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ia re Vikanes et al. BMC Pregnancy and Childbirth 2013, 13:169 http://www.biomedcentral.com/1471-2393/13/169maternal barrier [1,6-8].Institute of Public Health, Nydalen, P.O. Box 4404, Oslo 0403, NorwayFull list of author information is available at the end of the articleBackground Most women experience nausea and vomiting in early pregnancy (NVP) [1]. In contrast, hyperemesis gravidarum (HG) is a potentially life-threatening condition that occurs in between 0.8 % and 3.2 % [2,3]. The 10th edition of Inter-national Classification of Diseases (ICD 10) differentiates between mild (O21.0) and severe forms of HG (O21.1) [4]. The underlying mechanisms for HG remain unknown, al-though previous research has suggested genetic factors to be involved [5]. It is not yet clear whether maternal genes or environmental factors are the main contributing fac-tors. Increased levels of human chorionic gonadotropin (hCG), oestrogen and leptin have been found to be associ-ated with HG, as have increased levels of fetal DNA in maternal blood; the latter indicating damage of the feto-* Correspondence