Sexual intercourse frequency and fecundability in the Norwegian mother, father and child cohort study.

OBJECTIVE: The aim of this study was to explore the association between frequency of intercourse in the four weeks leading up to conception and fecundability among women trying to conceive in a pregnancy cohort. METHODS: The Norwegian Mother, Father and Child Cohort Study is a pregnancy cohort in Norway with recruitment between 1999 and 2008. This study includes self-reported questionnaire data from 64 193 women with planned, naturally conceived pregnancies. Mean time to pregnancy for women trying to conceive was calculated for different intercourse frequencies and the Jonckheere-Terpstra test for trend was performed for time to pregnancy across ordered groups of intercourse frequency. The cycle specific probability of conception (fecundability ratio) was calculated for intercourse frequencies of less than once, 1-2, 3-4, > 4 times per week through proportional probability regression with adjustment for the women's age and education level. Several supplementary analyses were performed to account for additional covariates such as BMI, parity, endometriosis and irregular menstrual cycles. RESULTS: The mean time to pregnancy decreased with intercourse frequency from 4.8 months for less than once per week to 3.7 months for > 4 times weekly, however the decrease was primarily evident for the group reporting intercourse > 4 times weekly compared to lower intercourse frequencies. There was a significant trend in TTP across ordered intercourse frequency groups (p < 0.0001), indicating progressively shorter TTP with higher intercourse frequencies. We found a 16% higher chance of conceiving in any cycle (fecundability ratio 1.16 (95% CI:1.14-1.19)) among women reporting intercourse >4 times per week compared to the reference group (1-2 times per week) within the analytic cohort. This association was consistent in supplementary analyses accounting for BMI, parity, menstrual cycle regularity and endometriosis. There were no clear differences in fecundability between intercourse frequencies of less than once, 1-2 and 3-4 times per week. CONCLUSION: Among women with planned pregnancies that resulted in natural conception, intercourse >4 times per week in the four weeks leading up to conception, was associated with a higher cycle probability of conceiving, whereas differences across lower frequency categories were limited.