Twinning rates in admixed Costa Rican populations.

The frequency of twinning in two admixed Costa Rican groups of women from the Limón province was investigated. One group was self-classified as Black, and the other group was self-classified as White. The twinning rate for each group was computed as the number of twin maternities divided by the total number of confinements multiplied by 1,000. Both groups have virtually identical twinning rates, even though Black subjects were expected to have higher rates than White women. The twinning rate of both groups is higher than that in other New World admixed Black groups. This is particularly surprising for White females, whose European ancestral population is largely from Spain, the European country with the lowest twinning rate. The virtually identical twinning rate is probably a result of the highly frequent gene flow between Blacks and Whites in the population. © 1994 Wiley-Liss, Inc.