Conception From Due Date—Reverse Calculation | Math Fix

If you have a due date and you’re trying to figure out when conception likely happened, you’re not alone. The catch: most due dates are estimates built from menstrual dating, not a known conception day. So the best answer is a date range with clear assumptions.

This guide shows the reverse calculation in plain steps, plus quick checks that tell you when your number is solid and when it’s a rough window. If you know ovulation or IVF dates, you can narrow it.

For quick reference, the phrase conception from due date—reverse calculation usually points to the same back-counting you’ll see below: due date minus 266 days, then a window. And keep these steps handy.

What You Know	What Your Due Date Likely Means	Reverse Calculation That Fits
Due date set by early ultrasound (first trimester)	Dating is anchored to measured gestational age	Conception window: due date − 266 days, then a small window
Due date set from last menstrual period (LMP)	EDD assumes a 28-day cycle with ovulation near day 14	Estimate ovulation: LMP + (cycle length − 14); conception near ovulation
Known conception date (tracked ovulation or one exposure)	EDD can be built forward from conception	EDD check: conception + 266 days should land near your due date
IVF embryo transfer date	Conception timing is known within embryo age	Use clinic dating; or transfer + 261 days (day-5 embryo) as a cross-check
Irregular cycles or recent birth control stop	LMP dating can drift	Use the earliest ultrasound date/gestational age for best back-calculation
Due date moved after a scan	Provider chose the dating method they trust more	Use the revised due date for all reverse math
Only a “weeks pregnant” number today	That count is gestational age from LMP, not fetal age	Conception is often about two weeks after the gestational age start
Twins or higher-order multiples	Care plans differ, but dating still uses gestational age	Back-calculate from the established due date, then confirm with chart notes

Conception From Due Date—Reverse Calculation With A Simple Rule

In most pregnancies, the due date is built from a 280-day count starting at the first day of the last menstrual period. That convention is widely used in obstetrics. ACOG’s methods for estimating the due date lays out that 280-day standard and how clinicians confirm dating.

Conception happens later than that “day one,” because the first days of gestational age are counted before ovulation. In a textbook 28-day cycle, ovulation falls near day 14. So a clean reverse rule is:

• Estimated conception date = due date − 266 days (38 weeks)
• Estimated LMP date = due date − 280 days (40 weeks)

If your due date came from an early ultrasound, the “minus 266 days” line often gets you close. If your due date came from LMP and your cycles run shorter or longer than 28 days, shift the window by the same amount.

What A Due Date Is Counting

There are two clocks that get mixed up in casual talk:

• Gestational age starts at the first day of the last menstrual period. It’s the clock used in prenatal care and due date dating.
• Fetal age starts near conception. It usually trails gestational age by about two weeks in people with a 28-day cycle.

That gap is the whole reason reverse calculation needs a bit of care. When someone says “I’m 10 weeks pregnant,” that is usually 10 weeks gestational age. Conception for that pregnancy often lands near 8 weeks earlier.

Why Your Result Should Be A Range

Even with a clean due date, biology isn’t a single dot on a calendar. Sperm can live several days in the reproductive tract, and the egg is viable for a shorter time after ovulation. Implantation also varies. So treat any calculated “conception date” as the center of a small window, not a guaranteed day.

Step-By-Step Reverse Calculation You Can Do On Paper

You don’t need a calculator. A few steps get you a solid estimate:

1. Write down the due date that your clinician is using now. If it was revised after a scan, use the revised one.
2. Subtract 266 days. That lands on an estimated conception day for a pregnancy dated from LMP conventions.
3. Build a window. Add a few days on either side to reflect ovulation timing and sperm survival.
4. If your cycle length differs from 28 days, shift the center. Add (cycle length − 28) days to the estimate if you ovulate later; subtract that amount if you ovulate earlier.

Quick LMP check: Subtract 280 days from the due date to get the estimated first day of the last period. Then add your usual cycle length minus 14 to estimate ovulation.

Cycle Length Adjustment Without Guesswork

If you track cycles and your “day 1 to day 1” length is steady, you can adjust with simple math:

• 30-day cycle: ovulation tends to land about 2 days later than the 28-day pattern, so shift conception about 2 days later.
• 26-day cycle: ovulation tends to land about 2 days earlier, so shift conception about 2 days earlier.

This is still an estimate. Stress, illness, travel, breastfeeding, and recent hormonal contraception can move ovulation dates. If any of that fits your timeline, lean on early ultrasound dating if you have it in your chart.

When Ultrasound Dating Beats Calendar Math

If you had a first-trimester ultrasound that set or confirmed your due date, it tightens the back-calculation. Early scans estimate gestational age using fetal measurements, then anchor the due date with ACOG’s methods for estimating the due date.

If your clinic notes say “dating by ultrasound,” use your established due date and keep your conception window tighter. If your notes say “dating by LMP,” keep the window wider, especially with irregular cycles.

The UK’s NHS also frames due date as an estimate and notes the typical range of normal birth timing, which helps when you’re mapping a conception window back from a single calendar day. See the NHS due date calculator page for how the LMP method is used and the usual term range.

Special Cases That Change The Math

IVF And Embryo Transfer

With IVF, clinics know the egg retrieval day, fertilization timing, and embryo age at transfer. That makes conception dating much tighter than typical cycle dating. If you have a transfer date, use the clinic’s calculated due date as the anchor and ask for the “gestational age on transfer day” from your paperwork if you want to back-calculate cleanly.

Known Ovulation Or LH Surge

If you used ovulation predictor kits, basal body temperature, or cervical mucus tracking, you may have a solid ovulation day. In that case, you can treat ovulation as the center of your conception window. Back-calculate from the due date, then compare it with your tracked ovulation day. If the gap is large, your due date may be based on a later ultrasound adjustment.

Irregular Cycles

If cycle length swings month to month, “LMP plus 280 days” can miss by a lot. In that case, your best clue is often the earliest ultrasound that produced a gestational age. Use that due date, then apply the 266-day reverse rule and keep a wider range.

Worked Calendar Checks That Catch Common Mistakes

These quick checks stop the two biggest mix-ups: confusing LMP with conception, and using an outdated due date.

Check	What To Do	What It Tells You
EDD-to-conception check	Due date − 266 days	Center of the conception window for many pregnancies
EDD-to-LMP check	Due date − 280 days	Estimated first day of last period used in gestational dating
Cycle length shift	Add (cycle length − 28) days to the conception estimate	Moves the center toward your usual ovulation timing
Ultrasound anchor check	Use the due date listed after your first trimester scan	Matches how clinicians date the pregnancy
“Weeks pregnant” sanity check	Conception is often two weeks less than gestational age	Prevents mixing gestational age with fetal age
Term window reality check	Birth can be normal across a span of weeks	Reminds you why the result is a window, not a promise

What To Write Down For A Clean Timeline

If you’re building a timeline for your own records, write down the inputs and the assumptions. It makes the result easier to defend later if dates matter for legal or medical paperwork. Save it later.

Checklist For Your Notes

• The due date used in your chart right now
• How it was set: LMP, ultrasound, IVF dating, or unknown
• Your typical cycle length, if you know it
• Any known ovulation or LH surge day
• The reverse-calculated conception center (due date − 266 days)
• Your chosen window width (narrow for IVF, wider for irregular cycles)

How Accurate Is The Reverse Calculation?

Accuracy depends on how the due date was set. Early ultrasound dating can be tight, while LMP dating depends on recall and cycle regularity. Most births don’t land exactly on the due date, so conception from due date—reverse calculation is best treated as a range.

If you need a timeline for a sensitive reason, use your medical record dates. Ask your clinic for the dating basis, the ultrasound gestational age used, and any date revisions. That paperwork beats back-of-the-napkin math.

Simple Wrap-Up For Today

Use your current due date. Subtract 266 days for the center, then widen the window for cycle length and dating method. Anchor to early ultrasound if you have it.