The use of twinning rates as a reproductive health monitor in England and Wales 1988-1996.
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D000328:Adult; D000367:Age Factors; D004739:England; D005260:Female; D006801:Humans; D011247:Pregnancy; D011256:Pregnancy Outcome; D011786:Quality Control; D012099:Reproductive Techniques; D014427:Twins; D014852:Wales