Objective: To clarify the effect of the using the intrauterine device on fertility in nulliparous women.
Design: Prospective cohort study of two groups of nulliparous women, one recruited while using an intrauterine device and the other while using an oral contraceptive.
Setting: Seventeen family planning clinics in England and Scotland.
Sample: 1,071 nulliparous, married women, aged 18-40 years, 558 of whom contributed information to the main objective of the study.
Methods: The women were recruited between 1982 and 1985 and followed up annually to 1994. Dates and reasons for any contraceptive method changes (which were most frequently to barrier methods) were recorded, together with the outcome of any pregnancies, at each follow up.
Main outcome measures: The number of nulliparous women giving birth at term after stopping contraception (oral contraceptive, intrauterine device or barrier method) in order to conceive.
Results: Women who stopped using a barrier method to achieve a planned pregnancy conceived most quickly: 54% were delivered after one year vs 39% of intrauterine device and 32% of oral contraceptive users (log rank P = 0.002). There was no association between fertility and duration of oral contraceptive use: However, short term intrauterine device users (< 42 months) showed a fertility pattern more favourable than seen in those discontinuing oral contraceptives, with increasing duration of intrauterine device use being associated with decreasing fertility (linear trend P = 0.005); the fertility of women who had used the intrauterine device for 78 + months was the most impaired (28% were delivered by 12 months vs 46% of short term users; at 36 months the corresponding figures were 79% vs 91%). This association remained after adjusting for potential confounding factors, including maternal age, husband's social class, and history of gynaecological illnesses, factors which themselves had independent associations with fertility.
Conclusions: Long term intrauterine device use in nulliparous women appears to be associated with an increased risk of fertility impairment.