Research question: Is low-grade inflammation, detected by C-reactive protein (CRP), a marker of IVF outcome addressing both blastocyst quality and pregnancy outcome? Design: This sub-study of a multicentre randomized controlled trial included 440 women undergoing IVF treatment with a gonadotrophin-releasing hormone (GnRH) antagonist protocol. Serum CRP was measured on cycle day 2–3 (baseline) and on the day of ovulation triggering. The association between CRP concentrations and reproductive outcomes (number of retrieved oocytes, number of good-quality blastocysts, pregnancy, pregnancy loss and live birth), were analysed, adjusting for relevant confounders. Results: A negative association was found between higher baseline CRP concentrations and live birth rate (odds ratio [OR] 0.77, 95% confidence interval [CI] 0.62–0.96, P = 0.02) and higher CRP concentrations at baseline were associated with pregnancy loss among women who conceived (OR 1.37, 95% CI 1.07–1.76, P = 0.01). When testing for a specific cut-off, CRP concentrations above 2.34 (the highest quartile) were more likely to be associated with pregnancy loss (P = 0.02) and a lower chance of live birth (P = 0.04) compared with the lowest quartile. No associations were found between CRP concentrations and pregnancy outcomes on the day of ovulation triggering, and there were no associations between CRP concentrations and the number of good-quality blastocysts. Conclusions: Higher CRP concentrations at cycle day 2–3, before starting ovarian stimulation, are negatively associated with chance of live birth, possibly because of an increased risk of pregnancy loss. No association was found between the number of good-quality blastocysts and CRP concentration. More studies are needed to investigate the impact of low-grade inflammation.