Lower fertility for twins

Johan Tollebrant, Statistics Sweden

The percentage of children who are born as a twin has increased since the mid of the 20th century. During year 2010, 2.8 percent of the children in Sweden were born as a twin or triplet. In 1950 the same proportion was 1.8 percent. This study compares twins and non-twins in Sweden using administrative register data. The database contains 8.6 million individuals born between 1932 and 2010 and 181 000 of them are twins, triplets or quadruples. The fertility in Sweden is characterized of big fluctuations during the 20th century. The study shows that both twins and non-twins have the same trends during time. But, twins have a lower fertility rate than the non-twins all years between 1980 and 2010. Also the ultimate number of children for women in birth cohort 1950-1965 was lower for twins than non-twins. There is not only between twins and non-twins there are differences. When a comparison is made between twin women with a twin brother and with a twin sister, the study shows that they with a sister have a lower fertility rate than they with a brother. The lower fertility for twins is confirmed by the proportion still childless at age 45. A higher proportion of the twins than the non-twins are childless when they are 45 years old. Also the mean age at birth of the first child is higher for twins than for non-twins, both for men and for women.