Conception Calculator

Select the estimated due date method and enter the date from your pregnancy record. The calculator subtracts 266 days to estimate fertilization, shows possible intercourse dates from five days before that estimate through the estimated day, and uses the same due date to calculate pregnancy weeks. Because the original due date is itself an estimate, the back-calculated conception date is also an estimate. If you want to go the other way and estimate a due date from your last period, ovulation, conception, or an ultrasound, use our due date calculator.

No. Natural conception timing cannot usually be reduced to one certain day. Ovulation can shift, sperm may remain viable for up to five days, and fertilization can occur for about a day after ovulation. A due date, LMP, ovulation date, or ultrasound can narrow the timing, but the result remains an estimate.

Not necessarily. Intercourse can happen several days before fertilization because sperm may remain viable while waiting for an egg. The calculator therefore separates estimated fertilization timing from possible intercourse dates. IVF results do not show an intercourse window because embryo-transfer dating does not infer one.

Pregnancy weeks use gestational age, which starts from the first day of an actual or equivalent last period rather than from fertilization. In standard dating, that starting point is about 14 days before conception. This is why a person may be dated at roughly 2 weeks pregnant when fertilization occurs.

The LMP method estimates ovulation by adding the cycle length and subtracting the luteal phase from the first day of the last period. With the defaults, that is cycle day 14. It then estimates fertilization from ovulation through the following day and estimates the due date 266 days after ovulation.

With the luteal phase held constant, a longer cycle moves estimated ovulation, fertilization, and the due date later; a shorter cycle moves them earlier. For example, a 30-day cycle with a 14-day luteal phase places estimated ovulation near cycle day 16. Advanced options let you adjust the luteal phase for the LMP method.

Open Advanced options in the LMP method and enter up to three recent cycle lengths. The calculator uses their rounded average for the main estimate and the shortest and longest entries to create fertilization, intercourse, and due-date ranges. Wider variation means wider uncertainty, so use the result as a timing range.

Yes. Select the ovulation-date method and enter the known or tracked date. The calculator treats that date through the next day as the likely fertilization window, shows possible intercourse dates from five days before ovulation through the following day, and adds 266 days for the estimated due date.

Enter the ultrasound date and the gestational weeks and days printed on the report. The calculator counts backward to an equivalent LMP date, places estimated fertilization 14 days later, and calculates the due date from the same dating anchor. It converts report data only and does not analyze the scan or change a clinical due date.

Select IVF transfer, enter the transfer date, and choose the documented embryo age: Day 3, Day 5, or Day 6. The calculator subtracts embryo age from the transfer date to estimate fertilization and adds 263, 261, or 260 days to estimate the due date. IVF results do not infer possible intercourse dates; use your clinic's recorded dates if they differ.

No. A conception estimate cannot identify which intercourse event led to fertilization and cannot prove biological parentage. Overlapping possible dates are common because sperm may remain viable for several days. Only appropriate genetic testing can address paternity; date math cannot.