Conception Calculator

A conception calculator runs the reverse of a due date calculator. Given when the baby is expected, it backs out the most likely conception date using the same math obstetricians use to estimate due dates in the first place. Standard math: due date = LMP + 280 days, and conception = LMP + (cycle length − 14). For a typical 28-day cycle, ovulation/conception happens on day 14 after LMP, and due date is exactly 266 days later. So if your due date is October 15, conception was likely around January 22 of the same year. The calculator on this page accepts three input modes — due date, LMP, or ultrasound — because not every pregnant person has reliable access to all three pieces of information. Pick the date you know most confidently and you'll get the same conception estimate any OB would calculate.

If you have a due date — from a pregnancy test result, prenatal app, or OB appointment — the conception date is straightforward: due date − 266 days (38 weeks). Alternatively: due date − 280 days = LMP, then LMP + 14 days = conception. Worked example: due date October 15, 2026 → LMP January 8, 2026 → conception January 22, 2026. This is the mode our conception date calculator from due date uses by default because most pregnant people learn their due date first (from a pregnancy app or the first OB visit) and then want to figure out the conception backstory. Note: this assumes a standard 28-day cycle. For cycles outside the 26-32 day range, adjust the cycle length input to recalibrate ovulation timing.

The traditional obstetric method dates pregnancy from the first day of the last menstrual period (LMP). This is the start of the cycle that resulted in pregnancy — NOT the day of intercourse or fertilization. Conception happens approximately on cycle day 14 (mid-cycle ovulation) and due date is LMP + 280 days. This is called Naegele's rule, formulated in 1812 by German obstetrician Franz Karl Naegele, and it remains the global standard. Why date from LMP instead of conception? Because most women remember the start of their last period precisely but rarely know the exact day they conceived. Our pregnancy conception calculator with LMP mode runs the LMP + (cycle length − 14) formula to estimate conception, then adds 280 days to estimate due date.

Ultrasound dating in the first trimester (8-13 weeks) is the most accurate pregnancy dating method available — typically accurate to within 3-5 days, vs 7-14 days for LMP-based estimates. The ultrasound measures the embryo's crown-rump length (CRL), which corresponds tightly to gestational age in the first trimester. If LMP dating and ultrasound dating disagree by more than 7 days in the first trimester, OBs typically use the ultrasound date going forward (this is called 'dating override'). Our calculate conception date from ultrasound mode takes the scan date and reported gestational age (e.g. '8 weeks 3 days') and reverse-calculates the conception date by subtracting gestational days from the scan date and adding 14 days. This bypasses the cycle-length variable entirely — ultrasound dating assumes the standard 14-day post-LMP conception regardless of actual cycle length.

These three terms refer to distinct biological events that happen days apart. CONCEPTION (also called FERTILIZATION): the moment sperm penetrates the egg, forming a zygote with 46 chromosomes. Happens in the fallopian tube within 24 hours of ovulation. IMPLANTATION: the zygote (now a blastocyst, 5-7 days into rapid division) embeds in the uterine wall lining. Happens 6-12 days after conception. This is when hCG hormone production starts, which is what home pregnancy tests detect. INTERCOURSE DATE: the day sperm was deposited. Can be 0-5 days BEFORE conception because sperm survives in fertile cervical mucus. So 'when did I get pregnant?' can have three different answers depending on which event you mean: the date of conception (fertilization), the date of intercourse, or the date of implantation. Our conception calculator outputs the FERTILIZATION date — the standard obstetric definition.

Each menstrual cycle has a fertile window of 5-6 days during which pregnancy is possible. It includes the 5 days BEFORE ovulation plus the day of ovulation itself. Peak fertility is the 2-3 days immediately before ovulation. Why is the window 5 days when the egg only survives 24 hours? Because sperm can survive 3-5 days in fertile cervical mucus, waiting for the egg. Practical implication: sex on Monday with ovulation on Friday can still result in conception, with fertilization happening Friday — not Monday. Our conception window calculator outputs this range as a 5-day span centered on the estimated ovulation date. For active conception planning (versus retrospective dating), our [ovulation calculator](/ovulation-calculator) projects this window forward across multiple cycles.

Naegele's rule and most conception calculators assume a 28-day cycle with ovulation at day 14. But cycles vary — 26-32 days is normal range, and many women have cycles outside this range. Critical insight from reproductive endocrinology: the LUTEAL phase (ovulation to next period) is biologically constant at ~14 days for almost all women. What VARIES is the FOLLICULAR phase (LMP to ovulation). So if your cycle is 32 days, ovulation likely happens around day 18 — not day 14. If your cycle is 24 days, ovulation is around day 10. Our conception calculator adjusts for this by accepting your cycle length input and computing ovulation as (cycle length − 14) days post-LMP. This dramatically improves accuracy for women with non-standard cycles vs the default 28-day assumption.

LMP-based dating and first-trimester ultrasound dating sometimes give different gestational ages. Common reasons: (1) the woman ovulated earlier or later than day 14, throwing off LMP-based estimates; (2) the LMP date was misremembered or there was breakthrough bleeding mistaken for a period; (3) measurement error in the CRL ultrasound (usually small, ±2 days). When the two methods differ by more than 7 days in the first trimester, ACOG/RCOG/SOGC/RANZCOG all recommend using the ultrasound date as the official pregnancy dating going forward. This 'dating override' affects everything downstream: the due date, scheduling of NIPT and combined screening, monitoring for preterm or post-term labor, and induction timing. Our conception calculator's ultrasound mode reflects this preference — when used, it bypasses LMP entirely.

Scenario 1: Due date October 15, 2026, 28-day cycle. → LMP Jan 8, conception Jan 22, conception window Jan 19-23. Scenario 2: LMP February 1, 2026, 32-day cycle. → Ovulation/conception ~Feb 19 (LMP + 18, not +14), due date Nov 8. Scenario 3: 8 weeks 3 days at ultrasound on April 1, 2026 (standard 28-day cycle assumed). → LMP Feb 6, conception Feb 20, due date Nov 13. Scenario 4: 12 weeks 0 days at ultrasound on May 15, 2026. → LMP Feb 21, conception Mar 6, due date Nov 28. The estimate conception date calculator handles all four automatically — switch input modes and the underlying math adjusts. For real OB care, your provider will pick whichever method (LMP or ultrasound) gives the most accurate dating based on your specific situation.

IVF (in-vitro fertilization) pregnancies use a different dating convention because the conception date is precisely known. Fertilization happens in the lab on the day of egg retrieval. The embryo is then transferred 3-5 days later (day-3 cleavage embryo OR day-5 blastocyst). Standard IVF dating: due date = embryo transfer date + 263 days (for day-5 transfer) or + 261 days (for day-3 transfer). LMP for IVF pregnancies is back-calculated as due date − 280 days for compatibility with standard obstetric tracking. Our conception calculator is not IVF-specific — for IVF dating, use our [pregnancy due date calculator](/pregnancy-due-date-calculator) which has dedicated IVF mode. Frozen embryo transfers (FET) follow the same date math but the 'biological conception date' (original egg retrieval) may have been months or years earlier, irrelevant for pregnancy dating.

Most home pregnancy tests detect human chorionic gonadotropin (hCG) starting 10-14 days after conception. hCG isn't produced at conception itself — it starts at IMPLANTATION (6-12 days post-conception) and rises rapidly thereafter. Early-detection tests (claiming '6 days before missed period') work because they detect at lower hCG thresholds (10-25 mIU/mL) than standard tests (25-50 mIU/mL). Practical guide: if you conceived around day 14 of a 28-day cycle, expect a reliable positive test by day 28-30 (around or just after the expected period). Testing before day 25 has high false-negative rates because hCG hasn't risen high enough yet. Blood tests at a clinic detect hCG earlier (as low as 5 mIU/mL) and can confirm pregnancy 8-10 days post-conception.

Mistake 1: Confusing conception date with date of intercourse. Sperm can survive 5 days in fertile mucus, so the conception (fertilization) date can be up to 5 days AFTER the sex that caused it. Mistake 2: Assuming day-14 ovulation regardless of cycle length. Ovulation is 14 days BEFORE the next period, not 14 days after the last one. For non-28-day cycles, this matters significantly. Mistake 3: Thinking pregnancy 'starts' at conception. Obstetrically, pregnancy is dated from LMP, which is ~2 weeks BEFORE conception. So at 'week 8 of pregnancy', the embryo is biologically 6 weeks old. Mistake 4: Trusting calculator results when cycles are irregular. If cycles vary by more than 7 days, LMP-based conception estimates can be off by 1-3 weeks. Use ultrasound dating instead. Mistake 5: Confusing conception with implantation. Implantation bleeding happens ~7-10 days post-conception, not at conception itself.