How to Calculate Your Conception Date: From Your Last Period, Due Date or Ultrasound

To calculate your conception date from your last menstrual period (LMP), count 14 days forward from the first day of that period if your cycles are about 28 days long. If you already know your due date, count back 266 days (38 weeks). If you've had an early ultrasound, use the scan's gestational age to work back to the same date. In all three cases the result is an estimate, and the honest answer is a window of several days, not a single date.

This guide walks through each method with real dates, shows how irregular cycles change the answer, explains the IVF shortcut, and covers the question many couples actually care about: why the date on the ultrasound report doesn't match the date they remember.

First, what "conception" means

Conception (fertilisation) happens when a sperm fuses with an egg. That is not the same as the day you had sex.

• An egg survives for roughly 12 to 24 hours after ovulation.
• Sperm can survive inside the female body for up to about 5 days.
• So sex up to five days before ovulation, and on the day of ovulation, can lead to pregnancy. Sex after ovulation has passed doesn't.

This is why the "fertile window" is about 6 days long, and why a conception date calculated from the calendar is only ever the likeliest day within that window. Your actual fertilisation might have happened on the day you had sex, or up to five days later.

Pregnancy dating in medicine doesn't even start at conception. It starts at the first day of your last period, about two weeks before conception. That is why a pregnancy is called "40 weeks" even though the baby has only been developing for about 38.

Method 1: from your last menstrual period (LMP)

The 28-day cycle shortcut

In a textbook 28-day cycle, ovulation falls on about day 14, counting the first day of bleeding as day 1. Fertilisation typically follows within hours.

Conception date ≈ LMP + 14 days (or, more precisely, LMP + 13 days if you count the first day as day 1).

Worked example

Your LMP started on 10 March 2026.

• Ovulation (day 14): around 23 March
• Likely conception: 23 to 24 March
• Realistic window: roughly 18 to 24 March (the fertile window, which starts five days before ovulation)
• Due date: 15 December 2026 (LMP + 280 days)

If your cycle isn't 28 days

Ovulation usually happens about 14 days before the next period, and the second half of the cycle (the luteal phase) is fairly constant, while the first half varies. So:

Ovulation day ≈ cycle length − 14

| Cycle length | Expected ovulation day | Fertile window (cycle days) | |---|---|---| | 21 days | Day 7 | Day 2 to 8 | | 24 days | Day 10 | Day 5 to 11 | | 26 days | Day 12 | Day 7 to 13 | | 28 days | Day 14 | Day 9 to 15 | | 30 days | Day 16 | Day 11 to 17 | | 32 days | Day 18 | Day 13 to 19 | | 35 days | Day 21 | Day 16 to 22 | | 38 days | Day 24 | Day 19 to 25 | | 40 days | Day 26 | Day 21 to 27 |

Worked examples with different cycle lengths

Same LMP of 10 March 2026:

• 24-day cycle: ovulation around day 10, which is 19 March. Conception around 19 to 20 March. Adjusted due date: 11 December 2026.
• 28-day cycle: ovulation 23 March. Due date 15 December 2026.
• 35-day cycle: ovulation around day 21, which is 30 March. Conception around 30 to 31 March. Adjusted due date: 22 December 2026.

You can see how a seven-day change in cycle length moves conception by a full week and the due date by a week.

When this method works badly

• Irregular cycles (more than about 7 to 9 days of variation between cycles)
• You're not sure of your LMP date
• You conceived within a few months of stopping hormonal contraception, or soon after a miscarriage or childbirth, while cycles are still settling
• You were breastfeeding
• Bleeding in early pregnancy was mistaken for a period

In these cases, ultrasound dating is more reliable.

Method 2: from your due date (EDD)

If you have the due date and want to know the conception date, subtract.

Conception date ≈ due date − 266 days (38 weeks)

Worked example

Due date: 15 December 2026. 15 December − 266 days = about 24 March 2026.

Your LMP, if the due date was calculated from a regular 28-day cycle, would be 280 days before the due date: 10 March 2026.

Which "due date" are you using?

Doctors use the term EDD (estimated date of delivery) for the date they've settled on. It may have come from the LMP or from an early scan. If your doctor changed the date after a scan, use the new one. The conception date you calculate from it is the one the medical record is built around, even if it doesn't match your memory.

Naegele's rule and the 280-day count

You may have heard of Naegele's rule: subtract 3 months from the first day of the LMP and add 7 days. It assumes a 28-day cycle and ovulation on day 14. The 280-day count gives almost the same answer, and sometimes differs by a day or two because months have different lengths. With an LMP of 10 March 2026, Naegele's rule gives 17 December, while counting 280 days gives 15 December. Neither is "wrong." They're two ways of rounding the same estimate.

Method 3: from an ultrasound

An early scan measures the embryo or foetus and compares its size with standard growth charts to estimate gestational age. Once you know gestational age, working back to conception is straightforward.

Conception date ≈ scan date − (gestational age in days − 14)

Worked example

On 5 May 2026, a scan shows 8 weeks 0 days. That is 56 days from the (dating) LMP, so the dating LMP is 10 March 2026. Conception, assuming ovulation at day 14, is about 23 to 24 March.

How accurate are scan dates?

Crown-rump length (CRL), measured between roughly 8 and 14 weeks, is accurate to about 5 to 7 days (StatPearls review of estimated date of delivery). That's the best dating method we have in routine care, and it's most accurate the earlier it's done. After about 22 weeks, the American College of Obstetricians and Gynecologists (ACOG) considers dating by ultrasound less reliable, because babies vary more in size as they grow.

When does the scan date replace the LMP date?

Doctors follow rules like these (ACOG, as summarised in StatPearls):

• Up to about 9 weeks, if the scan and the LMP differ by more than 5 days, the scan date is used.
• In the rest of the first trimester, if they differ by more than 7 days, the scan date is used.
• Later in pregnancy, the allowed gap gets bigger (up to around 3 weeks in the third trimester), because ultrasound is less precise.
• If you've had more than one scan, the earliest one that gives a gestational age is used.

That is why the "estimated conception date" on your report may not match your memory or your LMP. The scan has probably moved the date by a few days, and your clinic follows that date from then on.

Method 4: IVF and assisted conception

Here the conception date is known exactly, which makes dating easier. Count from the embryo transfer date.

• Day 5 (blastocyst) transfer: due date = transfer date + 261 days
• Day 3 transfer: due date = transfer date + 263 days

Worked example

A day 5 transfer on 1 June 2026 gives a due date of 17 February 2027. A day 3 transfer on the same date gives 19 February 2027.

Fertilisation in the lab happened 5 days before a day 5 transfer (for example, 27 May), but pregnancy dating counts as if conception happened 14 days after an imaginary LMP, so the "gestational age" is calculated as 19 days plus the embryo age at transfer. Your fertility clinic will give you the dates; follow theirs.

Why the conception date matters

For most people, it's curiosity. Medically, though, it can matter in a few situations:

• Dating the pregnancy accurately determines when screening tests are done, and when to decide on induction if you go past your date.
• Checking whether growth is on track. An error of a week or two can make a normal baby look small or large.
• Paternity questions. A conception window may support or rule out certain possibilities, although only DNA testing can answer them.
• Planning around a contraceptive failure or an assault. If you may need emergency contraception or other medical care, contact a doctor straight away. Calendar methods aren't a substitute.

Why you might not get a single date

• Sperm survive for days, so one act of intercourse can lead to fertilisation up to five days later.
• Ovulation can shift by several days from one cycle to the next, even in women with regular periods, because of stress, illness, travel or lack of sleep.
• Implantation occurs about 6 to 10 days after conception, and the pregnancy test turns positive only after that. It doesn't tell you when you conceived.
• Early pregnancy dating by symptoms (nausea, tiredness) isn't reliable.

A realistic answer sounds like "most likely around 23 March, give or take about five days."

Quick calculation guide

| You know | Do this | Accuracy | |---|---|---| | LMP and a regular 28-day cycle | LMP + 14 days | Within about a week | | LMP and a different regular cycle | LMP + (cycle length − 14) days | Within about a week | | Irregular cycles | Use ultrasound dating | Less reliable by calendar | | Due date | Due date − 266 days | As good as the due date | | Early scan | Scan-based gestational age, then back-calculate | About 5 to 7 days | | IVF | Transfer date − embryo age; due date = transfer + 261 (day 5) | Exact |

Frequently asked questions

How to find conception date from due date?

Subtract 266 days (38 weeks) from your due date. If your due date is 15 December, your estimated conception date is about 24 March.

How to calculate conception date from LMP?

For a 28-day cycle, add 14 days to the first day of your last period. For other cycle lengths, ovulation is about 14 days before your next expected period.

What is the estimated conception date on my ultrasound report?

It's the date the report calculates from the gestational age found at the scan, assuming ovulation about two weeks after the start of that gestational age. It can be several days different from your own calculation, especially if your cycles are irregular.

Can the conception date be different from the date of intercourse?

Yes. Sperm can survive for up to about 5 days, so fertilisation may happen days after intercourse.

How accurate is a conception date calculator?

It's an estimate. Expect to be within about a week with regular cycles and further off with irregular ones.

How do I calculate the month of conception?

Subtract about 38 weeks (roughly 8.7 months) from your due date. A due date in mid-December means conception in the second half of March.

Does the conception date change the due date?

It can, indirectly. If an early ultrasound shows a baby is smaller or larger than your LMP suggests, your doctor may adjust the due date and therefore the working conception date.

When to see a doctor

Book an early visit if you have missed a period and think you're pregnant, especially if you have severe one-sided pain, heavy bleeding or fainting, which can be signs of an ectopic pregnancy and need immediate care. Also speak to a doctor if your cycles are very irregular, you've been trying for a year (six months if you're over 35) without success, or you have a history of miscarriage.

Care at Prakash Hospital Noida

Whether you live in Sector 18, Sector 62, Greater Noida West, or anywhere nearby, Prakash Hospital Noida is a trusted name for antenatal care and women's health in Noida. Our doctors can confirm your dates with an early scan and guide you through each stage of pregnancy.

To book a consultation, call the number.

Sources: Estimated Date of Delivery. StatPearls, National Center for Biotechnology Information (NBK536986), summarising ACOG dating guidance and Naegele's rule.

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