How Pregnancy Due Dates Are Calculated: LMP, Conception, IVF and Ultrasound

Your due date, or EDD (estimated date of delivery), is a calculated date rather than a prediction. Four methods are in common use: your last menstrual period (LMP), a known conception date, an IVF transfer date, or a first trimester ultrasound. This guide explains each one, shows how to calculate LMP based dates by hand, and covers why the numbers sometimes disagree.

Important: This is general information only. It cannot replace advice from your midwife, obstetrician or fertility team. Always discuss your own dates and care plan with a healthcare professional.

The short answer

Add 280 days, or 40 weeks, to the first day of your last period. That is the LMP method, and it is what almost every pregnancy due date calculator does by default, including the SoftBump due date calculator.

If you know your conception date instead, add 266 days. If you had an embryo transfer, use the IVF due date calculator, which subtracts the embryo age. If you have had a dating scan, use the ultrasound due date calculator.

Method comparison at a glance

Method	What you need	Calculation	Typical accuracy
LMP (Naegele's rule)	First day of last period	LMP + 280 days	Plus or minus 1 to 2 weeks with regular cycles
Conception or ovulation	Date of ovulation or conception	Date + 266 days	Good if ovulation was tracked
IVF transfer	Transfer date and embryo age	Transfer + 266 days minus embryo age	The most precise before a scan
First trimester scan	Crown rump length measurement	Gestational age from the scan, counted back	The clinical gold standard up to 14 weeks

Gestational age vs fetal age

Two terms come up constantly and they are not the same thing.

Gestational age counts from the first day of your last menstrual period. This is the number used in appointments, scan reports and most due date conversations.

Fetal age, sometimes called conception age or embryonic age, counts from fertilisation. Because ovulation usually happens about two weeks after your period starts, fetal age runs about two weeks behind gestational age.

So at 10 weeks gestational age, the baby's fetal age is around 8 weeks. Clinicians use gestational age because it links cleanly to calendar dates for scheduling scans and screening tests.

Method 1: Last menstrual period (LMP)

This is the traditional calculation, known as Naegele's rule.

• Take the first day of your last menstrual period, not the day it ended.
• Add 280 days, which is 40 weeks.

An equivalent shortcut is to add one year, subtract three months and add seven days. If your LMP was 10 March 2026, that gives 17 December 2026.

The method assumes a regular 28 day cycle with ovulation on day 14. That assumption is doing a lot of work, which is why the next section matters.

Adjusting for cycle length

If your cycles are consistently longer or shorter than 28 days, you ovulate later or earlier than day 14, and the plain LMP date drifts by the same amount.

• Adjusted due date = LMP + 280 days + (your cycle length minus 28)

A 35 day cycle pushes the due date seven days later. A 24 day cycle pulls it four days earlier. The SoftBump calculator applies this adjustment for you when you enter your cycle length.

If your cycles are irregular, or you were breastfeeding, recently stopped hormonal contraception, or had bleeding you were unsure about, LMP dating becomes unreliable and a scan is the better source of truth.

Finding the first day of your LMP

The first day of your LMP is the first day of full bleeding, not spotting. If you do not know it, you are not unusual. Roughly one in four people cannot recall it confidently. In that case, an early scan replaces the estimate entirely.

You can also work backwards. If you already have a due date and want to know your LMP or likely conception window, the reverse due date calculator does that in one step.

Method 2: Known conception or ovulation date

If you tracked ovulation with tests, temperature charting or a fertility app, you may know the conception window within a day or two.

• Due date = conception date + 266 days, which is 38 weeks.

The 14 day difference from the LMP method is exactly the gap between the start of your period and ovulation in a textbook cycle. You can work this out with the conception due date calculator, and check ovulation timing with the ovulation calculator.

Even with careful tracking, sperm can survive several days, so fertilisation may not happen on the day of intercourse. Clinicians usually treat conception dating as helpful context and still confirm with a scan.

Method 3: IVF and embryo transfer dates

IVF removes most of the guesswork, because the lab knows the exact age of the embryo on the day it was transferred.

• Due date = transfer date + 266 days minus embryo age

A day 5 blastocyst transferred today is due 261 days from now. A day 3 embryo is due 263 days from now. A day 6 blastocyst is due 260 days from now. Fresh and frozen transfers are dated identically, because what matters is the embryo's age, not when it was created or thawed.

This is usually the most precise date available before the first scan. The IVF due date calculator handles day 3, day 5 and day 6 transfers, and the IVF gestational age calculator tells you how many weeks pregnant you are on any given day after transfer.

Method 4: Ultrasound dating

A first trimester scan measures the crown rump length, the distance from the top of the head to the bottom of the torso, and compares it with reference growth charts.

• The scan gives a gestational age on the day it was taken.
• Counting forward to 40 weeks gives the due date, and counting backwards gives an implied LMP.

Early growth follows a tight pattern, so dating between roughly 8 and 14 weeks is accurate to within about five days. After 14 weeks, natural size variation widens and dating becomes less reliable, which is why late scans are used to check growth rather than to re-date a pregnancy.

When the scan overrides your LMP date

Most guidelines re-date the pregnancy when the scan and the LMP date differ by more than a set threshold, commonly five to seven days in the first trimester. Below that threshold, the LMP date usually stands. Once a due date has been set by an early scan, it is not normally changed again.

Why the methods disagree

• Cycle length and ovulation timing. Anything other than a 28 day cycle with day 14 ovulation shifts the LMP estimate.
• Recall. Period start dates are easy to misremember, and irregular bleeding blurs the starting point.
• Natural variation. Babies are not identical in size at the same age, so scan based estimates carry their own spread.
• Rounding. Clinics round gestational age differently, which explains many one or two day gaps between an app and a clinic letter.

A difference of a few days between two methods is normal and rarely changes anything about your care.

A due date is a window, not an appointment

Only a small minority of babies arrive on the calculated date. Most births happen across a range of weeks either side of it, and terms like early term, full term and late term exist precisely because of that spread. There is more on that in early, full and late term explained.

Once you have a date you trust, the how many weeks pregnant am I calculator shows where you are today, and the trimester calculator turns it into the exact date ranges for each trimester.

A gentle reminder

If you are ever worried about your dates, your baby's movements, bleeding, pain or any other symptom, contact a healthcare professional urgently. No online calculator or article, including this one, can safely diagnose or manage those situations.