BACKGROUND: The increased risk of low birth weight associated with the use of assisted reproductive technology has been attributed largely to the higher rate of multiple gestations associated with such technology. It is uncertain, however, whether singleton infants conceived with the use of assisted reproductive technology may also have a higher risk of low birth weight than those who are conceived spontaneously. METHODS: We used population-based data to compare the rates of low birth weight (less-than-or-equal 2500 g) and very low birth weight (