Pregnancy Planning

Due Date CalculatorFind Your Baby's Arrival Date

Enter your last period, conception date, ultrasound results, or IVF transfer date to instantly calculate your due date and current pregnancy week with a free due date calculator.

Free4 input methodsPrivateNo sign-up

Reviewed against ACOG, NHS, and MedlinePlus due date guidance.

How would you like to calculate?

Choose the dating method that fits your situation

Private · Local only

Calculate from Last Period

First day of last menstrual period

Enter the first day of your most recent period.

Average cycle length

21 days45 days

This due date calculator adjusts ovulation timing when your cycle is shorter or longer than 28 days.

Results update instantly as you edit your dates.

Your Estimated Due Date

December 22, 2026

Tuesday

280 days to go · Approximately 9 months remaining

Only about 4% of babies arrive on their exact due date. Most are born within about 2 weeks before or after the estimate.

Current Week

Week 0Day 0

Gestational age 0w 0d

Fetal age about 0w 0d

Pre-viability

Trimester

1️⃣ First trimester

Weeks 1–13

First trimester ends: June 16, 2026

Conception

March 31, 2026

Estimated conception date

Based on the selected dating method

Your Pregnancy Progress

Track where you are in the 40-week timeline

Week 0, day 0 · 0% complete · Pre-viability

Week 0Week 4Week 8Week 13Week 17Week 22Week 27Week 32Week 37Week 40

1st Trimester · Weeks 1–13

2nd Trimester · Weeks 14–26

3rd Trimester · Weeks 27–40

1st TrimesterWeeks 1–13

2nd TrimesterWeeks 14–26

3rd TrimesterWeeks 27–40

Pregnancy Milestones

Key pregnancy milestones

Each milestone date is generated from your estimated LMP, so the entire timeline shifts with the method you select above.

Milestone timeline will load as you scroll into this section. Your next major milestone is Week 4: Positive pregnancy test possible on April 14, 2026.

Understanding Your Trimesters

Open each trimester for the key changes and appointments

First Trimester

Weeks 1–13 · March 17, 2026 – June 16, 2026

What's happening

Major organs and body systems begin forming.
The heartbeat may become visible on ultrasound around week 6.
Nausea and fatigue are especially common during weeks 6 to 12.

Key appointments

First prenatal visit often happens around weeks 8 to 10.
Dating ultrasound usually falls between weeks 8 and 14.
Nuchal translucency or NIPT may happen between weeks 11 and 14.

Second Trimester

Weeks 14–26 · June 16, 2026 – September 15, 2026

What's happening

Many people feel more energy and steadier symptoms.
Quickening often appears between weeks 16 and 22.
The anatomy scan usually happens between weeks 18 and 20.

Key appointments

Anatomy scan and placenta review.
Cervical length or growth checks if indicated.
Planning for glucose screening later in the trimester.

Third Trimester

Weeks 27–40 · September 15, 2026 – December 22, 2026

What's happening

The baby gains weight quickly and lung maturity continues.
Braxton Hicks contractions and position changes become more relevant.
Appointments often become weekly from week 36 onward.

Key appointments

Glucose screening is often completed between weeks 24 and 28.
Group B strep testing often happens between weeks 35 and 37.
Labor planning intensifies as full term approaches.

How It Works

How to Use This Due Date Calculator

This period calculator family page is built to serve four common pregnancy dating scenarios with one due date calculator. The first and most common path is the last period method, also called LMP dating. If you know the first day of your most recent period, you can use it as the anchor date and let the calculator estimate your due date, current pregnancy week, and trimester in real time.

If you know the conception date instead, switch to the conception tab. This is often useful when ovulation was tracked carefully, when the fertile window was narrow, or when a person wants to confirm what week they are in with more targeted timing. If conception timing is unclear, the due date calculator still works best from LMP or ultrasound rather than guesswork.

The ultrasound tab is designed for people who already have a dating scan. Enter the ultrasound date plus the gestational weeks and days reported on the scan. This pregnancy due date calculator will convert that information into an adjusted LMP and estimate the due date from there. For many clinicians, that is the preferred method when last-period timing is uncertain or cycles are irregular.

The IVF tab adds one more layer of precision. Because embryo age is known, IVF dating can be extremely useful for confirming gestational age and due date planning. Across all four methods, the results update instantly, can be exported to calendar, and can be shared through a URL so you can re-open the same due date calculator state later.

Calculation Method

How Due Dates Are Calculated

The standard due date calculator formula comes from Naegele's Rule, a long-standing obstetric method first published in the nineteenth century. In plain terms, it adds 280 days, or 40 weeks, to the first day of the last menstrual period. That works because medical pregnancy dating uses gestational age rather than conception age. In other words, pregnancy week counting starts before fertilization actually occurs.

That is why someone can be described as 8 weeks pregnant even though conception likely happened about 6 weeks earlier. It is also why searches like how to calculate ovulation and due date planning are tightly related. Ovulation usually occurs around 2 weeks after LMP in a textbook 28-day cycle, which is why LMP plus 280 days and conception plus 266 days describe the same pregnancy from different starting points.

When cycles are not 28 days long, a smarter due date calculator can shift the estimate by the cycle difference. A 30-day cycle, for example, often means ovulation happened later, so the due date moves later as well. Ultrasound can refine things even further. A first-trimester scan is usually the most accurate pregnancy dating method because it measures fetal size when growth variation is still relatively small.

Even the best due date calculator still gives an estimate, not a promise. Pregnancy is considered early term at 37 weeks, full term at 39 to 40 weeks and 6 days, late term at 41 weeks, and post-term at 42 weeks or beyond. That is why doctors talk about a birth window rather than a single magic day.

Naegele's Rule

Core formula

Due Date = LMP Date + 280 days (40 weeks)

Or equivalently:
Due Date = LMP Date − 3 months + 7 days + 1 year

Example:
LMP = January 1, 2026
Due Date = October 8, 2026

For cycles not equal to 28 days:
Due Date = LMP + 280 + (cycle length − 28) days

Week By Week

Your Pregnancy Week by Week

Pregnancy week counting starts from the first day of the last menstrual period, not from the conception date. That detail matters because it explains why people often feel confused when they ask, “how many weeks pregnant am I?” or “what week am I in my pregnancy?” and get an answer that seems about 2 weeks ahead of when they think conception happened. The answer is that medical pregnancy dating uses gestational age, not fetal age.

If you want to estimate your weeks pregnant count manually, count the number of days from your LMP to today, divide by 7, and keep the remainder as extra days. A pregnancy week calculator simply automates that process. That same framework is used whether you are 6 weeks pregnant, 12 weeks pregnant, or 34 weeks pregnant. It keeps clinicians, scans, lab work, and trimester milestones aligned on the same clock.

Doctors prefer weeks over months because months are inconsistent. Four months pregnant could mean different things depending on how someone rounds, but 17 weeks pregnant is precise. That precision matters for ultrasound timing, screening windows, anatomy scans, and delivery planning. If you want a faster answer to “how many weeks pregnant am I?” use the calculator above and review the current week card.

Months Pregnant	Gestational Weeks	Trimester
1 month	Weeks 1–4	First
2 months	Weeks 5–8	First
3 months	Weeks 9–13	First
4 months	Weeks 14–17	Second
5 months	Weeks 18–22	Second
6 months	Weeks 23–26	Second
7 months	Weeks 27–30	Third
8 months	Weeks 31–35	Third
9 months	Weeks 36–40	Third

Accuracy

How Accurate Is a Due Date Calculator?

A due date calculator is best understood as a structured estimate. Only a small share of babies are born on the exact due date, and many pregnancies continue into a broad birth window around it. That is normal. Accuracy depends on how much precise information is available about ovulation, conception, or early fetal growth.

For people with regular cycles, the LMP method is often good enough for planning. When cycles are irregular, or when ovulation likely happened earlier or later than expected, the estimate can drift. That is why first-trimester ultrasound is usually considered the most accurate mainstream way to establish a pregnancy due date. IVF is even more targeted because embryo age is already known.

The practical takeaway is simple: the due date calculator is most useful when you pair it with the best available dating information. If the estimate from your last period and the estimate from ultrasound differ, clinicians often trust the early scan more, especially in the first trimester.

Calculation Method	Typical Accuracy	Best Used When
LMP (regular 28-day cycle)	± 1–2 weeks	Cycles are regular
LMP (irregular cycles)	± 2–4 weeks	No other dating data available
Conception date	About ± 1 week	Conception timing is known
1st trimester ultrasound	About ± 5–7 days	Before 14 weeks
2nd trimester ultrasound	About ± 1–2 weeks	14–28 weeks
IVF transfer date	About ± 1–3 days	IVF pregnancy

Special Cases

Due Date Calculator for Special Cases

Irregular Periods and Due Date Calculation

Irregular periods make last-period dating harder because ovulation may not have happened on the expected day. In that setting, the due date calculator still provides a useful planning range, but it should not be treated as precise until an early ultrasound confirms the pregnancy age. If you are not sure how variable your cycles are, track your cycles for a few months first so you can see whether a pattern exists.

The cycle-length adjustment in the LMP tab can help when you know your cycles tend to be longer or shorter than average. That does not replace a dating scan, but it gives the due date calculator a smarter starting point before you meet with a clinician.

Due Date for Twins and Multiples

Twin pregnancies and higher-order multiples usually deliver earlier than singleton pregnancies, often around 37 weeks rather than 40. This due date calculator uses the standard singleton 40-week model because that is the clearest starting framework for most pregnancy dating. For twins, ultrasound growth, chorionicity, and maternal-fetal medicine guidance matter much more than a simple fixed rule.

In other words, the due date calculator can still provide a reference date for twins, but management and delivery timing are individualized much more tightly.

IVF Due Date Calculation

IVF dating is one of the biggest strengths of this page because the embryo age is known. A day 5 transfer, day 3 transfer, and day 2 transfer do not use the same offset, which is why the IVF tab asks for embryo type explicitly. That makes this due date calculator more precise for assisted reproduction than a generic LMP-only approach.

The same logic applies to frozen embryo transfer cycles as well. If you know the transfer date and embryo age, you can estimate the pregnancy timeline accurately even when the natural cycle is irrelevant. If you also want to compare against expected ovulation timing, the ovulation calculator remains a useful educational reference.

FAQ

Frequently Asked Questions

How is a due date calculated?

A due date is calculated using Naegele's Rule: add 280 days, or 40 weeks, to the first day of your last menstrual period. Another way to express it is to subtract 3 months, add 7 days, then add 1 year. This assumes a 28-day cycle with ovulation around day 14.

How many weeks is a full-term pregnancy?

A full-term pregnancy is 40 weeks from the first day of the last menstrual period, or about 38 weeks from conception. Births between 39 weeks and 40 weeks 6 days are considered full term. Early term is 37 weeks to 38 weeks 6 days, late term is 41 weeks to 41 weeks 6 days, and post-term is 42 weeks or beyond.

How accurate is a due date calculator?

A due date calculator based on the last menstrual period is often accurate within 1 to 2 weeks for people with regular cycles. Only a small percentage of babies arrive on the exact due date, and most arrive within about 2 weeks before or after it. First-trimester ultrasound is usually the most accurate dating method and is often within 5 to 7 days.

What is the difference between gestational age and fetal age?

Gestational age counts from the first day of your last menstrual period and is the standard used in medicine. Fetal age, or conception age, counts from the actual date of conception and is usually about 2 weeks less. When someone says they are 10 weeks pregnant, they usually mean 10 weeks of gestational age.

Can I calculate my due date from my conception date?

Yes. If you know your conception date, add 266 days, or 38 weeks, to estimate your due date. This method can be more precise than LMP dating when conception timing is known clearly, such as with IVF, ovulation tracking, or a confirmed conception window.

How do I calculate my due date from an ultrasound?

Enter the ultrasound date and the gestational age shown on the scan in weeks and days. The calculator subtracts that gestational age from the scan date to estimate an adjusted LMP, then adds 280 days to project the due date. Early first-trimester scans are the most accurate for pregnancy dating.

When does each trimester start and end?

The first trimester runs from week 1 through week 13, the second trimester runs from week 14 through week 26, and the third trimester runs from week 27 to birth. Each trimester has different developmental milestones, symptoms, and appointment patterns, which is why doctors usually organize care around trimester timing rather than calendar months.

What if I have irregular periods?

If your periods are irregular, the last-period method can be less accurate because ovulation may not have happened on the expected day. In that situation, an early ultrasound is usually the best way to confirm a due date. You can still use this due date calculator as a planning tool, but it is smart to review the estimate with your clinician.

How is an IVF due date calculated?

IVF due dates are calculated from the embryo transfer date and embryo age. A day 5 blastocyst transfer uses transfer date plus 261 days. A day 3 transfer uses 263 days, and a day 2 transfer uses 264 days. IVF dating is often more precise than LMP dating because embryo age is known.

What week am I in my pregnancy?

To find what week you are in your pregnancy, count the number of days from the first day of your last menstrual period to today, then divide by 7. If your last period was 70 days ago, you are 10 weeks pregnant. This due date calculator automatically shows your current weeks pregnant count and the extra day within that week.

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Sources

Medical references and methodology

This due date calculator uses standard pregnancy dating rules and publicly available clinical guidance. The review label matches the rest of the site: Board-certified OB-GYN with 15+ years of clinical experience. Last reviewed: March 17, 2026.

This tool is for informational purposes only and is not a substitute for medical advice, diagnosis, or treatment. If your dating scan, bleeding history, IVF timing, or cycle pattern raises questions, review the pregnancy timeline with your prenatal clinician.