Estimated conception date: Monday, 7 September 2026
Likely conception range, from
Friday, 4 September 2026
Likely conception range, to
Monday, 14 September 2026
Uncertainty of this method
−3 to +7 days
Fertile window, from
Wednesday, 2 September 2026
Fertile window, to
Tuesday, 8 September 2026
Gestational age today
11 weeks 3 days
Every method gives an estimate. Assisted-reproduction pregnancy dating uses clinical records such as embryo age and transfer date; this calculator has no assisted-reproduction mode and does not verify an exact fertilization date.
Conception most likely happened around Monday, 7 September 2026, and between Fri 4 Sep 2026 and Mon 14 Sep 2026 allowing for the −3 to +7 days uncertainty of this method. Intercourse that led to it most likely fell between Wed 2 Sep 2026 and Tue 8 Sep 2026.
Likely conception range and fertile window
How it's calculated S
Subtract 266 days from the due date
Mon 31 May 2027−266d=Mon 7 Sep 2026
266 days is the 280-day count from the last period to the due date (ACOG CO 700) minus the 14 days from that period to ovulation in a 28-day cycle.
Likely range
3 days earlier to 7 days later: the 14-day luteal phase assumed by the due date against the 7–17 days seen in 95 % of cycles (Bull 2019).
Fertile window around the estimate
Wed 2 Sep 2026toTue 8 Sep 2026
Wilcox 1995: pregnancies followed intercourse only in the 5 days before ovulation and on the day itself; one day after is added as a margin.
About the conception calculator
Conception is dated to ovulation, which in a textbook 28-day cycle falls 14 days after the first day of the last period. Working back from a due date, conception is 266 days earlier. From the last period it is the next expected period minus a 14-day luteal phase. From an ultrasound it is the scan date minus the gestational age measured on the scan, plus 14 days. The fertile window shown runs from 5 days before that date to 1 day after: Wilcox and colleagues (1995) saw pregnancies only from intercourse in the 5 days before ovulation or on the day itself.
People use it to work out when they got pregnant. A due date of 1 August 2026 points to conception around 8 November 2025, with intercourse most likely between 3 and 9 November.
Each method carries its own error. Luteal phases vary: in 612,613 recorded cycles the middle 95 % lasted 7 to 17 days, so period-based dates can be 3 days early or 7 days late. ACOG puts first-trimester scan dating within 5 to 7 days.
When did I conceive?
Subtract 266 days (38 weeks) from your due date: a due date of 1 August 2026 points to conception around 8 November 2025. From the first day of your last period, conception is about 14 days before your next expected period; from an ultrasound, it is the scan date minus the gestational age, plus 14 days.
Every method gives an estimate with a range of several days. Only IVF, with a known transfer date and embryo age, dates conception exactly.
Why conception is 266 days before the due date
A due date is 280 days after the first day of the last period (ACOG Committee Opinion 700). In a textbook 28-day cycle, ovulation, and so conception, comes about 14 days after that period starts, which leaves 280 − 14 = 266 days.
The same 14 days explain why a pregnancy is already counted as 2 weeks 0 days at conception: pregnancy dating starts from the period, not from conception.
Conception date from your last period
Add your usual cycle length to the first day of your last period to get the next expected period.
Subtract the luteal phase, the days from ovulation to the next period, usually taken as 14.
For a period starting 1 December 2025 in a 28-day cycle, the next period is due 29 December and conception falls around 15 December. With a 32-day cycle it moves to 19 December; with 35 days, ovulation is on cycle day 22, on 22 December.
Real luteal phases vary. In 612,613 app-recorded cycles, the middle 95% lasted 7 to 17 days (Bull et al., 2019), so the true date can be up to 3 days earlier or 7 days later than the estimate: 12 to 22 December in the 28-day example.
Conception date from an ultrasound
Subtract the gestational age measured on the scan from the scan date to get an equivalent period date, then add 14 days.
A scan on 10 January 2026 at 8 weeks 3 days (59 days) gives an equivalent period date of 12 November 2025 and conception around 26 November 2025.
Early scans date a pregnancy most tightly. The ± ranges below are the gaps that ACOG Committee Opinion 700, Table 1, gives for how far scan and period dates must differ before the due date is changed:
Up to 8 weeks 6 days: ±5 days
9 weeks 0 days to 15 weeks 6 days: ±7 days
16 weeks 0 days to 21 weeks 6 days: ±10 days
22 weeks 0 days to 27 weeks 6 days: ±14 days
28 weeks or later: ±21 days
So a scan at 20 weeks on 20 December 2025 places conception around 16 August 2025, anywhere from 6 to 26 August. ACOG considers a pregnancy with no ultrasound before 22 weeks 0 days suboptimally dated.
How accurate is a conception date from a due date?
It depends on how the due date was set. A due date from the last period carries the luteal-phase spread, about 3 days earlier to 7 days later than the estimate. One set by a first-trimester scan is good to about ±7 days.
For the 1 August 2026 due date, that is 5 to 15 November 2025 from the period, or 1 to 15 November from an early scan.
When was the fertile window?
Pregnancy follows intercourse in the days up to and including ovulation.
In Wilcox and colleagues' 1995 study of 221 women, no pregnancy followed intercourse outside the six days ending on the day of ovulation, and the chance rose from 10% five days before to 33% on the day.
Counting from 5 days before the estimated date to 1 day after, the extra day as a margin, conception around 8 November 2025 gives a fertile window of 3 to 9 November.
Calendar dating cannot prove or rule out who the father is, because its range spans several days. It is also no basis for contraception.
Three worked cases
Due date example: due 1 August 2026, dated from the last period.
Period example: last period 1 December 2025, 28-day cycle, 14-day luteal phase.
Early scan example: scan on 10 January 2026 at 8 weeks 3 days.
Scenario
Central estimate
Modeled range from
Modeled range to
Due date example
2025-11-08
2025-11-05
2025-11-15
Period example
2025-12-15
2025-12-12
2025-12-22
Early scan example
2025-11-26
2025-11-21
2025-12-01
Entering your dates
Cycle length is the usual number of days from the start of one period to the start of the next, from 20 to 45. Cycles shorter than 21 or longer than 35 days are outside the usual adult range and harder to date from the calendar.
Luteal phase can be set from 10 to 16 days if you know yours from tracking ovulation.
Scan age is entered as weeks plus days, as written on the scan report.
Checked against: ACOG CO 700: first-trimester crown–rump length accuracy ±5–7 days
Last period 1 Dec 2025, 28-day cycle
Work it out from
First day of last period
First day of last period
2025-12-01
Usual cycle length
28 days
Estimated conception date
2025-12-15
Likely conception range, from
2025-12-12
Likely conception range, to
2025-12-22
Fertile window, from
2025-12-10
Fertile window, to
2025-12-16
Estimated due date
2026-09-07
Gestational age today
6 weeks 3 days
Checked against: LMP + 28 − 14 = 2025-12-15; LMP + 28 − 17 and LMP + 28 − 7 (Bull 2019); due LMP + 280 (ACOG CO 700)
Last period 1 Dec 2025, 32-day cycle
Work it out from
First day of last period
First day of last period
2025-12-01
Usual cycle length
32 days
Estimated conception date
2025-12-19
Likely conception range, from
2025-12-16
Likely conception range, to
2025-12-26
Estimated due date
2026-09-11
Checked against: LMP + 32 − 14 = 2025-12-19; due LMP + 280 + 4 d
Questions
How do I calculate when I conceived from my due date?
Subtract 266 days, or 38 weeks, from the due date. The due date is 280 days after the first day of the last period, and in a 28-day cycle ovulation, and so conception, happens about 14 days after that period starts. A due date of 1 August 2026 gives a conception date of 8 November 2025. If the due date came from the last period, the true date is most likely 3 days earlier to 7 days later.
How accurate is an ultrasound for dating conception?
Most accurate early in pregnancy. ACOG Committee Opinion 700 gives the crown–rump length measured up to 13 weeks 6 days an accuracy of ±5–7 days, ±7–10 days from 14 to 21 weeks 6 days, ±10–14 days from 22 to 27 weeks 6 days and ±21–30 days from 28 weeks. So a scan at 8 weeks dates conception to within about 5 days, and a third-trimester scan only to within 3 weeks.
Can you get pregnant from sex a week before ovulation?
Not in the best-known data. In Wilcox and colleagues' 1995 study of 221 women trying to conceive, no pregnancy followed intercourse outside the six days ending on the day of ovulation. Inside that window the chance of conception rose from 10 % five days before ovulation to 33 % on the day itself. The fertile window shown here adds one day after ovulation as a margin.
Why is my conception date two weeks after my last period?
Because pregnancy is counted from the first day of the last period, about two weeks before ovulation in a 28-day cycle. At conception a pregnancy is already called 2 weeks 0 days along. In longer cycles ovulation comes later: with a 35-day cycle and a 14-day luteal phase it falls on day 22, three weeks after the period started.
For information only, not medical advice. These formulas describe populations, not you; talk to a clinician before acting on a result.
Checked against references
This calculator includes 7 worked examples with independently sourced answers. They run in the test suite, and you can run them here too.
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