Dizygotic to monozygotic twinning ratio at The Royal Women's Hospital, Melbourne 1947-1997, compared with Australian national twinning incidence.

The incidence of dizygotic (Dz) twinning can be used as an index of natural human fertility. A retrospective study was done at The Royal Women's Hospital, Australia, to see whether the dizygotic to monozygotic (Mz) twinning ratio from one hospital can accurately reflect the national incidence of Dz twinning. The yearly twinning incidence from 1947-1997 was expressed as a Dz:Mz ratio, standardised for maternal age and plotted against previously published national statistics. The proportion of mothers born in Asia (of both singleton and multiples) between 1983-1997 was analysed to see whether different racial mixes might influence twinning trends. There were 5275 twins born of known sex and maternal age between 1947-1997. The age-standardised Dz:Mz ratio increased non-significantly from 1.39 in 1947 to 2.29 in 1953 (P = 0.08), underwent a significant decline to 0.73 in 1977, then remained stable until 1997 (P>0.05). The same trends were also apparent when the data was pooled into 2-year groups with the increase from 1947/48-1953/54 becoming highly significant (P<0.009). These trends observed in the hospital-based data were in close agreement with those found in the national statistics, with the exception of a rise in 1977-1982 only reflected in the Australia-wide data. In 1993, 2.6% of mothers were born in Asian countries; by 1997, this had risen to 10.6%. We found that the Dz:Mz ratio from one hospital closely reflects national twinning trends. Prospective studies must account for race, and would need around 200-300 twin pairs per year to minimise fluctuations of the ratio.