"When did I conceive?" sounds like it should have a one-line answer, and the arithmetic part does. But behind it sit three different clocks — the calendar, the medical chart, and biology — that don't tick together, which is why the doctor's "you're six weeks pregnant" never matches your own math. This guide untangles them.
Key Takeaways
- Conception ≈ due date − 266 days, with a ±5-day fertile window around it.
- Gestational age runs ~2 weeks ahead of time-since-conception, because medicine counts from your last period.
- Conception day ≠ intercourse day — sperm survive up to 5 days waiting for ovulation.
- First-trimester ultrasound is the tiebreaker: ±3–5 days accuracy, and it overrides LMP math when they disagree.
- No conception estimate proves paternity — windows overlap; only DNA settles it.
The three clocks
Clock 1 — the LMP clock (what your chart uses). Doctors date pregnancy from the first day of your last menstrual period, because it's the one date most people actually know. By this clock, pregnancy lasts 40 weeks (280 days). The strange consequence: during "weeks 1 and 2" of a medically-dated pregnancy, you weren't pregnant yet.
Clock 2 — the conception clock (what you're asking about). Fertilization happens at ovulation — around day 14 of a textbook cycle, so roughly LMP + 14 days. From conception to due date is about 266 days (38 weeks). This is why due date − 266 recovers the conception estimate.
Clock 3 — the intimacy clock (the one people confuse with #2). Sperm survive in the reproductive tract for up to five days. Conception can therefore trail intercourse by nearly a week — the encounter you remember and the biological start date can legitimately be different days. This is the entire reason honest calculators report a window.
The month-by-month shortcut
Due-date month maps to a conception window in a predictable way — each due month traces back roughly nine months, minus a week:
| Due in | Conceived around |
|---|---|
| January | Apr 10 – May 10 (previous year) |
| February | May 11 – Jun 7 (previous year) |
| March | Jun 8 – Jul 8 (previous year) |
| April | Jul 9 – Aug 7 (previous year) |
| May | Aug 8 – Sep 7 (previous year) |
| June | Sep 8 – Oct 7 (previous year) |
| July | Oct 8 – Nov 7 (previous year) |
| August | Nov 8 – Dec 8 (previous year) |
| September | Dec 9 – Jan 7 (the holidays — why September is the #1 US birth month) |
| October | Jan 8 – Feb 7 (same year) |
| November | Feb 8 – Mar 9 (same year) |
| December | Mar 10 – Apr 9 (same year) |
Each month links to a day-by-day table — for example due in September breaks down every September due date to its likely December–January conception date. For your exact date, the calculator handles leap years and lets you start from due date, LMP, or ultrasound.
When the estimates disagree, trust the ultrasound
LMP math assumes a 28-day cycle with day-14 ovulation. Real cycles run 21–35 days, ovulation moves, and about one in ten people can't pin their LMP anyway. That's why the first-trimester ultrasound (weeks 6–13) is the gold standard: the embryo's crown-rump length grows so uniformly early on that dating from it is accurate to ±3–5 days. Clinical practice follows accordingly — when early ultrasound and LMP dating disagree by more than about 5–7 days, the due date is re-dated from the scan, and your conception estimate should move with it: always compute from the most recently confirmed due date. (Planning forward instead of backward? The ovulation calculator maps the fertile window in an upcoming cycle, and the pregnancy calculator runs the week-by-week timeline from your dates.)
Special cases where the math changes
- IVF — the one case with a truly known answer: fertilization happened in the lab. Count from the retrieval/fertilization date; a day-5 embryo transfer means conception was 5 days before transfer.
- IUI — conception date ≈ the procedure date, give or take a day.
- Irregular cycles — skip LMP math entirely; date from ultrasound.
- Conceived while traveling / time zones — irrelevant at this precision; the window is ±5 days, a time zone is ±1.
What conception dating can and can't tell you
It can answer the questions people actually bring to it: reconstructing the timeline ("was it the trip in April?" — see the month table), satisfying curiosity about a positive test (home tests typically turn positive 12–15 days after conception), and syncing your own story with the medical chart's two-weeks-ahead numbering. It can't name the exact day, and it can't resolve paternity when the window overlaps more than one possibility — DNA testing exists precisely because date math never can. Treat every result — including our calculator's — as a well-reasoned window, and let the ultrasound be the referee.
Frequently Asked Questions
How do I work out when I conceived from my due date?
Subtract 266 days (38 weeks) from your due date — that's the most likely conception day, with a realistic window of about ±5 days because sperm survive up to five days before ovulation. Due January 15? Conception was around April 24 of the previous year.
Why does my doctor say I'm 6 weeks pregnant when I conceived 4 weeks ago?
Medicine counts pregnancy from the first day of your last menstrual period (LMP), which is about two weeks before ovulation and conception. So gestational age always runs roughly two weeks ahead of actual time since fertilization — '6 weeks pregnant' means about 4 weeks since conception.
Can a conception calculator tell me the exact day?
No method can. Conception happens at ovulation, which can follow intercourse by up to five days, and ovulation timing itself varies cycle to cycle. Every honest answer is a window of several days — the calculator gives you the most likely day plus that window.
Which is more accurate: LMP, due date, or ultrasound?
First-trimester ultrasound, by a wide margin — crown-rump length dating is accurate to ±3–5 days because early fetal growth is very uniform. LMP-based dating assumes a textbook 28-day cycle and can be off by 1–2 weeks. If ultrasound and LMP disagree by more than about 5–7 days early on, clinicians re-date from the ultrasound.
Can conception dating determine paternity?
No. Conception windows are estimates that can span multiple days and overlap more than one possibility. Only DNA testing establishes paternity — use conception math for dating the pregnancy, not for legal conclusions.