Conception Date Calculator | When Did I Conceive?

Q: Can I pinpoint an exact conception date?

No. Because sperm can survive up to 5 days and the egg is viable for 12–24 hours, conception could occur anywhere within a 6-day fertile window. A conception date calculator gives a most likely range, not a single exact day.

Calculate your estimated conception date using either your pregnancy due date or the first day of your last menstrual period.

Calculate Conception Date

Your pregnancy due date (EDD)

This subtracts 266 days (38 weeks) from your due date to estimate conception.

First day of last menstrual period (LMP) Average cycle length (days)

Adds cycle length minus 14 days to your LMP to estimate ovulation and conception date.

Estimated Conception Details

Remember: Conception dates are estimates. The fertile window spans 5–6 days, so the actual conception date may be up to 5 days earlier than the ovulation estimate. An early ultrasound is the most accurate way to date your pregnancy.

How Conception Date Calculators Work

There are two standard methods for working backwards to estimate when conception occurred. Both are estimates — the exact date of conception cannot be determined with certainty by a calculator alone.

Method 1: Working Back from the Due Date

Pregnancy is conventionally dated as 40 weeks from the first day of the last menstrual period. Since conception typically occurs around 2 weeks after the LMP (at ovulation), the fetal age at birth is approximately 38 weeks from conception.

Therefore: Conception date = Due date minus 266 days (38 weeks)

Example: If your due date is 1 December 2025, counting back 266 days gives an estimated conception date of approximately 8 March 2025.

Method 2: Working Forward from the Last Period

If you know the first day of your last menstrual period and your cycle length, you can estimate ovulation day — and therefore the likely conception window.

Ovulation day = LMP + (cycle length minus 14 days)

Conception most likely occurred on or within 5 days before the estimated ovulation day.

Example: LMP 1 February 2025, 28-day cycle → Ovulation estimated day 14 (15 February 2025). Conception window: 10–15 February 2025.

Why Exact Conception Dates Are Uncertain

Several biological factors make it impossible to pinpoint a single exact conception date:

Sperm survival

Sperm can remain viable in the female reproductive tract for up to 5 days. This means conception could result from intercourse up to 5 days before the egg was released.

Egg viability window

The released egg is only viable for 12–24 hours. Fertilisation must occur within this narrow window after ovulation.

Variable ovulation timing

Ovulation does not always occur on the same cycle day. Stress, illness, or cycle irregularities can shift ovulation by several days.

Implantation timing

After fertilisation, the embryo travels to the uterus and implants 6–12 days later. This is when hCG (the pregnancy hormone) begins to rise.

Gestational Age vs Fetal Age — What Is the Difference?

This distinction confuses many expectant parents:

Term	Counted From	Full Pregnancy	Used By
Gestational age	First day of LMP	40 weeks	Midwives, doctors
Fetal age	Conception date	38 weeks	Embryologists

When your midwife says "you are 12 weeks pregnant", they mean 12 weeks from your LMP — the baby is actually approximately 10 weeks old in terms of fetal development.

The Role of Early Ultrasound in Dating

The most accurate way to establish a pregnancy's gestational age is an early ultrasound scan. In the UK, the NHS offers a dating scan between 11 and 14 weeks of pregnancy. This scan measures the crown-to-rump length (CRL) of the fetus and compares it with standard growth charts to calculate gestational age.

Before 10 weeks, the CRL measurement can date a pregnancy to within 3–5 days
Between 11–14 weeks, accuracy is within 5–7 days
After 20 weeks, accuracy decreases to plus or minus 2 weeks
If the scan date differs from LMP-calculated date by more than 7 days (before 14 weeks), the scan date is used

NHS guidance: The NHS dating scan (also called the 12-week scan) is offered to all pregnant women in England. It is voluntary but strongly recommended as it confirms viability, checks for twins, and establishes the most accurate due date.

Multiple Possible Conception Dates

Because the fertile window spans approximately 6 days, there is genuinely a range of possible conception dates for any pregnancy. This is not just a matter of calculator accuracy — it is a biological reality. A calculator provides the most probable date, which is typically the day of or just before ovulation, but conception could have occurred several days earlier.

This has practical implications in situations where knowing the exact conception date matters, such as paternity questions. In these situations:

A conception window of 5–6 days should be considered, not a single date
Ultrasound dating adds further uncertainty of plus or minus 5–14 days depending on gestation at scan
Only DNA paternity testing can definitively establish biological paternity

Frequently Asked Questions

How do I calculate my conception date from my due date?

Subtract 266 days (38 weeks) from your due date to get the estimated conception date. This is because pregnancy is counted as 40 weeks from the LMP, but actual conception typically occurs 2 weeks later (at ovulation), giving 38 weeks from conception to full term.

What is the difference between gestational age and fetal age?

Gestational age counts from the first day of your last menstrual period (LMP) — this is the standard used by midwives and doctors in the UK. Fetal age (embryonic age) counts from the actual date of conception, which is usually about 2 weeks less than gestational age. A fetus described as 12 weeks gestational age is approximately 10 weeks old from conception.

Can I pinpoint an exact conception date?

No. Because sperm can survive up to 5 days in the reproductive tract and the egg is only viable for 12–24 hours, conception could occur within a 6-day fertile window. A calculator provides a most likely range — typically centred on the estimated ovulation day — not a single exact day.

What is the most accurate way to date a pregnancy?

An early ultrasound scan (ideally before 12 weeks) is the most accurate method. The NHS dating scan at 11–14 weeks measures the crown-to-rump length and can date the pregnancy to within 5–7 days. This is more accurate than LMP-based calculations, especially for women with irregular cycles.

Are conception dates used in paternity cases?

Conception date estimates can be relevant in paternity disputes as supporting evidence, but because conception windows span several days and ultrasound has its own margin of error, they cannot definitively establish or exclude paternity. DNA paternity testing — which can be done during pregnancy (non-invasive prenatal paternity test) or after birth — is the only reliable legal standard for paternity.

Why is pregnancy dated from the last period and not conception?

The date of the last menstrual period (LMP) is a known, easily recalled date, whereas the actual date of conception is usually not known with certainty. This standardised system, developed before ultrasound, allows consistent communication between healthcare professionals. It is why pregnancy is described as 40 weeks even though fetal development is only 38 weeks from fertilisation.

What if I have an irregular cycle?

If your cycle is irregular, the LMP-based calculation is less reliable. An early ultrasound provides more accurate dating in this case. You can adjust the cycle length in the "From Last Period" tab above to reflect your typical cycle duration, but the most reliable dating will come from your NHS 12-week scan.

Related Calculators

Due Date Calculator Fertile Window Calculator Pregnancy Weight Gain Calculator

Last updated: February 2026. This calculator provides estimates only and does not replace professional medical advice. Always discuss pregnancy dating with your midwife or GP. Written by Mustafa Bilgic.