Introduction

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It is important to identify modifiable causes of preterm delivery and fetal growth retardation, which are strong predictors of infants' later health and survival. Observations of high birth weights¹ and long gestations² in the fish eating community of the Faroe Islands suggested that intake of seafood rich in long chain n-3 fatty acids can increase birth weight by prolonging gestation² or by increasing the fetal growth rate.^3-6

Randomised controlled trials have shown that fish oil can delay spontaneous delivery and prevent recurrence of preterm delivery, ^{7 8} but the minimum amount of n-3 fatty acids needed to obtain this effect remains to be determined. No detectable effects on fetal growth rate were seen in these trials, ^{7 8} but fish oil was provided only in the second half of pregnancy, and several observational studies have found direct associations between measures of seafood intake in pregnancy and fetal growth rate. ^{5 9-12}

We tested whether a low intake of seafood in early pregnancy is a risk factor for preterm delivery and low birth weight and whether it is associated with a lower fetal growth rate. We related the findings to quantified intakes of fish and long chain n-3 fatty acids.

	Methods

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We invited all pregnant women receiving routine antenatal care in Aarhus, Denmark, to complete self administered questionnaires in weeks 16 and 30 of gestation.¹³ Only singleton, live born babies without detected malformations were included in the analysis. The local scientific-ethical committee approved the protocol, and we used an informed consent form.

Exposure variablesIn Denmark fish is eaten mainly as part of a hot meal, in an open sandwich, or cold in a green salad or pasta salad. Frequency of consumption of such meals has been shown to be a strong and independent predictor of variation in erythrocyte n-3 fatty acids.¹⁴ We posed four questions: "How often did you eat: (a) fish in a hot meal, (b) bread with fish, (c) green salad or pasta salad with fish, and (d) fish oil as a supplement?" The women were asked to understand the term "fish" as including roe, prawn, crab, and mussel and to let the responses represent the period from when they first knew they were pregnant until completion of the questionnaire. Each question had six predefined response categories: never, less than once a month, 1-3 times a month, 1-2 times a week, 3-6 times a week, every day. We then estimated daily intakes of fish and long chain n-3 fatty acids. ^{15 16} We defined six groups of exposure, with the lowest group consuming no fish and the other five groups being fifths of the remaining participants (designated QUANT0, QUANT1, QUANT2, QUANT3, QUANT4, QUANT5). An alternative strategy is presented on bmj.com.

Outcome variablesWe assessed gestational age by early ultrasonography in 71% of participants, and otherwise from menstrual data or best clinical judgment. We defined low birth weight as <2500 g and preterm as delivery before 259 days. We assessed intrauterine growth retardation below the 10th centile and birth weight expected from gestational age from the infant's birth weight, gestational age, and sex, on the basis of a Danish standard.¹⁷

CovariatesWe used covariates as previously described¹³: sex of infant (girl, boy); smoking (0, 1-9, 10 cigarettes a day) and alcohol consumption (<10 or 10 drinks a week) in pregnancy; maternal age (19, 20-29, 30-39, 40 years), parity (0, 1), height (159, 160-169, 170-179, 180 cm), and pre-pregnant weight (49, 50-59, 60-69, 70-79, 80 kg); length of education (7, 8-9, 10 years); and whether the mother had a cohabitant (0, 1).

Statistical analysesWe decided all analytical conditions a priori. We used the ² test, analysis of variance, and logistic regression. We included all suspected potential confounders (see covariate list) in the multivariate model simultaneously

	Results

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Of 8998 women returning the 16 week questionnaire, 8729 (97%) had not consumed fish oil supplementsresults refer to this restricted group. Mean birth weight was 3577 (SD 531) g, and duration of gestation was 280.0 (11.5) days. Low birth weight occurred in 2.7% (232/8707), preterm delivery in 3.4% (299/8707), and intrauterine growth retardation in 6.6% (572/8705) of participants. On the basis of the frequency of consumption of fish (table 1), we estimated that, on average, women consumed 15.8 (SD 13.9) g fish and 0.182 (0.161) g long chain n-3 fatty acids a day.

Estimated mean daily intakes for the six exposure groups QUANT0 to QUANT5 were 0, 3.3, 8.0, 13.4, 18.0, and 38.4 g fish; and 0, 0.038, 0.092, 0.146, 0.216, and 0.445 g long chain n-3 fatty acids. Low birth weight, preterm birth, and intrauterine growth retardation all tended to decrease with increasing fish consumption, and mean birth weight, duration of gestation, and birth weight adjusted for gestational age tended to increase with increasing fish consumption (table 2). These associations were mainly apparent at the lower end of the exposure scalethis was particularly the case for preterm birth and mean duration of gestation.

Smokers, primiparous women, teenagers, and women with low weight, short stature, and without high school education and cohabitant occurred more frequently in the low exposure groups (see bmj.com). The impression that the decline in risk occurred mainly at the lower end of the exposure distribution was confirmed on examination of adjusted odds ratios for low birth weight and preterm birth, with the highest intake group as reference (table 3). The association between intake of fish and risk of fetal growth retardation weakened but was not always fully abolished after adjustment for potential confounding.

	Discussion

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Strengths and weaknesses
Strengths of the study included that exposure data were collected in a concurrent fashion and long before occurrence of outcome among more than 8000 women, that exposure categories and other analytical conditions were decided a priori, and that analyses took account of nine potential confounding factors.

The main weakness of the study, as with any observational study, was the possibility of confounding that was not adjusted for. Adjustment had little impact on measures of association, but confounding by unmeasured factors cannot be ruled out.

Another weakness was that the assumed values for portion sizes, distributions of fish species in meals, and food contents of nutrients are only approximations to the true values. Imprecise estimates of quantified intake of n-3 fatty acids are thus inevitable. Although this imprecision is unlikely to explain the steep decline in risk at the low end of the exposure distribution, it may contribute to the observed "bending" of the relation if imprecision increases with increasing exposure, a possibility that cannot be ruled out. An alternative strategy, based solely on food frequency data, is presented on bmj.com.

Comparisons with other studies
Overall, the findings agree with the randomised trials showing that consumption of fish oil in pregnancy can increase birth weight by prolonging gestation and reduce the risk of recurrence of preterm delivery. ^{7 8} The finding that the dose-response relations were strong at low exposures corroborates two earlier studies. A reduction in early delivery was seen in women who had received only 0.1 g n-3 fatty acids (along with other substances) a day from week 20 of gestation.^18-21 An association was seen between duration of pregnancy and a biomarker for intake of marine n-3 fatty acids in Danish women, whereas no such association could be detected in Faroese women with a substantially higher intake, suggesting a stronger association at low exposures.²²

A case-control study in the same population could not detect any association between seafood intake in pregnancy and risks of preterm birth²³; unlike the present study, however, this study assessed dietary intake retrospectively after delivery, which may have distorted the results.

Several observational studies have found associations between measures of maternal seafood intake and fetal growth rate. ^{5 9-12} In the randomised trials, where fish oil was provided after week 16-20 of gestation, no effects were seen on fetal growth rate. ^{7 8} The observational data could therefore possibly be explained either by effects of n-3 fatty acids exerted before week 16-20 or by effects of other substances in seafood. Our study could substantiate neither of these two possibilities, as the associations between seafood consumption in early pregnancy and fetal growth rate tended to disappear after adjustment for potential confounders.

Randomised controlled trials to examine the dose-response relations between long chain n-3 fatty acids and timing of delivery and preterm risk are warranted.