Background: Early health interventions affecting child development can subsequently influence lifetime health and economic outcomes. These lifetime effects may be excluded from economic evaluation as empirical evidence covering the required time horizon is rarely available. One example is screening for postnatal depression where current guidelines do not account for lifetime effects despite evidence of a detrimental association between maternal depression and child development. Aims: To develop a methodological approach to estimate lifetime effects for economic evaluation and determine their influence on an evaluation assessing the cost-effectiveness of postnatal depression screening. Methods: Lifetime effects are estimated by linking results from two empirical studies. Firstly, growth curve models establish the effects of postnatal depression on development measures for children aged 3-11 using data from the Millennium Cohort Study. Secondly, child development measures are entered as explanatory variables in linear regression models predicting effects on lifetime health and economic outcomes using data from the 1970 British Cohort Study. An economic evaluation is conducted for scenarios which exclude/include lifetime effects to determine their influence on cost-effectiveness results. Findings: Postnatal depression was detrimentally associated with children's cognitive and socioemotional development up to age 11. Detrimental changes in cognitive and socioemotional development were negatively associated with lifetime outcomes. Postnatal depression exposure was predicted to reduce children's lifetime Quality Adjusted Life Years, increase healthcare and crime costs, and generate fewer monetary returns in education and employment. Cost-effectiveness results changed when including lifetime effects, leading to the recommendation of a screening strategy which treats a greater proportion of depressed mothers. Conclusions: Lifetime effects can influence cost-effectiveness results and their exclusion risks providing a partial analysis. This research demonstrates methods to estimate and include lifetime effects in economic evaluation. Similar approaches could be applied elsewhere to provide additional evidence for economic evaluation of other childhood interventions.