BACKGROUND: The aim of this study is to assess differences in human chorionic gonadotropin (hCG) values between singleton IVF/ICSI pregnancies after elective single (eSET) versus double embryo transfer (DET). METHODS: This is a unicentre, retrospective, non-randomized study of women with eSET or DET in years 2000-2006, giving birth to a singleton child with a birthweight of at least 500 g (N = 790). These women had between 1 and 6 hCG assessments in our centre. Log hCG values from Days 15 to 35 after oocyte pick-up were compared between eSET and DET patients in a multivariate analysis. RESULTS: We compared log hCG values in 456 eSET patients with log hCG values in 334 DET patients. Log hCG values were significantly higher after eSET than after DET from Day 15 until Day 35 after oocyte pick-up (P < 0.001). CONCLUSION The lower hCG values in early pregnancy are possibly due to a delay in implantation when two embryos are transferred and one of them does not implant, but the exact mechanism is unknown.