Fertility in twins.

From the 474 twins born in 1950 in 100 Flemish cities and towns in Belgium, 85% reached the age of 20 yr, against 37% in Austria a hundred years ago. More female than male twins are married (P = 0.03), in particular nonidentical female twins (P = 0.02). Fertility seems to be lower in male, especially male-male twins (P less than 0.03), and most strikingly so in identical male twins (P = 0.05). There seems to be no influence of premature birth or low birthweight.