Rachootin P, Olsen J
Scand J Soc Med. 1980;8(3):89-94. doi: 10.1177/140349488000800302.
The present study examines secular changes in the rate of twinning in Denmark during the period of 1931-77. Total twinning (i.e., mono- and dizygotic twinning combined) declined from a crude rate of 15.0 twin births per 100 deliveries in 1931 to 9.6 twin births per 100 deliveries at the end of the period. After direct adjustment for maternal age and parity (between the years 1935 and 1965), a 22% decline in total twinning rates was noted. Estimates of the rates of dizygotic twinning over the same period, again adjusted for maternal age and parity, show a decline of 20%. Thus there is evidence of a real decline in twinning, independent of the effects of changing patterns of maternal age and parity. These findings are consistent with reports of a general decline in twinning rates from a number of other countries. The interpretation of such secular changes is a matter for speculation.
本研究调查了1931年至1977年间丹麦双胞胎出生率的长期变化。双胞胎总数（即单卵双胎和双卵双胎之和）的粗出生率从1931年的每100次分娩中有15.0例双胞胎出生降至该时期末的每100次分娩中有9.6例双胞胎出生。在对母亲年龄和胎次进行直接调整后（1935年至1965年期间），双胞胎总数率下降了22%。同期双卵双胎率的估计值，同样经母亲年龄和胎次调整后，下降了20%。因此，有证据表明双胞胎出生率确实下降了，这与母亲年龄和胎次模式变化的影响无关。这些发现与其他一些国家双胞胎出生率普遍下降的报道一致。对这种长期变化的解释尚属推测。
