Two- and 3-month interactions were observed and 4-month Bayley MDI assessments carried out with 10 full-term, 14 preterm, and 9 small-for-gestational age (SGA) mother-infant dyads, to examine effects of neonatal status on interaction styles and the relationships between interaction variables and MDI scores. Two-month interaction patterns were most closely related to 4-month MDI scores in the full-term group, with state variables showing negative relationships and dyadic synchrony being positively related. For the preterm group, state-related variables were important to MDI scores. Preterm maternal interactive levels dropped from two to three months whereas preterm infant interactive levels rose from 2 to 3 months. SGA infant smile and maternal affection were related to MDI scores. SGA dyads differed from full-term dyads on state and synchrony variables. Findings are discussed with respect to the importance of state variables for development.