On average, twins have a lower birthweight than singletons as a result of a different pattern of fetal growth and shorter gestational duration. Therefore, investigations into the genetics of birthweight often exclude data from twins, leading to a reduction in sample size and remaining ambiguities concerning the genetic contribution to birthweight in twins.
The twinning genetics consortium has carried out a genome-wide association meta-analysis of birthweight in 42 212 twin individuals, and compared the results to those obtained in singletons. Our results indicate that a similar genetic architecture underlies birthweight in twins and singletons and that future genome-wide studies might benefit from including data from large twin registers.
The summary statistics of the GWAS of birthweight in twins are available here