There was an interaction ( P < 0.001 ) between the allocation group and maternal prepregnant body mass index ( BMI ; in kg/m ( 2 ) ) [ birth-weight effect : -23 , +34 , and +96 g in lowest ( < 18.6 ) , middle ( 18.6-21 .8 ) , and highest ( > 21.8 ) thirds of BMI , respectively ] .