- From LMP (Last Menstrual Period). Enter the first day of your last period and your typical cycle length. Conception typically occurs 14 days before your next period (around day 14 for 28-day cycles, later for longer cycles). Adjust cycle length for accuracy.
- From Due Date. Work backwards from a known or ultrasound-estimated due date. Due date is 280 days (40 weeks) from LMP, or 266 days (38 weeks) from conception. The calculator subtracts 266 days from the due date to estimate conception.
- From Birth Date. Estimate when conception occurred for a baby already born. Useful for genealogy research. The calculator subtracts the average gestation (266 days) from the birth date to give an approximate conception window.
Conception Calculator
Estimate when conception occurred based on your due date or last menstrual period.
28 days
Likely Conception Date
Saturday, February 28, 2026
| Conception Window | Feb 23, 2026 to Mar 1, 2026 |
| Estimated LMP | Feb 14, 2026 |
| Due Date (40 weeks) | Nov 21, 2026 |
Conception occurs approximately 14 days after the start of the last menstrual period (with a 28-day cycle). Actual conception date can vary by several days.
How to Estimate Conception Date
Conception Date and Pregnancy Timeline
From LMP: Ovulation day = LMP + (cycle length - 14) Conception window = Ovulation day ± 3 days From due date: Conception date ≈ due date - 266 days LMP date ≈ due date - 280 days Naegele's Rule (due date from LMP): Due date = LMP + 9 months + 7 days
Key pregnancy milestones from conception:
| Weeks from Conception | Gestational Age | Milestone |
|---|---|---|
| 2 | 4 weeks | Missed period, positive test possible |
| 6 | 8 weeks | Heartbeat detectable on ultrasound |
| 11 | 13 weeks | End of 1st trimester |
| 24 | 26 weeks | End of 2nd trimester |
| 36 | 38 weeks | Full term begins |
| 38 | 40 weeks | Estimated due date |
Frequently Asked Questions
Conception date estimates from LMP are typically accurate to within 3-7 days for women with regular cycles. Actual conception depends on when ovulation occurred (which varies cycle-to-cycle by 1-5 days) and how long sperm survived before fertilizing the egg (sperm can survive 3-5 days). An ultrasound before 12 weeks of pregnancy gives the most accurate due date estimate, which can then be used to calculate backward to approximate conception.
Related Calculators
Pregnancy Due Date Calculator
Calculate your estimated due date and pregnancy milestones from your last menstrual period or conception date.
Ovulation Calculator
Estimate your most fertile days and ovulation window based on your cycle length.
Age Calculator
Calculate exact age in years, months, and days from any birthdate to today or another date.