Background There is evidence to suggest a masculinizing effect on female intrauterine development in unlike-sexed twins.

The purpose of the present report was to examine the possible effects of male presence on fetal growth in females by comparing mean birthweights in members from dizygotic unlike-sexed (DZU) pairs with those from dizygotic like-sexed (DZL) pairs.

Methods The sample consisted of 1087 DZU and 1089 DZL twin pairs from the New Norwegian Twin Panel, which was established by identifying all twin births from 1967 to 1974 through the population-based Medical Birth Registry.

Results The mean birthweight of females from DZU pairs was 2684 ± 15 g (±SEM), as opposed to 2647 ± 19 g in females from DZL pairs (P=0.06).

For males, the mean birthweight was 2812 ± 16 g in DZU pairs and 2805 ± 20 g in DZL pairs (P=0.78).

Conclusion We found a tendency for the birthweight in females to be influenced by the presence of a male co-twin.

This observation may have a biological significance and should lead to a close follow-up of DZU and DZL females with respect to hormone-sensitive disorders and reproductive abilitv.