More than six out of ten unplanned pregnancies in Britain occur in women aged 20-34 years, a national study has found. Age under 16 at first intercourse, smoking and low educational attainment were all associated with unplanned pregnancy. This study was part of the third National Survey of Sexual Attitudes and Lifestyles (Natsal-3) which uses a stratified probability sample of 15,162 men and women aged 16-74 years in Britain.