Every method gives an estimate. Only IVF, with a known transfer date and embryo age, dates conception exactly.
Conception most likely happened around Saturday, 25 July 2026, and between Wed 22 Jul 2026 and Sat 1 Aug 2026 allowing for the −3 to +7 days uncertainty of this method. Intercourse that led to it most likely fell between Mon 20 Jul 2026 and Sun 26 Jul 2026.
Likely conception range and fertile window
计算方法 S
Subtract 266 days from the due date
Sat 17 Apr 2027−266d=Sat 25 Jul 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
Mon 20 Jul 2026toSun 26 Jul 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.
关于Conception calculator: when did I conceive?
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.
核验来源：ACOG CO 700: first-trimester crown–rump length accuracy ±5–7 days; Python 3.8 datetime ±7 d
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
核验来源：Python 3.8 datetime: 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
核验来源：Python 3.8 datetime: LMP + 32 − 14 = 2025-12-19; due LMP + 280 + 4 d
常见问题
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.
“Conception calculator: when did I conceive?”有多准确？
准确性取决于输入值和方法的假设。十进制运算使用50位有效数字，但估算、数值方法和源数据的精度可能较低；显示时的舍入并不能消除这些限制。 已按独立来源核验的计算示例：7。 例如，“Due 1 Aug 2026, dated from the last period”根据Python 3.8 datetime: 2026-08-01 − 266 d; luteal 7–17 d (Bull 2019) gives −3/+7 d; Wilcox 1995 window −5/+1 d; today 2026-01-15 − 2025-10-25 = 82 d进行核验。
这种方法出自哪里？
ACOG Committee Opinion No. 700 — Methods for estimating the due date (2017), Table 1; Bull JR et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. NPJ Digit Med 2019;2:83; Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517–21.