USE OF DIZYGOTIC TO MONOZYGOTIC TWINNING RATIO AS A MEASURE OF FERTILITY

Background Monozygotic (Mt, identical) twinning occurs at a rate of ar ound three per 1000 maternities in all populations, whereas dizygotic (Dz, fraternal) twinning is highly heritable, and varies with age and race, The Dz/Mz twinning ratio reflects the frequency of twin ovulatio ns, and can provide a useful measure of human fertility. Methods 1625 pairs of twins of known sex were born in Tsan Yuk Obstetric Hospital i n Hong Kong during the period 1960-95. The yearly Dz/Mz ratio was calc ulated, and trends were analysed by chi(2) test with the Bonferroni co rrection. Ages of mothers of all opposite-sexed twins were recorded an d trends analysed by ANOVA and linear regression. Findings The Dz/Mz r atio declined significantly from 1.12 in 1960 to 0.05 in 1978 (p<0.001 ), and then rose significantly to an average of 0.86 in 1994-95 (p<0.0 03). There was a significant declining trend in age of mothers of oppo site-sexed twins from 1960 to 1978 (p<0.001), but there were no signif icant changes in maternal age after 1978(p=0.38). Interpretation If we are correct in assuming that the frequency of Mt twinning remained co nstant during the study period, the declining Dz/Mz ratio from 1960 to 1978, which also occurred in many developed countries, could reflect some adverse environmental effect on human fertility. The increasing p roportion of Dz twins in the past two decades is probably due to incre asing use of ovulation-inducing drugs such as clomiphene citrate, whic h could mask a serious and continuing decline in human fertility. It i s therefore important to continue to monitor the Dz/Mz ratio in the fu ture in this and other subpopulations, after exclusion of any women wh o have taken drugs that stimulate fertility.