Published online by Cambridge University Press: 21 February 2012
Frequencies of recurrent multiple maternities were estimated using two sets of census data in Japan in 1990 and 1995. The repeat frequency (RF) of the twinning rate is the frequency of 2 sets of twins among families or couples who have already had 1 set of twins and 2 more siblings. The overall RFs were 9.6 per 1000 couples in 1990 and 9.3 in 1995. The RFs of the monozygotic (MZ) twinning rates were 5.9 per 1000 couples in 1990 and 5.5 in 1995. The RFs of the dizygotic (DZ) twinning rates were 3.7 in 1990 and 3.8 in 1995. For unlike-sexed propositus twins, the RF of MZ twins were 5.0 per 1000 couples in 1990 and 5.5 in 1995. The RF of DZ twins were 5.3 in 1990 and 4.6 in 1995. As for like-sexed propositus twins, the corresponding RFs were 6.2 and 5.5 for MZ twins, and 3.4 and 3.6 for DZ twins, respectively. In mothers who have experienced a twin maternity, the overall RF of twinning was 1.5 to 2 times as high as the average mother's chance of having twins. There was no RF for triplets for both census years. As for geographic variations of the overall RF, the rates in Okinawa (16.2) and Hokkaido (15.3) were significantly higher than those in the Tohoku (8.7), Kanto (8.0) and Kyushu (7.4) districts.