Background: as well as lifestyle habits during pregnancy have been shown to effect the risk of having a small-for-gestational-age (SGA) child. Most previous studies are based on a single assessment of these exposures, which does not take into account the possibility of different effects during early and late stages of pregnancy. The impact of psychosocial and lifestyle factors on the risk of giving birth to an SGA child (as measured by ultrasound) was examined among 747 nulliparous Swedish women who completed both a prenatal baseline, and a post-partum assessment. Results: Those registering low social participation on both assessments showed increased risk of giving birth to an SGA infant (OR=2.44 and 95% CI: 1.06–5.66), while at one assessment (OR=1.70 and 95% CI: 0.74–3.91). Maternal smoking confirmed by both or one assessments yielded an OR=2.72 and 95% CI: 1.37–5.39 and OR=1.60 and 95% CI: 0.58–4.46, respectively. During early pregnancy, poor instrumental support, maternal smoking, or passive smoking yielded increased risks of SGA, adjusted for confounding (OR=2.39 and 95% CI: 1.11–5.17; OR=2.38 and 95% CI: 1.27–4.49; OR=2.92 and 95% CI: 1.17–7.32, respectively). In late pregnancy, only maternal smoking yielded a significant association (OR=2.34 and 95% CI: 1.24–4.41). Conclusion: Scheduling repeated assessments of psychosocial resources and lifestyle factors during pregnancy yielded additional information. The findings suggest that there can be differential effects of such exposures depending on gestational stage. This information is of importance when designing appropriate intervention strategies for maternal health services as well as for public health relevant policy formulation (e.g. regarding exposure to environmental tobacco during pregnancy).