Reverse Due Date Calculator
Enter your due date (or your baby's birthday) and this calculator works backward to tell you the most likely conception date and the intercourse window that led to it. You can also flip to forward mode: enter a conception date to find the estimated due date. Pregnancy milestones and a full week-by-week timeline are shown below the result.
What is a reverse due date calculator?
A standard due date calculator takes the first day of your last menstrual period (LMP) and adds 280 days (40 weeks) to estimate when your baby will arrive. A reverse due date calculator runs the same arithmetic in the other direction: you start from the known delivery date (or the estimated due date from an ultrasound or a clinician) and work backward to estimate when conception most likely occurred and when the intercourse that led to it probably happened. The tool is useful for parents trying to piece together a conception timeline after the fact, for those curious about birth-month planning, or simply for anyone who did not track their cycle closely.
How the calculation works
Pregnancy length is conventionally measured from the first day of the last menstrual period, not from fertilisation. Because ovulation on a standard 28-day cycle happens around day 14 after the period begins, the gap between LMP and actual conception is roughly 14 days. That is why a 40-week (280-day) pregnancy is only about 38 weeks (266 days) from conception to birth. To reverse the due date: subtract 280 days from the due date to find the estimated LMP, then add the expected ovulation day for the cycle length to find the conception date. For a 28-day cycle that is 14 days; for a 35-day cycle it is 21 days. The intercourse window extends from 5 days before that conception estimate (sperm can survive up to 5 days in the reproductive tract) to about 2 days after (late ovulation). This calculator automatically adjusts the ovulation offset when you enter a non-standard cycle length.
Using the forward modes
If you already know an approximate conception date, you can flip to forward mode. Enter the conception date and the calculator adds the ovulation offset back to recover the LMP, then adds 280 days to produce the due date. The LMP mode follows Naegele's rule directly: LMP + 280 days = due date, which is what most clinicians and ultrasound reports use. All three modes share the same underlying math; they simply choose a different starting point.
Accuracy and limitations
Conception date estimates carry inherent uncertainty for three reasons. First, ovulation does not always fall precisely on cycle day 14, even in regular cycles. Second, sperm can survive 3 to 5 days inside the reproductive tract, so the actual fertilisation might occur days after intercourse. Third, the embryo takes another 6 to 12 days to implant after fertilisation. An early ultrasound (performed at 7 to 10 weeks of gestation) is more accurate than any date-based calculation for pinpointing gestational age, because it measures the actual embryo rather than assuming a textbook cycle. Use this calculator as a plausible estimate, not a definitive answer.
Pregnancy trimester and term definitions
| Gestational age | Period | Key features |
|---|---|---|
| 0-13 weeks 6 days | First trimester | Organogenesis, highest miscarriage risk |
| 14-27 weeks 6 days | Second trimester | Rapid growth, anatomy scan at 20 weeks |
| 28-36 weeks 6 days | Third trimester / preterm | Lung maturation, viability established |
| 37-38 weeks 6 days | Early term | Lungs nearly mature, usually good outcomes |
| 39-40 weeks 6 days | Full term | Optimal gestational age for delivery |
| 41-41 weeks 6 days | Late term | Monitoring increases |
| 42+ weeks | Post-term | Induction typically considered |
Standard gestational age milestones used by ACOG (American College of Obstetricians and Gynecologists).
Frequently asked questions
How accurate is a reverse due date calculator?
The calculation is accurate given the inputs, but the inputs themselves carry uncertainty. A due date from an early ultrasound is generally within 5 to 7 days of the true delivery date, so the back-calculated conception date inherits that same margin. The intercourse window shown (roughly 7 days wide) captures most of the biological variability. For a more precise answer, an early first-trimester ultrasound is the gold standard.
Why does the calculator show a range of dates for intercourse instead of one date?
Because sperm can survive inside the female reproductive tract for up to 5 days, intercourse does not have to happen on the day of ovulation to result in conception. Ovulation can also occur slightly later than expected. The window shown runs from 5 days before the estimated conception date to 2 days after, covering the most biologically plausible range.
What if my cycle is not 28 days?
You can enter your actual average cycle length in the input field. The calculator adjusts the ovulation offset accordingly: ovulation is assumed to occur at cycle length minus 14 days. A 35-day cycle places ovulation around day 21, so the LMP-to-conception gap is 21 days rather than 14. For very irregular cycles the estimate becomes less precise, and tracking basal body temperature or using an ovulation predictor kit gives better data.
Can I use this calculator if my baby has already been born?
Yes. Enter the actual birthday in the due date field and select "Due date or birthday". The calculator treats the birthday the same as a due date and works backward to the conception window. This is useful for parents who want to understand the timing of their pregnancy retrospectively.
Why do doctors count pregnancy from the LMP rather than from conception?
The LMP is an objectively known date for most people, whereas the exact day of ovulation and fertilisation is usually unknown. Counting from the LMP also aligns pregnancy into neat 40-week periods that match the historical conventions used in obstetric research, drug approval studies and clinical guidelines. The trade-off is that "two weeks pregnant" by the standard count actually predates fertilisation.
How do I find my due date without knowing my LMP?
If you have an ultrasound report, use the gestational age given there: add the remaining weeks to today's date to project the due date. If you tracked ovulation, enter the ovulation date in the conception-date mode of this calculator. Without any of these, a clinician can estimate gestational age from a first-trimester ultrasound measurement of the crown-rump length.