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Singletons born after IVF are at risk of adverse pregnancy outcome, the cause of which is unknown. The present study investigated the influence of ovarian stimulation and IVF laboratory procedure on birthweight. Birthweight of singleton pregnancies resulting from IVF treatment with (n = 161 ) and without ovarian stimulation (using a modified natural cycle (MNC) protocol; n = 158), and spontaneous conceptions in subfertile patients (n = 132) were compared. Mean ± SD birthweight of singletons after conventional IVF with ovarian stimulation, MNC-IVF and natural conception were 3271 ± 655, 3472 ± 548 and 3527 ± 582 g (P = 0.001). After adjustment for biological and social confounders, the difference in birthweight between conventional IVF and MNC-IVF was reduced to 88 g and the differences between conventional IVF and MNC-IVF versus spontaneous conceptions to 123 and 23 g, respectively. The results lead to three conclusions. First, a major part of the crude differences in birthweight between the three groups is related to patient and pregnancy characteristics. Second, the IVF laboratory procedure has no influence on birthweight. Third, although a trend towards tower birthweight after ovarian stimulation was found, an adverse effect of ovarian stimulation on birthweight was not substantiated. [ABSTRACT FROM AUTHOR]