The rise in twinning rates previously reported for U.S. between 1964 and 1987 after adjustment for maternal age and birth order occurred in each of the nine conventional geographic divisions except the Pacific States. Differences and consistencies in rank order among the divisions with respect to crude and adjusted twinning rates and other demographic parameters may hold clues to yet unidentified influences in twinning.
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Gordon Allen (1988) studied this question.
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