BACKGROUND Both maternal and paternal factors have been suggested to influence a couple's fecundity. To investigate this, we examined the role of several maternal and paternal lifestyle and socio-demographic factors as determinants of time to pregnancy (TTP) in a Dutch birth-cohort. METHODS Groningen Expert Center for Kids with Obesity (GECKO) Drenthe is a population-based birth-cohort study of children born between April 2006 and April 2007 in Drenthe, a province of The Netherlands. Both partners received extensive questionnaires during pregnancy. Univariable and multivariable Cox regression analyses were used to determine the impact of the investigated factors on TTP. RESULTS A total of 4778 children were born, and the parents of 2997 children (63%) gave their consent to participate. After excluding unintended pregnancies and pregnancies as a result of fertility treatment, the data of 1924 couples were available for analysis. Hazards ratios and 95% confidence intervals of factors influencing TTP in multivariable Cox regression analysis were: maternal age 1.23 (0.98–1.54) for age <25 years, 1.17 (1.03–1.32) for age 25–30 years and 0.72 (0.61–0.85) for age >35 years (reference category: 30–35 years); paternal age: 1.31 (0.94–1.82) for age <25 years, 1.11 (0.97–1.28) for age 25–30 years and 0.91 (0.80–1.04 for age >35 years (reference category: 30–35 years); nulliparity: 0.76 (0.68–0.85) versus multiparity; menstrual cycle length: 1.12 (0.95–1.30) for 3 weeks, 0.72 (0.62–0.83) for 4–6 weeks, 0.68 (0.40–1.16) for >6 weeks and 0.66 (0.54–0.81) for irregular cycle (reference category: 4 weeks); prior contraceptive use: 0.78 (0.67–0.91) for no contraception, 1.68 (1.45–1.95) for condom use, 1.08 (0.89–1.33) for condom use combined with oral contraception, 1.40 (1.16–1.70) for intrauterine device and 0.50 (0.25–1.01) for contraceptive injection (reference category: oral contraception); and maternal educational level 0.75 (0.62–0.92) for low education level and 0.81 (0.73–0.90) for medium educational ...