BACKGROUND: Data show that differences exist in the birthweight of singletons after frozen embryo transfer (FET) compared with fresh transfer or gamete intra-Fallopian transfer (GIFT). Factors associated with low birthweight (LBW) after assisted reproduction technology (ART) were studied. METHODS: Birthweight, distribution of birthweight, z-score, LBW (<2500 g), gestation and percentage preterm (<37 weeks) for singleton births >19 weeks gestation, conceived by ART or non-ART treatments

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... n induction and artificial insemination) between 1978 and 2005 were analysed for one large Australian clinic. RESULTS: For first births, the mean birthweight was significantly (P < 0.005) lower, and LBW and preterm birth more frequent for GIFT (mean 5 3133 g, SD 5 549, n 5 109, LBW 5 10.9% and preterm 5 10.showed ART treatment before 1993 and fresh embryo transfer were negatively related to birthweight after including other covariates: gestation, male sex, parity, birth defects, Caesarean section, perinatal death and socio-economic status. CONCLUSIONS: Birthweights were lower and LBW rates higher after GIFT or fresh embryo transfer than after FET. Results for FET were similar to those for non-ART conceptions. This suggests IVF and ICSI laboratory procedures affecting the embryos are not causal but other factors operating in the woman, perhaps associated with oocyte collection itself, which affect endometrial receptivity, implantation or early pregnancy, may be responsible for LBW with ART.