- 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
Friday, June 19, 2026
| Conception Window | Jun 14, 2026 to Jun 20, 2026 |
| Estimated LMP | Jun 5, 2026 |
| Due Date (40 weeks) | Mar 12, 2027 |
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.
Getting pregnant just before or during your period is very unlikely for most women with regular 28-day cycles because ovulation occurs much earlier (around day 14). However, sperm can survive 3-5 days, and some women with shorter cycles ovulate early (around day 8-10), meaning intercourse during the last days of a period could theoretically lead to pregnancy. With irregular cycles, fertile windows shift unpredictably.
Gestational age is counted from the first day of the last menstrual period (LMP), not from actual conception. Since conception typically occurs about 14 days after LMP, gestational age is always about 2 weeks more than embryonic age (time since conception). At 8 weeks gestational age, the embryo is actually about 6 weeks old. Doctors and ultrasounds use gestational age because LMP is a known date while conception is estimated.
A full-term pregnancy is 39-40 weeks from LMP (37-38 weeks from conception). Deliveries between 37 and 42 weeks are considered normal. Before 37 weeks is preterm. After 42 weeks is post-term. Only about 4% of babies are born exactly on their due date; most births occur within 2 weeks before or after. The estimated due date is a midpoint, not a deadline.
Home pregnancy tests detect hCG (human chorionic gonadotropin), produced after the fertilized egg implants in the uterine wall, typically 6-12 days after conception. Most tests become positive 12-15 days after conception (around the time of a missed period). Sensitive early-detection tests may show positive 10-12 days after conception. Blood tests (quantitative hCG) can detect pregnancy as early as 6-8 days after 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.