How is the conception date calculated from the LMP?

Ovulation — and thus conception — typically occurs 14 days before the end of a 28-day menstrual cycle, or equivalently about 14 days after the start of the LMP for a standard cycle. For cycles longer or shorter than 28 days, the ovulation day shifts accordingly. This calculator adds (cycle length − 14) days to your LMP date to estimate conception.

What is the fertile window?

The fertile window spans roughly 6 days: the 5 days before ovulation and the day of ovulation itself. Sperm can survive up to 5 days in the female reproductive tract, so intercourse before ovulation can still result in pregnancy. This calculator shows ±5 days around the estimated conception date as an approximation of this window.

How accurate is this calculator?

These are estimates based on average cycle timing. Actual ovulation day varies from person to person and from cycle to cycle. For precise tracking, methods like basal body temperature (BBT) charting, ovulation predictor kits (OPKs), or ultrasound dating are more reliable.

Why is the due date 266 days after conception?

Human gestation is approximately 38 weeks (266 days) from the date of conception, or 40 weeks (280 days) from the last menstrual period. The standard Naegele's rule adds 280 days to the LMP, which is equivalent to adding 266 days to the conception date when ovulation occurs at day 14 of a 28-day cycle.

Can I use this to determine paternity timing?

This calculator provides an estimated conception window, not a precise moment. Conception can occur several days after intercourse because sperm can survive in the reproductive tract. For medical or legal questions about paternity, consult a healthcare professional.

Conception Date Calculator

Understanding when conception occurred is one of the most common questions during pregnancy. Whether you want to estimate how far along you are, cross-check an ultrasound date, or simply satisfy your curiosity, this calculator uses established obstetric principles to give you a reliable estimate of your conception date, fertile window, and expected due date.

What Is a Conception Date Calculator?

A conception date calculator uses two key pieces of information — either your last menstrual period (LMP) or your known due date — to estimate when fertilisation most likely occurred. It is based on the same arithmetic that obstetricians and midwives use to establish gestational age.

The calculator also outputs your fertile window: the range of days during which intercourse was most likely to result in pregnancy. This is useful for understanding the timing of conception even if the exact date of intercourse is uncertain.

The Science Behind the Calculation

Human ovulation typically occurs approximately 14 days before the end of a menstrual cycle. For a standard 28-day cycle, this means ovulation on day 14 after the LMP. However, menstrual cycles vary: if your cycle is 32 days long, ovulation typically happens around day 18 (32 − 14 = 18). This calculator accounts for your specific cycle length.

Conception can only occur during a brief window: the egg is viable for roughly 12–24 hours after ovulation. However, sperm can survive in the female reproductive tract for up to 5 days. This creates the fertile window: approximately 5 days before ovulation through the day of ovulation itself.

Once fertilised, the egg takes about 6–10 days to implant in the uterine wall. Pregnancy tests become positive only after implantation, when hCG hormone begins to rise.

How to Use This Calculator

Choose your calculation mode — select “From last menstrual period (LMP)” if you know the first day of your last period, or “From due date” if you already have an established due date.
Enter the date — input your LMP start date or due date in the required field.
Enter your cycle length — this is the number of days from the first day of one period to the first day of the next. The default is 28 days. If your cycles are irregular, use your best average estimate.
Review your results — the calculator displays your estimated conception date, fertile window, and (if you calculated from LMP) your estimated due date.

Examples

Example 1: From Last Menstrual Period

Scenario: Your last period started on 1 March 2024. Your average cycle length is 28 days.

Days to ovulation: 28 − 14 = 14 days
Estimated conception date: March 1 + 14 days = 15 March 2024
Fertile window: 10 March – 20 March 2024
Estimated due date: 15 March + 266 days = 5 December 2024

Example 2: Longer Cycle

Scenario: LMP was 1 April 2024, cycle length is 35 days.

Days to ovulation: 35 − 14 = 21 days
Estimated conception date: April 1 + 21 days = 22 April 2024
Estimated due date: 22 April + 266 days = 14 January 2025

Example 3: From Due Date

Scenario: Your confirmed due date is 20 September 2024.

Estimated conception date: 20 September − 266 days = 28 December 2023
Fertile window: 23 December 2023 – 2 January 2024

Frequently Asked Questions

Why do doctors count pregnancy from the LMP and not from conception? Doctors use the LMP because it is a concrete, observable date — conception itself cannot be directly observed. The standard system counts 40 weeks (280 days) from the LMP for a due date. Since ovulation occurs roughly 14 days into a 28-day cycle, this is equivalent to 38 weeks (266 days) from actual conception. This is a clinical convention, not a biological claim that pregnancy lasts 40 weeks from an embryo’s perspective.

Can conception happen on multiple days? Technically, fertilisation happens at a single moment when one sperm penetrates the egg. However, the sperm that causes the pregnancy could have been deposited up to 5 days before ovulation. This is why we speak of a “fertile window” rather than a single conception date.

My ultrasound date differs from this calculator — which is correct? Ultrasound dating, especially in the first trimester, is considered the gold standard for gestational age. If your ultrasound date differs by more than 5–7 days from LMP dating, clinicians typically adjust to the ultrasound date. This calculator provides an estimate; always defer to clinical measurements.

Does this calculator work for IVF pregnancies? IVF pregnancies use a different dating system because the exact date of embryo transfer is known. For IVF, the gestational age at embryo transfer is assigned based on the embryo’s development stage (typically 3 or 5 days post-retrieval). Consult your fertility specialist for precise IVF dating.

What if my periods are irregular? This calculator is less accurate for people with highly irregular cycles. If your cycles vary by more than 5–7 days from month to month, tracking methods like basal body temperature (BBT) charting, cervical mucus monitoring, or ovulation predictor kits (OPKs) will give more reliable estimates.