Rapid infant growth is defined as upward crossing through one major centile band on a growth chart. Babies who demonstrate rapid weight gain are at higher risk for obesity. However, this association is likely to differ according to whether or not they also demonstrate rapid length growth. For example, an infant with rapid weight gain and rapid length growth is likely to have lower obesity risk than an infant with rapid weight gain but non-rapid length growth. This is an important question for infant growth monitoring practice, but no studies have been published on this topic. Our project will provide novel evidence on which infants are at greatest risk for obesity according to both their weight gain and length growth.