Introduction: Pregnancy is a time of profound changes in a woman's life. These changes can influence her sexual behaviour and the relationship with her partner. The purpose of this article is to study sexual behaviour modification before and during pregnancy in a cohort of healthy Spanish pregnant women. Methods: This is a prospective cohort study in which 111 pregnant women took part. They completed a questionnaire about their sexual habits before pregnancy and at 20, 30 and 38 weeks (frequency of intercourse and orgasm, satisfaction with their relationships and pain or discomfort). Results: Intercourse frequency decreased progressively during pregnancy at all times studied relative to pre-pregnancy periods, to a significant degree (p < 0.0001), accompanied by a reduction in sexual satisfaction. The incidence of orgasm was lightly modified, except in the weeks before parturition in which it underwent a notable decrease (p < 0.0001) with an increase in the rate of hypoorgasmia/ anorgasmia from 19% pre-pregnancy to 43% at 38th week. Conclusions: The longitudinal monitoring of this cohort permits the conclusion that during pregnancy, sexual behaviour undergoes a general decline with respect to prepregnancy levels. The differences vary depending on the sexual parameter considered, with intercourse frequency being the most affected variable in contrast to dyspareunia, which is the least affected.