Introduction 6 3 7 49 20 52 56 4 20 5 17 either 32 15 29 37 Methods Participants 38 47 36 t P t P 40 t P 8 47 Measures 27 16 51 31 11 11 48 57 t P 38 59 23 51 33 2 26 21 34 less 13 14 38 7 15 32 Statistical analysis 45 46 22 56 0 1  2  3 4  5  0 12 b P b P b P b P b P b P 9 Results Descriptives t P 2 P 2 P 2 P 2 P 2 P 2 P 1 Table 1 Birth-weight in grams and gestation in weeks by zygosity, gender and birth order for twins with complete data points  N Mean Standard deviation First-born twin birth-weight 889 2484.55 544.03 Second-born twin birth-weight 889 2457.62 562.17 MZ first-born birth-weight 381 2404.32 531.55 MZ second-born birth-weight 381 2392.41 545.44 DZ first-born birth-weight 508 2544.65 546.01 DZ second-born birth-weight 508 2506.10 570.11 Girl first-born birth-weight 489 2445.43 544.59 Girl second-born birth-weight 489 2426.00 560.75 Boy first-born birth-weight 400 2532.18 540.06 Boy second-born birth-weight 400 2494.76 562.45 MZ duration of gestation (weeks) 371 36.44 3.00 DZ duration of gestation (weeks) 497 35.99 2.75 Girls weeks gestation 483 36.31 2.87 Boys weeks gestation 385 36.18 2.87 Main effect of birth-weight for gestation—standard regression analysis b P n b P Genetic regression analysis 2 2 P n P n 1 Table 2 Genetic risk for depression and birth-weight  Beta (birth-weight as a continuous variable) P Beta (low birth-weight for gestation as a categorical variable) P Interaction between birth-weight corrected for gestation & genetic risk a 0.096 * 0.001  Birth-weight for gestation * 0.006 * 0.032 Co-twin depressive symptoms * 0.001  * 0.002 Twin gender a 0.130 a 0.080 Zygosity 0.027 0.349 * 0.010 Maternal pre-natal smoking (yes/no) * 0.033 * 0.021 Social class 0.011 0.705 0.011 0.700 Maternal age at birth −0.024 0.411 −0.018 0.529 a P * P Fig. 1 Genetic risk and depressive symptoms by birth-weight for gestation  Analyses of depression as a categorical variable—standard regression analysis N N N N P t P Discussion 18 19 2 18 21 26 34 lower 13 14 38 2 21 22 26 33 34 51 behavioural and cognitive 25 52 56 25 42 42 41 44 1 35 43 58 r twin 1 birth-weight P r twin 2 birth-weight P 10 24 39 9 42 16 54 50 55 53 28 30 38 In summary, the present results suggest that LBW for gestation and genetic/familial factors co-act and interact in influencing early parent-rated depressive symptoms. These observations suggest that it may be useful to monitor children in cases where there is a family history of depression and the child was small for gestational age.