[Frequency of multiple pregnancies in various Italian regions: 1955-1983].

The trend in frequency of multiple birth in Italy has been analyzed on the basis of the number of single and multiple births, according to age and regions, published annually by the Central Institute of Statistics. The rates and ratios of twin and triple or more pregnancy were computed for region, children sex and calendar period. Specific and standardized rates by maternal age are presented here too. Between 1955 and 1983 the frequency of multiple births declined from 12.6/1000 to 9.6/1000 deliveries. The downward trend was constant till the late 70's, when multiple pregnancy rates flattened out, being constant till the early 80's. This was largely attributable to decreasing trend in dizygotic multiple pregnancies, the monozygotic rates were generally constant over the considered period. Rates of triple births decreased slightly till the early 70's and increased in relative and absolute terms from late 70's on, thus if in the quinquennium 1955-1959 only 1 out of 99 multiple births was a triplet, this ratio increased to 1 out of 70 in 1980-1983. There was no noticeable variation in the North/South multiple birth frequency ratio, which was constantly about 0.7. Similarly the national trends were generally reproduced in various regions, the regional differences were largely attributable to differences in dizygotic multiple pregnancy rates.