How is the conception date calculated?

The conception date is estimated based on ovulation timing. For a standard 28-day menstrual cycle, ovulation occurs around day 14. The calculator adjusts this based on your actual cycle length. From a due date, it works backward by subtracting 266 days (38 weeks) since that's the average time from conception to birth.

Can I find out my conception date from my birthday?

Yes, the Birth Date method estimates when you were conceived based on your birthday. It assumes a standard 40-week pregnancy and works backward to find the estimated conception date. You can adjust the pregnancy duration (36–42 weeks) if you know the actual gestation length. Keep in mind this is a rough estimate since exact pregnancy durations vary.

What is the fertile window and why does it span 7 days?

The fertile window is the range of days during which intercourse could have led to pregnancy. It spans about 7 days because sperm can survive in the female reproductive tract for up to 5 days before ovulation, and the egg remains viable for approximately 24 hours after release. Conception itself occurs at ovulation, but intercourse anywhere within this window could result in that conception.

Is conception date the same as the date of intercourse?

Not necessarily. Conception (fertilization) occurs when a sperm meets the egg, which happens at ovulation. Since sperm can survive 3–5 days inside the body, intercourse could have happened several days before the actual fertilization. The conception date estimates when fertilization likely occurred, not when intercourse took place.

Why is pregnancy counted from the last menstrual period instead of conception?

Healthcare providers count pregnancy from the LMP (last menstrual period) because it's a more reliably known date — most women know when their last period started, but may not know exactly when they ovulated or conceived. This is why pregnancy is said to last 40 weeks (from LMP) even though the actual time from conception to birth is about 38 weeks.

How does cycle length affect the conception date estimate?

Cycle length shifts the estimated ovulation day. In a 28-day cycle, ovulation occurs around day 14. In a 32-day cycle, it shifts to about day 18 (ovulation typically happens 14 days before the next period, not 14 days after the last one). Entering your actual cycle length makes the conception estimate more accurate.

Which calculation method should I use for the most accurate result?

If you've had a first-trimester ultrasound, use the Ultrasound method — it's the most accurate for pregnancy dating. If not, the LMP method with your correct cycle length is the next best option. The Due Date method is useful if your doctor has already given you an estimated due date. The Birth Date method is least precise and best for curiosity (e.g., 'When was I conceived?').

Conception Calculator

Calculate From

First Day of Last Period

The date your most recent period started

Average Cycle Length28 days

20 days45 days

28 days is the average. Leave as-is if unsure.

Estimated Conception Date

March 25, 2026

10 weeks, 0 days·First Trimester

Based on Last Menstrual Period

Fertile Window

The 7-day window during which intercourse could have led to this conception, accounting for sperm survival (up to 5 days) and egg viability (~24 hours)

Mar 20Mar 25Mar 26

Earliest intercourseLikely conceptionLatest

Key Dates

Important dates for your pregnancy timeline

Estimated LMP

Mar 11, 2026

First day of last period

Conception Date

Mar 25, 2026

Most likely date of fertilization

Fertile Window Start

Mar 20, 2026

Earliest intercourse that could lead to conception

Fertile Window End

Mar 26, 2026

Latest intercourse that could lead to conception

Estimated Due Date

Dec 16, 2026

40 weeks from estimated LMP

210 days until estimated due date

All dates are estimates. The actual conception date can vary by several days depending on ovulation timing and individual cycle variations.

How This Was Calculated

Naegele’s Rule with cycle length adjustment

Ovulation typically occurs about 14 days before the next period. With a 28-day cycle, ovulation is estimated on day 14 of the cycle.

Formula

Conception = LMP + 14 days

How the Conception Calculator Works

Medically accepted formulas to estimate when fertilization occurred

Conception is the moment a sperm fertilizes an egg, typically occurring during ovulation — about 14 days before the start of your next menstrual period. This calculator uses established obstetric formulas to estimate that date based on the information you provide.

LMP MethodLast period + cycle length

Due DateReverse from EDD

UltrasoundGestational age scan

Birth DateReverse from birthday

Why not just count back 9 months?

The "9 months" rule is a rough estimate. A full-term pregnancy is actually 280 days (40 weeks) from the last menstrual period, or 266 days (38 weeks) from conception. Your cycle length also shifts the ovulation day, making a personalized calculation more accurate than simple month-counting.

Calculation Methods & Formulas

Four approaches to estimate your conception date

Last Menstrual Period (LMP) — The most common method used by healthcare providers. Ovulation typically occurs 14 days before the next period, so for a 28-day cycle, ovulation is on day 14. Longer or shorter cycles shift this day accordingly.

LMP method

Conception = LMP + 14 + (cycle length − 28)

Due Date (Reverse) — Works backward from your estimated due date. Since the interval from conception to birth is 266 days (38 weeks):

Due date method

Conception = Due Date − 266 days

Ultrasound — Uses the gestational age measured during an ultrasound to back-calculate the LMP date, then estimates conception 14 days later. First-trimester ultrasounds are the most accurate for dating (±5–7 days).

Ultrasound method

LMP = US Date − GA days, then Conception = LMP + 14

Birth Date (Reverse) — Estimates when conception occurred based on an actual birth date and known pregnancy duration. Useful for answering "When was I conceived?"

Birthday method

LMP = Birthday − (gestation weeks × 7), then Conception = LMP + 14

Which method is most accurate?

First-trimester ultrasound is the gold standard (±5–7 days). LMP with correct cycle length is next best. Due date reversal is reliable if your EDD was ultrasound-confirmed. The birthday method has the widest margin because exact gestation length varies.

How to Use This Calculator

Step-by-step guide to finding your conception date

Choose your method

Select the method that matches the information you have. LMP is the most commonly used; Birth Date is for reverse-calculating from a birthday.

Enter your date

Pick the date from the calendar. For LMP, this is the first day of your last period. For due date, enter the EDD from your doctor.

Adjust settings (if applicable)

For LMP: set your average cycle length (20–45 days). For birthday: adjust gestation weeks if the baby was premature or overdue.

Review the fertile window

The calculator shows the most likely conception date and a 7-day fertile window showing which days intercourse could have led to this conception.

Check key dates

Review the estimated LMP, due date, and fertility window. Use the share button to save or share your results.

Understanding the Fertile Window

Why intercourse days before ovulation can still lead to conception

Conception (fertilization) happens at a single point — when sperm meets egg at ovulation. But because sperm can survive for days beforehand, intercourse across a wider window can lead to that same conception:

Sperm Viability

Sperm can survive 3–5 days in the female reproductive tract, waiting for an egg to be released during ovulation.

Egg Lifespan

Once released during ovulation, the egg remains viable for fertilization for approximately 12–24 hours.

Total fertile window

~6–7 days

Intercourse before ovulation

Up to 5 days

Intercourse day of ovulation

Highest chance

Peak fertility

1–2 days before ovulation

Intercourse date ≠ conception date

Because sperm can survive for days, the date of intercourse is often different from the actual date of fertilization. If intercourse occurred 3 days before ovulation, fertilization still happens at ovulation — not on the day of intercourse.

Accuracy by Method

How precise each calculation method is

Method	Accuracy	Best when
Ultrasound (1st tri)	±5–7 days	Scan done before 12 weeks
LMP + cycle length	±5–7 days	Regular, predictable cycles
Due date (reverse)	±5–7 days	EDD confirmed by ultrasound
Birth date (reverse)	±7–14 days	Known gestation length

Most Accurate

Ultrasound

1st trimester scan

Most Common

LMP

Regular cycles

For Curiosity

Birthday

Widest margin

Factors That Affect Accuracy

Why your actual conception date may differ from the estimate

Irregular cycles

Ovulation day unpredictable

Late or early ovulation

Can shift by 1–2 weeks

Stress, illness, or travel

Can delay ovulation

PCOS or hormonal conditions

Irregular ovulation timing

Cycle length estimate error

Each day off shifts result

Improve accuracy

Track cycle length over 3–6 months
Use early ultrasound dating
Track basal body temperature
Use ovulation predictor kits (OPKs)

Common mistakes

Assuming ovulation is always day 14
Confusing spotting with a true period
Using intercourse date as conception date
Guessing cycle length instead of tracking

Important Considerations

Medical context and when to consult a professional

This calculator provides estimates for educational purposes only. The actual date of conception cannot be determined precisely without clinical testing. Only about 4–5% of babies are born on their exact due date, and conception timing is similarly approximate.

For medical decisions

Consult your provider

Paternity questions

DNA testing required

Most reliable dating

Early ultrasound

Pregnancy confirmation

See your OB-GYN

Not for paternity determination

While this calculator can narrow down when conception likely occurred, it cannot determine paternity. The 7-day fertile window means multiple dates of intercourse could correspond to the same pregnancy. Only DNA testing can confirm biological parentage.

Frequently Asked Questions

Common questions and detailed answers

Conception calculators provide estimates, not exact dates. Accuracy depends on the method used: ultrasound dating in the first trimester is most accurate (±5–7 days per ACOG), LMP-based calculations assume regular cycles and can be off by a week or more, and birthday-based reverse calculations have the widest margin of error. The fertile window shown (7 days) reflects the range of intercourse dates that could have led to conception.

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Last updated Apr 3, 2026