Before launching a new study, pilot testing is often recommended, however, it is seldom described in depth. Here, we report extensively on a pilot study using the internet as a new method for recruitment and data collection in a prospective cohort study of women planning a pregnancy. We aimed to enroll 2500 participants in six months and attain more than 75 % after 12 months follow up. To test data completeness and validity we randomized participants to fill out either a long or a short version of the baseline questionnaire and compared self-reported data with registry-based data. We succeeded in enrolling 2288 participants, and the participation rate was 82 % after 12 months. We found high correlations (0.96) for self-reported vs. registry-based data and no difference in participation rate or data completeness according to questionnaire length. Overall, the internet-based methods seem promising and we plan to launch the full study.