Objective: To determine the combined effects of maternal and paternal preconception overweight and obesity on infant birthweight. Design: Retrospective data analysis, fresh cycles (2009-2017), Repromed, South Australia. Setting: Assisted Reproductive Technology. Population: Couples undergoing either in vitro fertilisation or intracytoplasmic sperm injection with their own gametes and transfer of a single blastocyst (N=1479). Methods: Maternal and paternal BMI were recorded prior to cycle initiation. Infant birthweight was recorded at delivery. The impact of paternal and maternal overweight and obesity and their interaction on infant birthweight was assessed using quantile regressions constructed at 5th, 10th, 50th, 90th and 95th birthweight percentiles based on Australian standards. Main Outcome Measures: First, singleton, term birth (≥ 37 weeks’ gestation) birthweight. Results: There was weak evidence for an interaction between parental BMI for median birth weight (β=-0.98; 95%CI=[-1.90, -0.05], p=0.04) with infants having increasing birth weight with increasing parental BMI, when one parent has normal weight. When either parent is overweight or obese, although birth weights are higher (maternal β=15.9; 95%CI=[1.63, 30.1], p=0.03; paternal β=7.33; 95%CI=[0.297, 14.4] p=0.04), they are not associated with increasing BMI of the other parent. Conclusions: Both maternal and paternal overweight and obesity at conception independently increase median infant birthweight. These findings necessitate the need for a family centered approach for preconception counselling on healthy BMI prior to pregnancy. Further studies are warranted in other ART or general population cohorts to support or refute our findings. Funding: NOM is the recipient of an NHMRC Early Career Fellowship.