Ovarian stimulation for IVF has no quantitative association with birthweight: a registry study

BACKGROUND: Singleton children born after IVF are of lower birthweight compared with their naturally conceived peers. It has been hypothesized that ovarian stimulation might be associated with low birthweight in children born after IVF. The aim of the present study was to explore whether or not a dose relationship exists between ovarian stimulation and birthweight in singletons born after ovarian stimulation for IVF. METHODS: Using a national IVF registry database with a coverage of 65-70%, parental demographic variables, treatment cycle variables and neonatal variables were retrieved from all IVF treatment cycles in women between 25 and 35 years of age in which gonadotrophins were used for ovarian stimulation and a fresh embryo transfer resulting in singleton live birth was performed. Birthweight was standardized as a z-score, adjusting for gestational week at delivery and fetal sex, using data from a large reference population. Multivariate regression analysis was used to investigate the association between the dependent variable z-score and the independent predictor variables maternal age (years), maternal weight (kg), maternal height (cm), maternal body mass index (BMI) (kg/m(2)), duration of infertility (years), number of embryos transferred (n), duration of stimulation (days), consumption of gonadotrophins (ampoules) and number of oocytes retrieved (n). RESULTS: Data retrieval yielded 32 416 singleton live births after IVF, with a mean (+/- SD) z-score of -0.25 (+/- 1.0) and -0.23 (+/- 1.0) for male and female neonates, respectively. Regression analysis indicated that maternal weight, maternal height, duration of infertility and the number of embryos transferred were statistically significant determinants of the birthweight of singletons after ovarian stimulation IVF. Parameters of ovarian stimulation (duration of stimulation, consumption of gonadotrophins, number of oocytes retrieved), maternal BMI and maternal age did not significantly predict birthweight. CONCLUSIONS: Features reflecting ovarian stimulation do not correlate with birthweight. Therefore, ovarian stimulation is unlikely to be a factor affecting birthweight of IVF pregnancies.