Fertility trends in European countries since the 1960s have been characterized by a notable decline in the completed fertility of successive cohorts (with an even greater decrease in the total fertility rate) and a rapid rise in age at first birth, of around 3-4 years over three decades. At the same time, recourse to assisted reproductive technology (ART) has increased substantially. One might thus assume that this is indicative of growing difficulties in conceiving, which could – at least in part – result from couples’ desire to postpone family formation. To evaluate the purely biological impact of birth postponement, and the potential catch-up made possible by ART, we used a microsimulation model that takes a large number of both biological and behavioural parameters into account. Our simulations show that the biological effect on completed fertility of the 3-4 year postponement of the first birth is quite limited, at between 0.1 and 0.2 children. Recourse to ART makes up for only a small part of this reduction, 10% at most.