Conception Calculator
Estimate when you most likely conceived, working from the first day of your last period, your due date, a dating ultrasound, or an IVF embryo transfer. You also get a realistic conception window, the fertile days that led to it, how far along you are now, and the projected due date, since the exact day is rarely certain.
Formula
Worked example
A last period starting 1 Nov 2025 with a 28-day cycle gives an estimated conception of 15 Nov 2025 (LMP + 14 days) and a due date of 8 Aug 2026. Working back from a due date of 8 Aug 2026 gives the same conception day: 8 Aug 2026 - 266 days = 15 Nov 2025. A day-5 blastocyst transferred on 20 Nov 2025 fertilised on 15 Nov 2025 (20 Nov - 5 days).
How conception is estimated
Conception occurs when a sperm fertilises an egg released at ovulation. In a textbook 28-day cycle, ovulation falls around day 14, so this tool estimates conception as the first day of your last menstrual period (LMP) plus 14 days. If you enter a due date instead, it works backwards: a full-term pregnancy lasts about 266 days from conception (38 weeks), while the due date is dated 280 days from the LMP, so subtracting 266 days from the due date returns the same conception day. The result is why both methods agree.
Four ways to date conception
This calculator supports the four datings a clinic might use. Last period and due date both rely on the 14-day ovulation assumption. A dating ultrasound is more reliable because it measures the developing baby directly: enter the scan date and the gestational age it reported, and the tool backs up to the LMP-equivalent point and adds 14 days. For IVF, the fertilisation date is known precisely, it is simply the embryo age (day 3, 5, or 6) counted back from the transfer date, so a day-5 blastocyst transferred on a given day was fertilised five days earlier.
Why the date is a window, not a single day
The exact moment of conception is almost impossible to pin down outside of IVF. Ovulation timing varies between people and from cycle to cycle, and sperm can survive in the reproductive tract for up to five days, so intercourse several days before ovulation can still result in pregnancy. Because of this, the calculator shows a fertile window of about six days and a conception range of roughly two days either side of the estimated ovulation date, which better reflects biology than a single calendar day. It also reports how far along you are now and the projected due date.
Adjusting for your cycle length
The standard day-14 assumption only holds for a 28-day cycle. The second half of the cycle, the luteal phase, is fairly fixed at about 14 days, so it is the time before ovulation that stretches or shrinks with cycle length. If your cycle runs longer than 28 days, ovulation and therefore conception happen later; if it is shorter, they happen earlier. The last-period method shifts the estimated conception day by the difference between your cycle length and 28 days to account for that. The ultrasound and IVF methods do not need this adjustment because they are anchored to a measured point.
Key pregnancy dating intervals
| From | To | Offset |
|---|---|---|
| Last period (LMP) | Conception (28-day cycle) | +14 days |
| Last period (LMP) | Due date (Naegele) | +280 days |
| Conception | Due date | +266 days |
| Conception | Fertile window start | -5 days |
| Day-5 blastocyst transfer | Conception (fertilisation) | -5 days |
The fixed offsets this calculator uses, all measured in days.
Frequently asked questions
Is the conception date the same as the day I had intercourse?
Not necessarily. Conception happens at ovulation, when the egg is fertilised. Because sperm can survive up to five days, intercourse a few days before ovulation can still cause pregnancy. The estimated conception date reflects the likely ovulation day, not the specific act of intercourse, which is why the tool also shows a fertile window.
How accurate is this estimate?
It is an approximation based on average cycle timing. Real ovulation can vary by several days, especially with irregular cycles, so use the date range rather than the single day. A dating ultrasound, which measures the developing baby, is more accurate, and the ultrasound mode here lets you use one. IVF dating is the most precise of all because fertilisation is known.
How do I calculate conception from an IVF transfer?
Count the embryo age back from the transfer date. A day-3 embryo was fertilised 3 days before transfer, a day-5 blastocyst 5 days before, and a day-6 blastocyst 6 days before. Select the embryo day and enter the transfer date and the calculator does this for you, then projects the due date 266 days from fertilisation.
Why do the last period and due date methods give the same answer?
They are two sides of the same standard dating. A due date is set 280 days after the LMP, and conception is estimated 14 days after the LMP, which is 266 days before the due date. So subtracting 266 days from the due date returns the same conception day that LMP + 14 days produces.