How Do You Calculate Your Pregnancy Due Date? | Date Math

Due dates feel like a single day on the calendar, but the math behind them is simple: you’re estimating when pregnancy will reach 40 weeks of gestational age. That estimate helps you plan appointments, track growth milestones, and set expectations for when labor might begin.

This article shows the main ways clinicians estimate a due date, how to do the calendar math at home, and why your date may shift after an ultrasound. You’ll get step-by-step methods for regular cycles, irregular cycles, known ovulation dates, and IVF timing.

What A Due Date Means In Real Life

A due date is an estimate of when you reach 40 weeks of gestational age. Gestational age is counted from the first day of the last menstrual period (LMP), not from the day you conceived. That’s why “40 weeks pregnant” usually means conception happened around two weeks after the LMP for someone with a 28-day cycle.

Most babies arrive within a range, not on the exact date. Many pregnancies go into labor between 37 and 42 weeks from the LMP, and your care team uses that window when judging what’s normal and when extra monitoring may be needed. The NHS due date calculator notes this 37–42 week range.

Calculating A Pregnancy Due Date With Calendar Rules

The classic method is often called Naegele’s rule. It assumes ovulation near day 14 of a 28-day cycle and uses the LMP as the anchor.

Method 1: The 280-Day Count From Your Last Period

Start with the first day of your last period. Add 280 days (40 weeks). If you prefer month math, subtract three months, add one year, then add seven days. The Johns Hopkins due date steps lay out that month-based version.

Example

If your LMP was March 2, 2026, counting forward 280 days lands on December 7, 2026. That’s your estimated due date using the standard LMP method.

Method 2: Adjusting For Cycle Length

The 280-day count fits a 28-day cycle. If your average cycle is longer, ovulation tends to happen later; if shorter, ovulation tends to happen earlier. A simple adjustment is:

• Take your average cycle length in days.
• Subtract 28.
• Add that number to the due date you got from the 280-day count.

So, if your cycle is 32 days on most months, the adjustment is +4 days. If your cycle is 26 days, the adjustment is −2 days. This is still an estimate, but it often matches what clinicians expect before an early scan.

Method 3: Using A Known Ovulation Or Conception Date

Sometimes you know the timing of ovulation from ovulation tests, basal body temperature tracking, or a dated procedure. In that case, you can count from conception rather than from the LMP.

• Estimated due date = conception date + 266 days (38 weeks).

This method skips the “two-week offset” built into gestational age. It can feel more intuitive, but it still depends on having a solid conception date.

Method 4: IVF And Embryo Transfer Dating

IVF dating uses the transfer date and the embryo’s age at transfer. Many clinics share a calculated due date on the transfer paperwork. If you want to sanity-check it:

• For a day-5 blastocyst transfer: due date ≈ transfer date + 261 days.
• For a day-3 transfer: due date ≈ transfer date + 263 days.

These are calendar shortcuts. Your clinic’s dating method is the one that matters for your chart.

Why Ultrasound Dating Can Change The Date

Early pregnancy measurements often give the cleanest dating because growth rates are tightly clustered in the first trimester. The ACOG due date dating guidance says the estimated due date should be set once LMP data and the first accurate ultrasound are available, then documented and changed only in limited situations.

Here’s the practical takeaway: if your early ultrasound measurement points to a different gestational age than your LMP suggests, your clinician may shift the due date. That shift is more common when cycles are irregular, the LMP date is uncertain, or ovulation happened later than day 14.

If you’re tracking at home, treat the LMP due date as a starting point. Once you have a dating scan, use the date your care team records.

Inputs That Affect Due Date Math

Due date math works best when you’re clear on your inputs. A small mistake in the LMP date, cycle length, or ovulation timing can move the estimate by days or even weeks.

Use this checklist before you do the calculation:

• Write down the first day of bleeding for your last period, not the last day.
• If spotting happened, note it separately. Many people don’t count light spotting as day 1.
• Use an average cycle length from at least three cycles if you can.
• If you used ovulation tests, note the first positive test and the day after.
• If conception was through IVF, keep the transfer date and embryo day.

For a quick official calculator, the NHS provides an LMP-based tool that also reminds you what details you need to enter.

Table: Common Due Date Methods And When Each Fits

The methods below are used in clinics and in home tracking. Pick the one that matches your situation, then let early ultrasound dating settle the final date when it’s available.

Method	What You Need	Best Fit
LMP + 280 days	First day of last period	Regular cycles, clear LMP date
LMP + 280 days, cycle adjusted	LMP + average cycle length	Consistent cycles that aren’t 28 days
Conception date + 266 days	Known ovulation or conception date	Well-dated conception timing
Ovulation date + 264 days	Ovulation date (often day after LH surge)	Cycle tracking with clear ovulation signal
IVF transfer shortcut	Transfer date + embryo day	IVF cycles with clinic dates
First-trimester ultrasound dating	Scan date + measurement/GA	Irregular cycles or unclear LMP
Combined approach	LMP plus first accurate scan	Most pregnancies after dating scan
Unknown LMP route	Scan dating and clinical history	No reliable LMP, irregular bleeding

How Do You Calculate Your Pregnancy Due Date? At Home In Five Steps

If you want a clean at-home workflow, use this sequence. It keeps the math simple and reduces common errors.

1. Record day 1 of your last period. Use the first day of true flow.
2. Add 280 days. Many phone calendars can count days forward.
3. Adjust for cycle length. Add or subtract the difference from 28 days.
4. Check against known ovulation. If ovulation is well dated, compare with conception+266 days.
5. Update after the dating scan. Use the date your clinician enters in your record.

When you’re doing the count manually, weeks are easier than days: 40 weeks from the LMP. You can mark “week changes” on your calendar to keep track of appointments and milestones.

What Can Make A Due Date Shift After Your First Appointment

A due date may change, and that can feel unsettling. In many cases, it’s just the math catching up to better data. Here are common reasons:

• Uncertain LMP. If you’re not sure of the date, the estimate starts on shaky ground.
• Irregular cycles. Ovulation may not line up with day 14.
• Recent hormonal contraception. Cycles can vary after stopping.
• Bleeding that looked like a period. Early pregnancy bleeding can be confusing.
• Early ultrasound dating. First-trimester measurements may override the LMP estimate.

ACOG’s guidance is that due dating should be established early and changes should be limited and documented. That’s why many practices treat the first accurate scan as a turning point for the official date on file.

Table: Quick Date Math Cheatsheet

This table gives calendar shortcuts you can use while you wait for a scan or while you’re double-checking a calculator result.

If You Know	Add This	Notes
First day of last period	40 weeks	Standard gestational age method
Conception date	38 weeks	Matches conception+266 days
Ovulation day	38 weeks minus 2 days	Some people ovulate after LH surge
Day-5 embryo transfer	261 days	Calendar shortcut, clinic date wins
Day-3 embryo transfer	263 days	Calendar shortcut, clinic date wins
Dating scan gestational age	40 weeks minus GA	Count remaining weeks from scan day

Planning Around The Due Date Without Stress

Once you have an estimated date, it helps to plan in ranges. Many people pick a “ready by” week for work leave, childcare, and travel rather than planning around a single day.

A practical way to do that is to mark three dates on your calendar:

• Early window: 37 weeks from your LMP-based start.
• Due week: the week that includes the estimated date.
• Late window: 41 to 42 weeks, when extra monitoring is common.

March of Dimes on due dates notes the date is a general guide and babies can arrive earlier or later. That mindset helps when plans need a little flexibility.

When To Reach Out For Medical Advice

Due date math is for planning, not for diagnosis. Reach out to your midwife, OB-GYN, or clinic if you have bleeding, severe pain, fever, or a sudden change in symptoms. If you’re unsure about your last period date, mention it at your first visit so dating can be set with the best information available.

If you want an online calculator for a starting estimate, you can use the NHS due date calculator for an LMP-based date, then compare it with your clinic’s dating scan once you have it.