Due Date Calculator from Last Period: How It Works

The moment you see a positive pregnancy test, your first question is likely “when is my baby due?” A pregnancy due date calculator from last period uses a simple but well-established formula called Naegele’s Rule to estimate your delivery date based on the first day of your last menstrual period (LMP). This guide explains exactly how the calculation works, how to adjust for irregular cycles, and how accurate due dates really are. Once you know your due date, explore our pregnancy week by week guide to follow along with baby’s development — so you understand the number your doctor gives you. Try our interactive due date calculator for an instant result.

📌 Key Takeaway: A pregnancy due date calculator from last period adds 280 days (40 weeks) to the first day of your last menstrual period using Naegele’s Rule. This assumes a 28-day cycle with ovulation on day 14. If your cycle is shorter or longer, an adjustment is needed. Only about 5% of babies arrive on their exact due date — most are born within a two-week window around it. Schedule your first prenatal appointment to confirm your dates with an ultrasound.

How the Due Date Calculator from Last Period Works: Naegele’s Rule

Naegele’s Rule is the standard formula used by obstetricians worldwide to estimate delivery dates. Named after German obstetrician Franz Naegele (1778-1851), it remains the foundation of the pregnancy due date calculator from last period method.

The Formula

Estimated Due Date (EDD) = First day of LMP + 280 days (40 weeks)

Or, as a shortcut:

EDD = LMP + 1 year - 3 months + 7 days

Step-by-Step Calculation Example

Let’s walk through a manual calculation using the pregnancy due date calculator from last period method:

Step	Action	Example
1	Write down the first day of your last menstrual period	January 15, 2026
2	Add one year	January 15, 2027
3	Subtract 3 months	October 15, 2026
4	Add 7 days	October 22, 2026

In this example, a last menstrual period starting January 15, 2026 produces an estimated due date of October 22, 2026.

💡 Tip: Don’t want to do the math? Our due date calculator does this instantly. Just enter the first day of your last period and your average cycle length for a personalized result with your full trimester timeline.

Why 280 Days?

Naegele’s Rule assumes:

• A 28-day menstrual cycle
• Ovulation on day 14 of the cycle
• Fertilization occurs within 24 hours of ovulation
• Full-term pregnancy lasts 266 days from conception (38 weeks)
• Since your LMP starts 14 days before conception: 266 + 14 = 280 days

This means a “40-week pregnancy” is really only 38 weeks from actual conception — the first two weeks of your “pregnancy” occur before you even conceive.

Adjusting for Cycle Length: What if Your Cycle Isn’t 28 Days?

Naegele’s Rule assumes a 28-day cycle, but cycle lengths vary significantly. If your cycle is consistently shorter or longer than 28 days, you need to adjust the pregnancy due date calculator from last period result.

Cycle Length Adjustment Table

Your Cycle Length	Adjustment	How to Adjust	Example (LMP Jan 15)
21 days	Subtract 7 days	Ovulated earlier (day 7 instead of 14)	Oct 15, 2026
23 days	Subtract 5 days	Ovulated day 9	Oct 17, 2026
25 days	Subtract 3 days	Ovulated day 11	Oct 19, 2026
26 days	Subtract 2 days	Ovulated day 12	Oct 20, 2026
27 days	Subtract 1 day	Ovulated day 13	Oct 21, 2026
28 days	No adjustment	Standard (ovulated day 14)	Oct 22, 2026
29 days	Add 1 day	Ovulated day 15	Oct 23, 2026
30 days	Add 2 days	Ovulated day 16	Oct 24, 2026
32 days	Add 4 days	Ovulated day 18	Oct 26, 2026
35 days	Add 7 days	Ovulated day 21	Oct 29, 2026

📊 Key Data: According to a study of over 4,000 pregnancies published in the journal Human Reproduction, the average cycle length is actually 29.3 days — not 28. Women with longer cycles tend to ovulate later, which means Naegele’s Rule may overestimate gestational age slightly for them. This is one reason ultrasound dating is often preferred for accuracy.

The Formula with Cycle Adjustment

Adjusted EDD = Standard EDD + (Cycle Length - 28)

For example, if your cycle is 32 days:

• Standard EDD: October 22, 2026
• Adjustment: 32 - 28 = +4 days
• Adjusted EDD: October 26, 2026

How Accurate Is a Due Date from Last Period?

Understanding the accuracy of the pregnancy due date calculator from last period is important for managing expectations.

Statistic	Data
Babies born on their exact due date	~5%
Babies born within 1 week of due date	~50%
Babies born within 2 weeks of due date	~80%
Average first-time pregnancy length	41 weeks, 1 day
Average subsequent pregnancy length	40 weeks, 3 days
”Full term” window (ACOG definition)	39 weeks 0 days through 40 weeks 6 days

⚠️ Important: Your due date is an estimate, not a deadline. ACOG defines the full-term window as 39-40 weeks, with “early term” at 37-38 weeks and “late term” at 41 weeks. Induction is typically discussed if you haven’t delivered by 41 weeks (7 days past due date).

Why Due Dates from LMP Can Be Inaccurate

Several factors can make the LMP-based calculation less reliable:

1. Irregular cycles — if your cycle length varies month to month, predicting ovulation day is difficult
2. Uncertain LMP date — if you’re not sure of the exact first day of your last period
3. Breakthrough bleeding — some women have light bleeding in early pregnancy that they mistake for a period
4. Late ovulation — even with regular cycles, ovulation doesn’t always happen on day 14
5. Recently stopping birth control — cycles may take several months to regulate

Ultrasound Dating vs. LMP Dating: Comparison

When your ultrasound due date differs from the LMP calculation, your provider needs to decide which is more accurate.

Factor	LMP (Naegele’s Rule)	First-Trimester Ultrasound
When used	From positive pregnancy test	Typically at 8-12 weeks
What it measures	Calculates from period start date	Measures baby’s size (crown-rump length)
Accuracy	± 2-3 weeks	± 3-5 days
Assumes	28-day cycle, day 14 ovulation	Embryos grow at predictable rates in early pregnancy
Best for	Women with regular 28-day cycles who know their exact LMP	Most accurate overall; standard for dating
Limitations	Inaccurate with irregular cycles or uncertain LMP	Less accurate after 13 weeks (individual variation increases)

💡 Tip: If your ultrasound date and LMP date differ by more than 7 days in the first trimester (or more than 10-14 days in the second trimester), your provider will typically use the ultrasound date as your official due date. This is because first-trimester ultrasound is more accurate than any calculation method.

When Each Method Is Preferred

Scenario	Recommended Method
Regular 28-day cycle, certain LMP date, no early ultrasound	LMP (Naegele’s Rule)
Irregular cycles or uncertain LMP	First-trimester ultrasound
IVF pregnancy	Transfer date (most precise — known conception date)
LMP and ultrasound dates agree (within 7 days)	Either; typically LMP is kept
LMP and ultrasound dates disagree (more than 7 days)	Ultrasound date used

Your Trimester Date Ranges Based on Due Date

Once you have your estimated due date from the pregnancy due date calculator from last period, here’s how the trimesters break down.

Trimester	Week Range	Duration	Key Milestones
First	Weeks 1-13	~13 weeks	Organ formation, heartbeat detectable, first prenatal visit
Second	Weeks 14-27	~14 weeks	Anatomy scan (week 20), feeling movement, gender visible
Third	Weeks 28-40	~13 weeks	Viability, fetal positioning, labor preparation
Early Term	Weeks 37-38	2 weeks	Baby ready but still building lung surfactant and brain connections
Full Term	Weeks 39-40	2 weeks	Optimal delivery window per ACOG
Late Term	Week 41	1 week	Induction typically discussed
Post Term	Week 42+	—	Induction usually recommended

Key Dates to Calculate from Your Due Date

Once you know your due date, you can work backward to plan your pregnancy.

Event	How to Calculate	Purpose
Conception date	Due date minus 266 days (38 weeks)	Estimated date of fertilization
End of first trimester	LMP + 13 weeks	Miscarriage risk drops significantly
Anatomy scan	LMP + 18-22 weeks	Detailed ultrasound; gender can be determined
Viability milestone	LMP + 24 weeks	Baby has chance of survival if born early
Third trimester starts	LMP + 28 weeks	Begin birth preparation, hospital bag planning
Full term begins	LMP + 39 weeks	Safe delivery window opens
Due date	LMP + 40 weeks	Estimated delivery date

Use our due date calculator to generate all of these dates automatically based on your LMP. The tool provides a complete pregnancy timeline so you can plan ahead for each milestone.

FAQ

How do I calculate my due date from my last period?

Use the pregnancy due date calculator from last period method (Naegele’s Rule): take the first day of your last menstrual period and add 280 days (40 weeks). Or use the shortcut — add 1 year, subtract 3 months, and add 7 days. Our due date calculator does this instantly and also adjusts for your cycle length.

How accurate is a due date based on last menstrual period?

A due date from the pregnancy due date calculator from last period method is accurate to within about 2-3 weeks for women with regular 28-day cycles. Only about 5% of babies are born on their exact due date, but approximately 80% are born within 2 weeks of it. A first-trimester ultrasound is more accurate (within 3-5 days).

What if I don’t know the date of my last period?

If you can’t remember your last period date, a first-trimester ultrasound (ideally between 8-12 weeks) can estimate your due date based on the baby’s size. This measurement — called crown-rump length — is the most accurate dating method available and is preferred over the LMP calculation when the period date is uncertain.

Can my due date change during pregnancy?

Yes. If an early ultrasound shows your baby is measuring significantly different from the LMP-based due date (more than 7 days difference in the first trimester), your provider will adjust your official due date to match the ultrasound. This happens in about 15-20% of pregnancies, most often in women with irregular cycles.

References

• American College of Obstetricians and Gynecologists. “Methods for Estimating the Due Date.” Committee Opinion No. 700. acog.org
• Jukic AMZ, et al. “Length of human pregnancy and contributors to its natural variation.” Human Reproduction. nih.gov
• Mayo Clinic. “Due Date Calculator.” mayoclinic.org
• March of Dimes. “Due Date.” marchofdimes.org
• National Institutes of Health. “How is gestational age determined?” nichd.nih.gov

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