Lower fertility for twins in Sweden

The percentage of children born as twins has increased since the middle of the 20th Century. During the year 2012, 2.7 percent of the children in Sweden were born as twins or triplets, whereas in 1950 that same proportion was 1.9 percent. This study compares twins and non-twins in Sweden using administrative register data. The database contains 8.8 million individuals born between 1932 and 2012 of which 187 000 were born as twins, triplets or quadruplets. During the 20th century, the fertility rate in Sweden was characterized by big fluctuations and this study shows that both twins and non-twins followed the same trends during that time. However, women twins have had a lower fertility rate than non-twins for all years between 1980 and 2012. In addition, a higher proportion of twins compared to non-twins were also childless by the age of 45. Curiously, discrepancies are not found only when comparing twins and non-twins. When a comparison is made between twin sisters and brother and sister twins, the study shows that the women with a twin sister had a lower fertility rate than those with a twin brother. Could different demographic and socioeconomic variables help explain the differences?