Twinning in Sweden between 1973 and 1990. The recurrence rate.
OBJECTIVE
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.


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).


CONCLUSIONS
For women with one twin birth, the likelihood of having twins in a subsequent birth increases two to three-fold.
Rydhstroem
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Department of Obstetrics and Gynecology, University Hospital, Lund, Sweden. Hakan.Rydhstrom@gyn.lu.se
D000328:Adult; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D010062:Ovulation Induction; D011247:Pregnancy; D011272:Pregnancy, Multiple; D012042:Registries; D013548:Sweden; D014427:Twins / Q000706:statistics & numerical data*