OBJECTIVE: This study tested the hypothesis that twins with a birth weight <2500 gm who subsequently had cerebral palsy or mental retardation had a birth weight distribution or birth weight discordance distribution differing from that of the total live-born twin population also weighting <2500 gm at birth.

STUDY DESIGN: All twins born in Sweden between 1973 and 1980 (n = 5382) and having a birth weight <2500 gm were identified by using information stored at the Medical Birth Registry, the National Board of Health and Welfare, Stockholm, or at Statistics Sweden. To identify twins with cerebral palsy or mental retardation, a questionnaire bearing the personal identification number (given to all newborns in Sweden shortly after birth) was distributed to all rehabilitation centers, all county school boards, all local boards of education, and to all County Councils for the Provisions and Services for the Mentally Retarded. The questionnaire was distributed in 1988 and 1989 when the twins were ≥8 years old.

RESULTS: Altogether 115 disabled twins resulting from 99 pregnancies were identified. The incidence of disabled twins per 1000 was 21.4 (95% confidence interval 17.5 to 25.2). No obvious difference was evident in the distribution of birth weight discordance when the twins with disability were compared with the population of all live-born twins having a birth weight <2500 gm. Disabled twins had a significantly lower birth weight for gestational age (t = −3.5, p < 0.001), but in fact only 10 (8.7%) twins had a birth weight < −2 SD. No difference in the incidence of disability was found for twin A versus twin B (relative risk 1.3, 95% confidence interval 0.8 to 1.9) or for like-sex versus unlike-sex twins (relative risk 1.0, 95% confidence interval 0.6 to 1.6). However, the larger twin in the pair had a significantly higher incidence of cerebral palsy than the smaller one did (relative risk 2.6, 95% confidence interval 1.4 to 4.8).

CONCLUSIONS: Birth weight discordance for twins seems not to be related to disability later in life. The great majority of twins with a birth weight <2500 gm who later became disabled were appropriate for gestational age at birth.