Twinning in Sweden between 1973 and 1990. The recurrence rate

Aim To ascertain the recurrence rate of twinning, by determining the repeat frequency in a population-based material of women who have already had one set of twins. Material and methods All singleton and twin births in Sweden between 1973 and 1990 were identified from the Medical Birth Registry (MBR), at the National Board of Health and Welfare, Stockholm. In the MBR fewer than 1% of all women with a singleton birth and fewer than 3% of all women with a twin birth are missing. When appropriate, 95% confidence limits (95% CL) were calculated. Results Altogether, 84 women with at least two twin births were identified. When the proband twins were like-sexed, the observed/expected ratio was 1.7 (95% CL 1.3, 2.2). For unlike-sexed proband twin pairs, the corresponding figures were 3.3 (95% CL 2.3, 4.7). Conclusion For women with one twin birth, the likelihood of having twins in a subsequent birth increases two to three-fold. (C) 1998 Elsevier Science Ireland Ltd.