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Association between usual menstrual cycle length and fecundability, fitted by restricted cubic splines, Denmark, 2007–2009. The reference level for the fecundability ratio (FR) is a cycle length of 28 days. The curves are adjusted for age, pack-years of smoking, body mass index, intercourse frequency, and last method of contraception. The spline graph has 4 knot points located at 24, 28, 29, and 32 days.
