased during recent years that maternal diet may influence the outcome of pregnancy as well as the long term health of the child," write Linda Englund-Ögge, MD, from the Department of Obstetrics and Gynecology, Institute of Clinical Sciences at Sahlgrenska Academy and Sahlgrenska University Hospital in Gothenburg, Sweden, and colleagues. "Several studies indicate associations between maternal diet and preterm delivery."

They continue, "The study of dietary patterns offers a broader view of food and nutrient consumption and overcomes the methodological limitations related to the study of single nutrients or foods."

The study goal was to determine whether there was any association between maternal dietary patterns and risk for preterm delivery (between 22 and <37 weeks). The study cohort consisted of 66,000 pregnant women in Norway between 2002 and 2008.

Inclusion criteria were singleton pregnancy, completion of food frequency questionnaire during the first 4 to 5 months of pregnancy, complete data regarding parity or previous preterm delivery, pregnancy duration between 22+0 and 41+6 gestational weeks, absence of diabetes, and first enrollment pregnancy.

The investigators defined a "prudent" dietary pattern as having a proportionately high intake of vegetables, fruits, oils, whole-grain cereals, and fiber-rich bread, with water as the predominant beverage. In contrast, the "Western" dietary pattern had high consumption of salty and sweet snacks, white bread, desserts, and processed meat products, and the "traditional" pattern emphasized boiled potatoes, fish, low-fat milk, and cooked vegetables.

The primary study endpoint was the hazard ratio (HR) for preterm delivery based on the level of adherence to these 3 distinct dietary patterns after adjustment for maternal age, history of previous preterm delivery, height, body mass index, marital status, parity, smoking, maternal education, household income, and total energy intake.

Certain Diets Linked to Lower Risk for Preterm Delivery

Preterm delivery occurred in 3505 (5.3%) of the 66,000 pregnant women. High scores for the prudent dietary pattern were associated with a significantly lower risk for preterm delivery (adjusted HR for the highest vs the lowest third, 0.88; 95% confidence interval [CI] 0.80 - 0.97), as well as with a significantly lower risk for late and spontaneous preterm delivery. Reduced risk for preterm delivery was especially apparent among women in their first pregnancy.

The traditional pattern was also associated with reduced risk for preterm delivery (adjusted HR for the highest vs the lowest third, 0.91; 95% CI, 0.83 - 0.99). In contrast, adherence to the Western pattern had no independent association with preterm delivery.

"This study showed that women adhering to a 'prudent' or a 'traditional' dietary pattern during pregnancy were at lower risk of preterm delivery compared with other women," the study authors write. "Although these findings cannot establish causality, they support dietary advice to pregnant women to eat a balanced diet including vegetables, fruit, whole grains, and fish and to drink water. Our results indicate that increasing the intake of foods associated with a prudent dietary pattern is more important than totally excluding processed food, fast food, junk food, and snacks."

Healthy Diet Important During Pregnancy

The investigators therefore recommend that clinicians emphasize dietary counseling for pregnant women, as maternal diet might actually affect risk for preterm delivery.

Limitations of this study include observational design precluding causal inference, possible residual confounding, and low participation rate (40.6%) in the Norwegian Mother and Child Cohort Study.

In an accompanying editorial, Lucilla Poston, PhD, Tommy's Professor of Maternal and Fetal Health and head of the Division of Women's Health at King's College London, United Kingdom, emphasizes the importance of healthy diet during pregnancy.

"[H]ow might a prudent diet protect against premature delivery?" Dr. Poston writes. "The consumption of probiotic milk products, and anti-inflammatory properties of fruit and vegetables are mentioned, somewhat speculative but perhaps more plausible than the suggested dampening of the hypothalamic pituitary axis, the role of which in the onset of labour in humans is not established.... Health professionals would therefore be well advised to reinforce the message that pregnant women eat a healthy diet."

This study was supported by the Freemasons Directorate Board for Children, the Adlerbertska Foundation, the Hjalmar Svensson Foundation, the Norwegian Research Council, the Jane and Dan Olsson Foundation, the Swedish Medical Society, and Swedish government. The Norwegian Ministry of Health and the Ministry of Education and Research, the National Institute of Environmental Health Sciences and the National Institute of Neurological Disorders and Stroke of the National Institutes of Health, and the Norwegian Research Council/Functional Genomics also supported the Norwegian Mother and Child Cohort Study. The authors have disclosed no relevant financial relationships. Dr. Poston's institution has received funds for the SCOPE study of nulliparous pregnant women, and she has received payment from the food and pharmaceutical industry for lecturing, for one piece of research unrelated to the subject of the editorial, and for another related to the subject of the editorial.

BMJ. Published online March 4, 2014.