To cite package ‘simPreg’ in publications use:

  Skodvin S, Gjerdevik M, Gjessing H (Unpublished manuscript).
  “simPreg: an R package for simulating pregnancy data with a
  time-varying exposure.” _Unpublished manuscript_.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {simPreg: an R package for simulating pregnancy data with a time-varying exposure},
    author = {Siri N. Skodvin and Miriam Gjerdevik and H\aa{}kon K Gjessing},
    journal = {Unpublished manuscript},
    year = {Unpublished manuscript},
  }