Reverse Due Date Calculator
Work backwards from any due date or birth date to find the likely conception date, fertile window, and key first-trimester milestones. Instant, private, no signup needed.
Reverse Due Date Calculator
Enter a due date or birth date to find the conception window
Enter the estimated due date (EDD) as given by your midwife or scan
An ultrasound-confirmed due date gives the most accurate result
Longer cycles shift ovulation — and therefore conception — later in the month
Your Conception Estimate
Estimated dates based on average 40-week pregnancy
Enter your due date or birth date and click Find Conception Date to see the estimated conception window and key pregnancy milestones.
Pregnancy Milestones
Key dates and windows during a typical 40-week pregnancy, calculated from the first day of the last menstrual period (LMP).
| Milestone | Weeks from LMP | Days from LMP | Notes |
|---|---|---|---|
| Last Menstrual Period (LMP) | Week 0 | Day 0 | Pregnancy dating starts here |
| Ovulation & Conception | Week 2 | Day 14 | ±2 days; adjusted for cycle length |
| Implantation | Week 3–4 | Day 20–26 | 6–12 days after fertilisation |
| Positive Pregnancy Test | Week 4–5 | Day 28–35 | hCG detectable after implantation |
| First Heartbeat (ultrasound) | Week 6 | Day 42 | Vaginal ultrasound may detect earlier |
| End of First Trimester | Week 13 | Day 91 | Miscarriage risk drops significantly |
| Anatomy Scan | Week 18–20 | Day 126–140 | Detailed structural ultrasound |
| Viability Threshold | Week 24 | Day 168 | Survival outside womb possible |
| Full Term | Week 39–40 | Day 273–280 | Optimal delivery window |
| Post-Term | Week 42+ | Day 294+ | Induction often recommended |
Reverse Due Date FAQ
Everything you need to know about working backwards from a due date to estimate conception.
A reverse due date calculator works backwards from either your due date or your baby’s birth date to estimate when conception likely occurred. Standard pregnancy lasts 280 days (40 weeks) from the first day of the last menstrual period. Since ovulation and conception typically happen around day 14 of a 28-day cycle, conception occurred roughly 266 days before the due date. The calculator subtracts 266 days from the due date to estimate conception, and adds a fertile window around it based on your cycle length.
The calculator provides an estimate based on average pregnancy lengths. Only about 5% of babies are born exactly on their due date, so results carry a natural margin of roughly ±2 weeks. The estimate is most accurate when using an ultrasound-confirmed due date from the first trimester, which is more precise than date-based calculations. Cycle length variations also affect accuracy — longer cycles mean later ovulation and a later conception date.
Conception is the moment the egg is fertilised by sperm, typically within 12–24 hours of ovulation. Implantation is when the fertilised egg attaches to the uterine wall, occurring 6–12 days after fertilisation. A pregnancy test only turns positive after implantation, when hCG levels begin to rise. This calculator estimates the conception (fertilisation) date, not the implantation date.
Yes. If your baby has already been born, simply toggle to Birth Date and enter the birth date. The calculator treats the birth date as the delivery date and works backwards using the same 266-day formula. Bear in mind that babies born prematurely or post-term will produce a conception estimate that’s offset from the true date — for greatest accuracy, use the actual gestational age at birth if you know it.
To manually calculate your conception date from a due date, subtract 266 days (38 weeks) from the due date. For example, if your due date is 1 January 2025, subtract 266 days to get approximately 9 April 2024 as the estimated conception date. Alternatively, subtract 280 days (40 weeks) to find the first day of your last menstrual period (LMP), then add 14 days to estimate ovulation and conception.
A normal full-term pregnancy lasts 39–41 weeks from the first day of the last menstrual period (LMP). The textbook average is 40 weeks (280 days). Babies born between 37 and 42 weeks are considered full term. Before 37 weeks is preterm; after 42 weeks is post-term. Research shows natural variation spans about 5 weeks around the average due date in uncomplicated pregnancies.
Yes, cycle length significantly affects when ovulation — and therefore conception — occurs. In a standard 28-day cycle, ovulation happens around day 14. For a 35-day cycle, ovulation occurs around day 21 — a week later. For a 21-day cycle, ovulation is around day 7. This calculator adjusts the estimated conception date based on your cycle length, shifting it later for longer cycles and earlier for shorter ones.