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[How often are twins born?]
1Institutt for samfunnsmedisin, Universitetet i Tromsø.
The frequency of twin births has declined over the last 50 years in Norway. The probability of a twin birth varies with the age of the mother, and the rates increase regularly up to the age of 35-39 years and afterwards decline. The age distribution of the mothers has changed, and a larger proportion of the children are born by younger mothers. But this alone does not explain the decline in the frequency of twin births. The probability of twin births also varies with the number of births, and the rate increases with each birth. The mean number of children from one woman has also declined during the same period.
The frequency of twin births has declined over the last 50 years in Norway. The probability of a twin birth varies with the age of the mother, and the rates increase regularly up to the age of 35-39 years and afterwards decline. The age distribution of the mothers has changed, and a larger proportion of the children are born by younger mothers. But this alone does not explain the decline in the frequency of twin births. The probability of twin births also varies with the number of births, and the rate increases with each birth. The mean number of children from one woman has also declined during the same period.