OBJECTIVE Maternal weight gain has been consistently linked to birth weight but, beyond maternal energy intake, no macronutrient has been associated with either of them. We have examined whether maternal energy-adjusted intake of macronutrients is associated with either maternal weight gain or birth-size parameters. METHODS Cohort study. METHODS University hospital in Boston, USA. METHODS A total of 224 pregnant women coming for their first routine prenatal visit. The women were followed through delivery. METHODS None. Pregnant women's dietary intake during the second trimester was ascertained at the 27th week of pregnancy through a food frequency questionnaire. RESULTS Intake of neither energy nor any of the energy-generating nutrients was significantly associated with birth size. In contrast, maternal weight gain by the end of the second trimester of pregnancy was significantly associated with energy intake (+0.9 kg/s.d. of intake; P approximately 0.006) as well as energy-adjusted intake of protein (+3.1 kg/s.d. of intake; P<10(-4)), lipids of animal origin (+2.6 kg/s.d. of intake; P<10(-4)) and carbohydrates (-5.2 kg/s.d. of intake; P<10(-4)). CONCLUSIONS Although maternal weight gain is strongly associated with birth size, the indicated nutritional associations with weight gain are not reflected in similar associations with birth-size parameters. The pattern is reminiscent of the sequence linking diet to coronary heart disease (CHD) through cholesterol: diet has been conclusively linked to blood cholesterol levels and cholesterol levels are conclusively linked to this disease, even though the association of diet with CHD has been inconclusive and controversial.