Direct evidence linking ambient temperature to human fecundity is sparse. We aimed to evaluate the potential impact of ambient temperature on time to pregnancy (TTP) and identify the optimal temperature range for initiating conception attempts.Our analysis included 576 927 couples from the Chinese National Free Preconception Health Examination Project (NFPHEP) in Yunnan Province, with a one-year follow-up post-enrollment. Each female partner's cycle-specific average temperatures (Tmean) at the couple residences were aggregated and summarized by daily concentrations with a resolution of 0.1° × 0.1°. We used discrete-time Cox regression nested with distributed lag non-linear models to estimate the fecundity odds ratio (FOR) for Tmean concerning one-, two-, or three-cycle preceding pregnancy attempts.Among the 576 927 couples (mean [SD] age: female partner, 27.6 [5.5] years; male partner, 30.1 [5.8] years), 193 710 couples conceived within 12 cycles, among which 52.1% were pregnant within 3 TTPs and 78.9% were pregnant within 6 TTPs. The cumulative pregnancy rate in 12 menstrual cycles was 38.87%. An inverted U-shaped exposure-response relationship between TTP and Tmean was identified for the cycles preceding the pregnancy attempt. The optimal temperature interval (TI) for conception attempts was determined to be 7.9 °C to 14.5 °C, correlating with a 0.3% (FOR: 1.003, 95%CI: 0.987-1.020) to 3.8% (FOR:1.038, 95%CI: 1.031-1.047) increase in fecundity, compared to the median Tmean of 15.9 °C. Temperatures below or above this interval were linked to a significant reduction in fecundity, ranging from 1.2% (FOR: 0.988, 95%CI: 0.977-1.000) to 6.8% (FOR: 0.932, 95%CI: 0.910-0.953) for the lower TI (<7.9 °C), 2.3% (FOR: 0.977, 95%CI: 0.970-0.984) to 6.6% (FOR: 0.934, 95%CI: 0.921-0.948) for the higher TI (14.5 °C-24.6 °C), respectively, compared to the optimal TI. These findings were robust after stratifying by age and BMI of female or male partners.Exposure to temperatures within the 7.9 °C to 14.5 °C, one to three menstrual cycles preceding pregnancy attempts, was associated with enhanced fecundity and a reduced TTP, suggesting that the optimal ambient temperature could be pivotal for conception success.Copyright © 2025. Published by Elsevier GmbH.