Journal of Biosocial Science

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Short Report

IS THE NATURAL TWINNING RATE NOW STABLE?


M. F. G. MURPHY a1, K. HEY a1, D. WHITEMAN a1, M. O'DONNELL a2, B. WILLIS a2 and D. BARLOW a3
a1 ICRF General Practice Research Group
a2 Department of Public Health, Institute of Health Sciences, Old Road, Headington, Oxford OX3 7LF
a3 Nuffield Department of Obstetrics & Gynaecology, John Radcliffe Hospital, Oxford OX3 9DU

Abstract

As contribution to a recent debate (James, 1998; Murphy et al., 1997, 1998) the proportion of twins following ovulation induction (OI) or assisted conception (AC) in 1994 in Oxfordshire and West Berkshire was estimated, and by extrapolation the natural twinning rate in England and Wales was judged to have maintained a plateau phase since the 1970s. Similar figures for 1995 and 1996 from the same study, and hence a more stable local estimate, are now provided. The proportions, as before, were estimated from women's responses to a questionnaire within a case[hyphen]control study, with ascertainment from general practitioners' records or hospital case[hyphen]notes for non[hyphen]responders or for those excluded from the study originally. In 1994, 1995 and 1996 the proportion of twins following OI/AC was overall 27% (24%, 30% and 27% respectively). Restriction to the 87% locally resident made no difference. The national crude twinning rate for those years was overall 13·3 per 1000 maternities (12·8, 13·6 and 13·4 respectively).