Fertility & Conception Calendar

1. Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med. 1995;333(23):1517–1521. — Prospective study of 221 women (625 cycles, 192 pregnancies). Observed conception probabilities: Day –5: 10%, Day –4: 16%, Day –3: 14%, Day –2: 27%, Day –1: 31%, Day 0: 33%. Fertile window = 6 days ending on ovulation day. Note: ovulation estimated by urinary hormone metabolites without error correction.
2. Dunson DB, Baird DD, Wilcox AJ, Weinberg CR. Day-specific probabilities of clinical pregnancy based on two studies with imperfect measures of ovulation. Hum Reprod. 1999;14(7):1835–1839. — Error-corrected re-analysis of two independent datasets (London NFP study and North Carolina Early Pregnancy Study). After correcting for measurement error in ovulation identification: peak probability shifts to Day –1 (day before ovulation) in both datasets; probability falls close to zero after ovulation. This tool uses these error-corrected estimates as primary values.
3. Dunson DB, Weinberg CR, Baird DD, Kesner JS, Wilcox AJ. Assessing human fertility using several markers of ovulation. Stat Med. 2001;20(6):965–978. — Bayesian semi-parametric model with multiple ovulation markers confirming Day –1 as peak fecundability day.
4. Colombo B, Masarotto G. Daily fecundability: first results from a new data base. Demogr Res. 2000;3:5. — Large European Natural Family Planning database. Six-day fertile window confirmed; consistent with Dunson 1999 error-corrected estimates.
5. Wilcox AJ, Dunson D, Baird DD. The timing of the fertile window in the menstrual cycle: day specific estimates from a prospective study. BMJ. 2000;321(7271):1259–1262. — 696 cycles; fertile window variability; only ~30% of women have fertile window within the clinically assumed Days 10–17.
6. Ecochard R, Gougeon A. Side of ovulation and cycle characteristics in normally fertile women. Hum Reprod. 2000;15(4):752–755. — Ultrasound-confirmed ovulation timing data.
7. Kidd SA, Eskenazi B, Wyrobek AJ. Effects of male age on semen quality and fertility: a review of the literature. Fertil Steril. 2001;75(2):237–248. — Age effects on fertility relevant to conception probability context.
8. Bull JR, Rowland SP, Berglund Scherwitzl E, et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. NPJ Digit Med. 2019;2:83. — Large digital cohort; mean cycle length 29.3 days; confirms luteal phase ~14 days.
9. Freis A, Freundl-Pedersen C, Freundl G, et al. Plausibility of menstrual cycle apps claiming to support conception. Front Public Health. 2018;6:98. — Mobile app conception data consistent with established fertile window biology.
10. Shettles LB, Rorvik D. How to Choose the Sex of Your Baby. New York: Doubleday; 1997. — Shettles method basis for sex-selection timing recommendations. Evidence Level III.
11. Grimes DA, Gallo MF, Grigorieva V, et al. Fertility awareness-based methods for contraception. Cochrane Database Syst Rev. 2004;(4):CD004860. — Typical-use failure rate calendar method ~24%/year.
12. Frank-Herrmann P, Heil J, Gnoth C, et al. The effectiveness of a fertility awareness based method to avoid pregnancy. Hum Reprod. 2007;22(5):1310–1319. — Symptothermal method perfect-use ~0.4%/year.

Note on probability values: Conception probabilities shown in this tool are error-corrected estimates from Dunson et al. (1999), which re-analyzed both the London NFP study (Barrett & Marshall 1969) and the Wilcox/North Carolina cohort. These values differ from the uncorrected Wilcox 1995 raw data. The key correction: peak fecundability is on Day –1 (day before ovulation, ~29%), not Day 0 as the uncorrected data suggest. All values represent single-act-of-intercourse per-cycle probabilities in healthy fertile couples.