Early Pregnancy

Conception Date Calculator

When did you conceive? Work it out from last period, due date, or IVF transfer. Plus the 4-6 day fertile window explained, gestational age vs conception age, and what the dating scan actually says.

Last reviewed 29 May 2026

Estimated conception window

When did I most likely conceive?

What you know

Pick a method and enter the relevant date to see your conception window.

When did I conceive?

Conception happens at fertilisation — when sperm meets egg — usually within 24 hours after ovulation. For a regular 28-day cycle: ovulation on day 14, so conception is around day 14-15.

For other cycle lengths:

  • Conception day = LMP + (cycle length − 14)
  • 32-day cycle → conception day 18.
  • 25-day cycle → conception day 11.

Why is conception a window, not a single date?

  • Sperm survival: 3-5 days inside the female reproductive tract in fertile cervical mucus.
  • Egg viability: only 12-24 hours after ovulation.
  • Result: fertilisation can happen anywhere in a 4-6 day window centred on ovulation.

Unless conception was via IVF (when the day is known exactly), the actual fertilisation date can be a few days before, on, or just after the cycle’s ovulation day.

How accurate is working backwards from due date?

Working back from an LMP-based EDD assumes 28-day cycle with ovulation day 14. Real cycles vary, so the back-calculated conception date is approximate — usually within ±3-5 days.

A first-trimester CRL ultrasound between 6-14 weeks is the most accurate dating tool. If your dating scan moved your EDD, use the scan date to work back from. For IVF pregnancies, the conception date is known exactly.

Gestational age vs conception age

  • Gestational age: counted from first day of LMP. Used in all clinical documentation.
  • Conception age (embryonic age): counted from fertilisation. About 2 WEEKS LATER than gestational age.

So at “12 weeks pregnant” (gestational), the embryo is actually about 10 weeks old (conceptional). The 280-day (40-week) due-date count includes those 2 pre-conception weeks.

What is implantation?

Fertilised egg embedding in the uterine wall. Happens 6-12 days after ovulation (peak 9-10 days). This is when hCG production begins. Some women notice implantation bleeding — light pink/brown spotting lasting hours to 2 days. Most don’t notice anything. Failed implantation (~30% of all conceptions) results in a slightly late period — often unnoticed.

Different scenarios — how to estimate

Scenario 1: Regular 28-day cycle, LMP 1 January

Ovulation ~15 January. Conception window: 10-16 January. Approximate conception date: 15 January.

Scenario 2: Cycle 35 days, LMP 1 January

Ovulation ~22 January (LMP + 35 − 14). Conception window: 17-23 January. Approximate conception date: 22 January.

Scenario 3: IVF day-5 blastocyst transfer 15 February

Conception (fertilisation in lab) = transfer date − 5 days = 10 February. Known exactly.

Scenario 4: Dating scan moved EDD from 8 October to 15 October

New EDD − 266 days = new conception date about a week later than originally calculated. If LMP suggested conception on 15 Jan, scan suggests conception ~22 January.

Scenario 5: Paternity question — need to know exact conception

Date calculations can’t answer this with certainty — the 4-6 day fertile window means intercourse with different partners during that window can both biologically lead to the pregnancy. Definitive answer requires DNA testing — non-invasive prenatal paternity testing (NIPP) from 7-8 weeks via maternal blood; or postnatal DNA testing after delivery.

Can I tell exactly when I conceived?

Only with specific data:

  • IVF — embryologist documented fertilisation date.
  • One-off intercourse — you know the day, but still 24h window for fertilisation.
  • OPK + BBT tracking — narrows window to 2-3 days.

Otherwise it’s an estimate based on cycle date. For practical purposes (paternity, work timing, etc.), use the wider 4-6 day window unless you have IVF or scan data.

How to confirm pregnancy

  • Home pregnancy test: from 10 days post-ovulation; most reliable from missed period.
  • Blood test: 6-8 days post-ovulation can detect early hCG.
  • Ultrasound: gestational sac at 4 weeks 3 days; heartbeat at 5.5-6.5 weeks.

See /calculators/pregnancy-test-timing for detailed guidance.

Care guidance — after possible conception

  • Start prenatal vitamin immediately. Folic acid 400 mcg/day (5 mg if higher risk).
  • Stop alcohol immediately.
  • Stop smoking.
  • Reduce caffeine to under 200 mg/day.
  • Check medications for pregnancy safety.
  • Eat well — Mediterranean-style.
  • Take a pregnancy test from day of missed period.
  • Book GP / midwife visit after positive test.

Limitations of this calculator

  • Conception is a window (4-6 days), not a single day.
  • Working back from LMP assumes 28-day cycle — less accurate for irregular cycles.
  • Dating ultrasound is more accurate than calculator-based estimates.
  • Doesn’t determine paternity — DNA testing needed.
  • Educational only.

Sources

  • Wilcox AJ, et al. Time of implantation of the conceptus and loss of pregnancy. N Engl J Med 1999.
  • Wilcox AJ, et al. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995.
  • ACOG Committee Opinion 700. Methods for Estimating the Due Date.
  • NICE NG201. Antenatal care.
  • Robinson HP, Fleming JE. A critical evaluation of sonar “crown-rump length” measurements. Br J Obstet Gynaecol 1975.

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Frequently asked questions

When did I conceive?
Conception happens at FERTILISATION — when sperm meets egg — which is usually within 24 hours after ovulation. For a regular 28-day cycle: ovulation on day 14 of cycle (counting from first day of last period), so conception is around day 14-15. For OTHER cycle lengths: conception = LMP + (cycle length - 14). So 32-day cycle → conception day 18. 25-day cycle → conception day 11. Use the calculator with what you know (LMP, EDD, or IVF transfer date) and it gives you the date range.
Why is conception a window, not a single date?
Sperm can survive 3-5 DAYS inside the female reproductive tract waiting for an egg. The egg itself is viable for only 12-24 HOURS after ovulation. So fertilisation can happen any time within a 4-6 day window centred on ovulation. Unless conception was via IVF (when the day is known exactly), the actual date of fertilisation can be a few days before, on, or just after the cycle's ovulation day.
How accurate is working backwards from due date?
Working back from an LMP-based EDD assumes a 28-day cycle with ovulation on day 14. Real cycles vary, so the back-calculated conception date is APPROXIMATE — usually within ±3-5 days. A FIRST-TRIMESTER CRL ULTRASOUND between 8 and 13 weeks is the most accurate dating tool. If your dating scan moved your EDD, use the scan date to work back from. For IVF pregnancies, the conception date is known exactly.
Can I tell exactly when I conceived?
Only if you have specific data: (1) IVF — embryologist documented fertilisation date. (2) ONE-OFF intercourse — you know the exact day; but still could be earlier (sperm survival) or later (fertilisation up to 24h post-ovulation). (3) OPK + BBT tracking — narrows window to 2-3 day range. Otherwise it's an estimate based on cycle date. For practical purposes (paternity questions, work timing), use the WIDER 4-6 day window unless you have IVF or scan data.
Why does my dating scan say a different conception date?
Ultrasound CRL between 6-14 weeks is the most accurate dating tool — accurate to ±5 days vs LMP-based dating which can be off by a week or more (especially with irregular cycles or breakthrough bleeding misread as a period). ACOG Committee Opinion 700 says: if CRL scan EDD differs from LMP EDD by more than 5-7 days early on, the scan EDD becomes the official one. Conception date works back from the EDD — so if scan moved your EDD, your conception date also shifts.
What if I had intercourse with different partners around conception time?
Paternity questions need lab testing, not date calculations. The 4-6 day fertile window means conception could have been from intercourse 5 days BEFORE the cycle's ovulation day, ON the day of ovulation, or up to 24h AFTER. So intercourse with different partners within that 6-day window can both biologically lead to the pregnancy — only DNA testing can determine paternity. Prenatal paternity testing (cell-free fetal DNA from maternal blood) available from 7-8 weeks. Postnatal paternity testing (baby's DNA) definitive.
How does conception date relate to gestational age?
GESTATIONAL AGE is counted from first day of LMP. CONCEPTION is about 2 WEEKS LATER (around ovulation, typically day 14 of 28-day cycle). So when clinicians say 'you're 12 weeks pregnant', the embryo / fetus is actually about 10 weeks old (conceptional age). All clinical documentation uses gestational age. The 280-day (40-week) due-date count includes those 2 pre-conception weeks.
What is implantation and when does it happen?
Implantation = fertilised egg embedding in the uterine wall. Happens 6-12 days after ovulation (peak 9-10 days). This is when hCG production begins. Some women notice IMPLANTATION BLEEDING — light pink/brown spotting lasting hours to 2 days. Most don't notice anything. The pregnancy 'starts' biologically at fertilisation but isn't really 'a pregnancy' until implantation completes 9-10 days later. Failed implantation (~30% of all conceptions) results in a slightly late period — often unnoticed.
Is gestational age the same as how old my baby is?
No. GESTATIONAL AGE counted from LMP. AT BIRTH (typically 39-40 weeks gestational age), baby is born. ACTUAL CHRONOLOGICAL AGE after birth restarts from delivery date. So your baby born at exactly 40 weeks gestation is 0 days old chronologically. CONCEPTIONAL AGE (fertilisation to birth) is about 38 weeks at term-baby birth. The terminology varies: 'pregnancy weeks', 'how far along', 'how many weeks', 'gestation' all generally mean GESTATIONAL age from LMP.
Can sperm really survive 5 days inside me?
Yes — in fertile-quality cervical mucus (clear, stretchy, slippery near ovulation). Average sperm survival 2-3 days; outer range up to 5 days; very rarely 7 days documented. Outside fertile mucus window, sperm survival is much shorter (1-2 days max). Fresh sperm motility 'window of opportunity' for fertilisation is typically the 6 days ending one day after ovulation. This sperm-survival window is why timing intercourse a few days BEFORE ovulation is as effective as on the day itself.
When can I confirm my pregnancy after possible conception?
HOME PREGNANCY TEST: from 10 days post-ovulation with sensitive test (4 days before missed period); most reliable from missed period. BLOOD TEST: ~6-8 days post-ovulation can detect early hCG. ULTRASOUND: gestational sac at 4 weeks 3 days; yolk sac at 5 weeks; fetal pole at 5.5-6 weeks; heartbeat at 5.5-6.5 weeks. See /calculators/pregnancy-test-timing for detailed test-timing guidance.
What happens to my body around conception?
Most women feel nothing at conception itself. After implantation (6-12 days post-ovulation), some early signs may appear: light spotting (implantation bleeding) in ~25%; mild cramping; breast tenderness from rising progesterone; fatigue; mild nausea start around 4-6 weeks; missed period (most reliable). Most early pregnancy symptoms aren't from conception itself but from rising hCG and progesterone over 1-4 weeks post-conception.
Can I get pregnant on the first try?
About 30% of couples conceive in the first cycle of trying. 60% by 6 months; 84-85% by 12 months; 92% by 18 months for couples under 35. AGE 35+: roughly halve those rates. Fertility declines from 30s onwards but most healthy under-35 women conceive within a year. Failure to conceive after 12 months (or 6 months if 35+) = see GP / fertility specialist. Failure first cycle is normal — try again next cycle.
Why didn't I conceive during the fertile window?
Many reasons. (1) TIMING — peak fertility is the 2 days BEFORE ovulation, not the day of. Many couples mistime. (2) NOT EVERY CYCLE IS OVULATORY — even in regular cycles, some are anovulatory. (3) EGG QUALITY — even with healthy ovulation, only ~30% of attempts result in conception per cycle. (4) SPERM ISSUES (semen analysis can clarify). (5) TUBAL / UTERINE ISSUES (less likely if regular cycles). (6) UNDIAGNOSED endometriosis, fibroids, thyroid, hormonal issues. NORMAL FERTILE COUPLES conceive on ~30% of monthly cycles.
Is the conception date what's on the birth certificate?
No — birth certificates show DATE OF BIRTH, not conception. Conception date is rarely officially recorded (except for IVF). For legal / paternity purposes, the relevant data is intercourse dates within the fertile window + DNA testing where needed. UK / US birth certificates show DOB, parents' names, and (where included) gestational age at delivery — not conception date.
How does this relate to other calculators on BumpBites?
Companion: /calculators/due-date for forward calculation (LMP/conception → EDD); /calculators/ovulation for fertile window planning; /calculators/pregnancy-test-timing for when to test; /calculators/implantation for implantation timing; /calculators/baby-size-by-week for fetal development from conception; /calculators/hcg-calculator for tracking early pregnancy.

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