Pregnancy Calculator: Easy Guide to Your Due Date, Weeks, and Trimesters (2026)

Important note: This article is for general information only. It is not medical advice, diagnosis, or treatment. Always confirm your due date and pregnancy care with your own clinician or midwife.

A pregnancy calculator gives you an estimated due date, your current week of pregnancy, and a rough timeline for the months ahead. It does not predict the exact day your baby will be born, and it does not replace prenatal care. Used well, though, a pregnancy calculator is one of the most useful tools in early pregnancy, because it helps you book the right appointments at the right time, understand screening windows, and plan without guessing.

This guide explains how a pregnancy calculator works, shows the math behind each method, and helps you choose the method that fits your situation, whether your cycles are regular, irregular, or you conceived through IVF.

Try a pregnancy calculator

To get a quick estimate, you can use the Calculator.net Pregnancy Calculator – enter your due date, last period date, ultrasound date, conception date, or IVF transfer date, and it will estimate your pregnancy schedule. Use the result as a starting point, then read the sections below to understand how accurate it is and what to do next. Always confirm the result with your own clinician or midwife. This is an external tool, not a tool provided by this site.

What a Pregnancy Calculator Shows You

A pregnancy calculator shows your estimated due date, your current week, and your trimester, so you can see the whole timeline at a glance.

What Is a Pregnancy Calculator?

A pregnancy calculator is a tool that estimates your due date, also called your estimated date of delivery, or EDD. Most calculators also tell you how many weeks and days pregnant you are today, which trimester you are in, and how many days remain until your due date. In short, a pregnancy calculator turns one known date into a full pregnancy timeline.

You usually enter one of these dates:

The first day of your last menstrual period, called LMP
The date of conception, if you know it
The date of an IVF embryo transfer
The date and measurement of an early ultrasound

The calculator then applies a standard rule for that method. The result is a starting point for prenatal care, not a final answer. Your provider may adjust it after an early ultrasound, especially in the first trimester.

How Does a Pregnancy Calculator Work?

Most pregnancy calculator results use gestational age, not fetal age. That distinction confuses many people at first, so it is worth explaining clearly.

Gestational age is counted from the first day of your last menstrual period. Fetal age is counted from conception, which usually happens about two weeks after the LMP in a 28-day cycle. That is why you can be described as "two weeks pregnant" before conception has happened. Clinicians use gestational age because the LMP is usually easier to know than the exact day of conception, and because it creates a consistent timeline for appointments and tests.

A full pregnancy is usually counted as 40 weeks, or 280 days, from the LMP. That equals about 38 weeks from conception. If that sounds long, remember that the first two weeks are counted before conception in this system.

The core formula: Naegele’s rule

The most common method is called Naegele’s rule. In its simplest form:

Estimated due date = First day of LMP + 280 days

A handy calendar shortcut gives almost the same result:

Add 7 days to the first day of your LMP.
Subtract 3 months.
Add 1 year.

Because months have different lengths, the shortcut can differ from the 280-day count by a day or two. The 280-day count is the standard to use when you want the most consistent result.

This rule assumes a 28-day cycle with ovulation around day 14. If your cycle is regularly longer or shorter, the calculator should adjust for that, as explained below.

Calculate From Your Last Period

The LMP method is the most common starting point for a pregnancy calculator, and it is the method most providers use at a first visit when no early ultrasound is available yet.

Formula: Due date = LMP + 280 days + (your cycle length – 28 days)

In plain terms:

If your cycle is 28 days, add 280 days and make no adjustment.
If your cycle is 35 days, add 7 extra days, because ovulation likely happened later.
If your cycle is 21 days, subtract 7 days, because ovulation likely happened earlier.

How to find your LMP

Your LMP is the first day of your last period, not the last day, and not the day your period ended. If you track periods in an app or calendar, look for the first day of bleeding for your most recent period before pregnancy. If you are unsure by more than a few days, an early ultrasound is usually a better dating method.

When the LMP method works best

Your cycles are fairly regular.
You remember the first day of your last period with confidence.
You have not recently stopped hormonal birth control, which can make early cycles less predictable. In that case, tell your provider, because dating may need extra care.

Calculate From Conception

If you tracked ovulation closely, used ovulation test strips, or know the date of conception from fertility treatment, a pregnancy calculator using conception can be more precise than LMP for irregular cycles.

Formula: Due date = Conception date + 266 days

That is 38 weeks from conception, which lines up with 40 weeks from LMP when ovulation happened on day 14 of a 28-day cycle.

Conception dating is most reliable when:

You tracked ovulation with tests, basal body temperature, or fertility monitoring.
You had timed intercourse or intrauterine insemination on a known date.
Your cycles are irregular, so LMP alone would be misleading.

It is less reliable when conception is estimated from symptoms alone, because ovulation symptoms and implantation symptoms can be vague and can overlap with other causes.

Calculate From IVF Transfer

For IVF, the transfer date is known exactly, which makes a pregnancy calculator using IVF transfer one of the most precise dating tools.

Day-3 embryo transfer: Due date = Transfer date + 263 days
Day-5 embryo transfer (blastocyst): Due date = Transfer date + 261 days

Some calculators use a slightly different presentation, such as adding 266 days from the estimated conception date implied by the transfer. The key point is the same: because the embryo’s age at transfer is known, there is no need to guess ovulation timing.

If you had IVF, use the transfer method first, and share the transfer date and embryo age with your provider. Do not mix IVF dating with LMP dating, because that can create a confusing two-week gap.

Calculate From Ultrasound

An early ultrasound, especially in the first trimester, is often the most accurate way to date a pregnancy. The clinician measures the embryo or fetus, most commonly crown-rump length in early pregnancy, and compares that measurement with standard growth charts. A pregnancy calculator based on ultrasound can be more accurate than one based on LMP alone.

Why ultrasound can beat LMP:

It does not depend on remembering a period date.
It does not assume a 28-day cycle.
It measures actual development, not just calendar time.

Timing matters. First-trimester ultrasound dating is generally more accurate than later ultrasound dating, because early growth is more consistent from one pregnancy to another. Later in pregnancy, babies grow at more different rates, so ultrasound dating becomes less precise.

If your ultrasound date differs from your LMP date by more than a few days in early pregnancy, your provider may update your due date. That is normal, and it does not mean something is wrong. It means you now have better information.

Which Method Should You Use?

Use this decision guide to choose the pregnancy calculator method that fits your situation. Each pregnancy calculator method below starts from a different known date.

Your situation	Best starting method	Why it fits
Regular 28-day cycles, LMP known	LMP	Simple, standard, and usually close
Longer or shorter regular cycles	LMP with cycle-length adjustment	Corrects for later or earlier ovulation
Irregular cycles, ovulation tracked	Conception date	Avoids the 28-day assumption
IVF pregnancy	IVF transfer date	Transfer date and embryo age are exact
LMP unknown or very uncertain	Early ultrasound	Measures development directly
LMP and ultrasound disagree	Ask your provider, usually early ultrasound wins in the first trimester	Early growth is more consistent

If you have two different results, do not average them yourself. Bring both dates to your first prenatal visit and let your provider decide which one to use for scheduling.

Worked Examples

These pregnancy calculator examples use the formulas above, so you can check the math yourself.

Example 1: LMP method, 28-day cycle

Hypothetical Example: LMP is January 10, 2026. Add 280 days. January 10 + 280 days = October 17, 2026. That is the estimated due date, assuming a regular 28-day cycle.

Example 2: LMP method, 35-day cycle

Hypothetical Example: LMP is June 1, 2024, with a regular 35-day cycle. First, June 1 + 280 days = March 8, 2025. Then add the cycle adjustment: 35 – 28 = 7 extra days. Estimated due date = March 15, 2025. Without the adjustment, the date would be a week too early.

Example 3: Conception method

Hypothetical Example: Conception is February 14, 2026. Add 266 days. February 14 + 266 days = November 7, 2026.

Example 4: IVF day-5 transfer

Hypothetical Example: A day-5 transfer happens on March 3, 2026. Add 261 days. March 3 + 261 days = November 19, 2026.

These are estimates, not appointments. Your provider’s dating ultrasound, if you have one, can refine them.

How Accurate Is a Due Date?

A due date is a window, not a deadline. Only a small share of babies, often cited around 4 to 5 percent, are born on their exact estimated due date. Most births happen within about two weeks before or after that date, and a normal pregnancy term is generally considered to be 37 to 42 weeks from LMP. That is why a pregnancy calculator result should be read as a window, not a deadline.

ACOG, the American College of Obstetricians and Gynecologists, uses these term categories:

Early term: 37 weeks 0 days through 38 weeks 6 days
Full term: 39 weeks 0 days through 40 weeks 6 days
Late term: 41 weeks 0 days through 41 weeks 6 days
Postterm: 42 weeks 0 days and beyond

That is why clinicians often talk about a "due month" or "due window" in conversation, even when the chart shows one specific date. The specific date is useful for scheduling tests and visits. The window is a better way to think about birth itself.

What makes a due date less accurate?

Irregular cycles or an uncertain LMP date
Recent hormonal birth control, which can delay ovulation
No early ultrasound to confirm dating
A later ultrasound used for dating, when growth varies more
Multiple pregnancies, where growth and timing need closer monitoring

None of these automatically means a problem. They mean your provider may need more information, and may update your date as that information arrives.

Can my due date change?

Yes, especially in the first trimester. Your provider may change it if:

An early ultrasound shows a gestational age that differs meaningfully from the LMP estimate
Your cycle length is very different from 28 days
New fertility or transfer records become available

Later changes are less common and usually need a clear reason, such as a growth measurement that does not fit the current timeline. If your date changes, ask what changed and how it affects your screening and visit schedule. A changed date is usually a sign of better information, not a cause for alarm on its own.

Trimesters Explained

A pregnancy calculator usually divides pregnancy into three trimesters:

Trimester	Weeks	What to expect
First trimester	Week 0 through week 13, day 6	Dating ultrasound, first prenatal visit, early screening discussions, nausea and fatigue are common
Second trimester	Week 14 through week 27, day 6	Anatomy scan, often more energy, fetal movement usually felt between 18 and 22 weeks
Third trimester	Week 28 through birth	More frequent visits, growth checks, birth planning, and preparation for labor

These boundaries are conventions for care and communication. Development does not switch neatly at a trimester boundary, and symptoms vary widely from person to person.

A note on "how far along am I?"

To find your current gestational age, count weeks and days from your LMP, or from your provider’s confirmed due date working backward 280 days. For example, if your due date is 20 weeks away, you are about 20 weeks pregnant in the usual counting system, with 20 weeks remaining. If you are using a calculator, enter today’s date as well as your LMP or due date, so it can show both your current week and your countdown.

Your Prenatal Timeline

Your exact schedule depends on your health history, location, and provider, but a typical low-risk timeline looks like this:

As soon as pregnancy is confirmed: Call for a first prenatal appointment. In the US, many providers aim for a first visit around 8 weeks, but call early because booking can take time.
First trimester: Dating ultrasound if needed, blood tests, screening discussions, and a review of medications, supplements, and health history.
Second trimester: Anatomy scan, usually around 18 to 22 weeks, plus routine checks at each visit.
Third trimester: More frequent visits, growth and position checks, and a birth plan discussion.

First prenatal visit checklist

Take this list to your first appointment:

Your LMP date, cycle length, and any ovulation or fertility tracking records
IVF transfer date and embryo age, if applicable
Any ultrasound reports you already have
A list of medications, vitamins, and supplements, including folic acid
Your health history, allergies, and previous pregnancies, if any
Questions about screening, vaccines, travel, exercise, and work
Your insurance and preferred hospital or birth setting, if you know them

Do not wait for a "perfect" question list. Providers expect first-visit questions, and asking early is better than worrying quietly.

Common Mistakes

Using the last day of your period instead of the first day. This can shift your date by several days.
Ignoring cycle length. A 35-day cycle and a 21-day cycle should not use the same 280-day result without adjustment.
Mixing methods. Do not average an LMP date and an ultrasound date yourself. Let your provider choose the dating method.
Treating the due date as a guaranteed birth day. Plan for a window, not a single day, especially for work leave, childcare, and travel.
Delaying prenatal care while waiting for a "better" pregnancy calculator result. If you are pregnant, booking care matters more than perfect dating.
Trusting a calculator that asks for no method at all. A good calculator should show which input it used and which rule it applied.
Sharing personal health dates in an untrusted app without checking privacy settings. Check what a calculator site stores, and avoid entering more detail than you need.

Best Practices

Use the most reliable date you have, not the most convenient one.
Write down the method you used, so you can explain it at your first visit.
Recheck your pregnancy calculator result after an early ultrasound, and update your personal timeline if your provider changes the date.
Use your due date to plan screening windows and appointments, not to judge whether your pregnancy is "on track" day by day.
Keep a simple record: LMP, cycle length, conception or transfer date if known, ultrasound date, and provider-confirmed due date.

When to Seek Care Sooner

A pregnancy calculator cannot tell you whether you need urgent care. Contact your clinician promptly, or seek urgent care in the US by calling emergency services if needed, if you experience:

Vaginal bleeding, especially heavy bleeding or bleeding with pain
Severe abdominal pain or cramping that does not ease
Leaking fluid from the vagina
Severe headache, vision changes, or sudden swelling of the face or hands
Reduced fetal movement later in pregnancy, compared with your baby’s usual pattern
Fever, severe vomiting, or inability to keep fluids down
Signs of an allergic reaction or severe illness

If you are unsure whether a symptom needs urgent care, call your provider’s nurse line or after-hours number rather than waiting for your next routine visit. That advice applies whether your calculator result looks "normal" or not.

Who Should Use Which Calculator Result?

A pregnancy calculator is useful for almost everyone in early pregnancy, but the way you use it should match your situation:

Trying to conceive: Use it to understand fertile windows and cycle patterns, but remember that ovulation timing varies, and a missed period plus a positive test still needs clinical confirmation.
Recently pregnant with regular cycles: LMP dating is a good starting point, and an early ultrasound can confirm it.
Irregular cycles or fertility treatment: Conception or IVF dating is usually better than LMP alone.
Unsure of dates: Do not guess repeatedly. Book prenatal care and ask about dating ultrasound.
High-risk pregnancy: Follow your specialist’s dating and visit schedule, even if it differs from a general calculator.

Conclusion

A pregnancy calculator is a simple tool with a powerful job: it turns one date you know into a timeline you can plan around. The key is to use the right method, adjust for cycle length when needed, and treat the result as an estimate. Your due date helps schedule care, screening, and preparation, but your baby’s actual arrival will fall in a window, not on a single predicted day.

Start with the most reliable date you have, write down the method you used, and bring that information to your first prenatal visit. Keep your pregnancy calculator result with your prenatal records. If an early ultrasound changes your date, that is usually good news: you now have a clearer picture. And if anything worries you along the way, your provider, not a calculator, is the right next step.

Frequently Asked Questions

1. How does a pregnancy calculator work?

It applies a standard rule to a date you enter. The most common rule is Naegele’s rule: add 280 days to the first day of your last menstrual period, adjusted for cycle length. Other methods use conception date, IVF transfer date, or ultrasound measurements.

2. What is my due date if my last period started on January 10, 2026?

Using a 28-day cycle, January 10, 2026 + 280 days = October 17, 2026. If your cycle is longer or shorter, adjust by the difference from 28 days, and confirm with your provider.

3. How many weeks pregnant am I?

Count weeks and days from the first day of your last period, or work backward 280 days from your provider-confirmed due date. That count is gestational age, which is about two weeks longer than fetal age from conception.

4. Can my due date change?

Yes, especially in the first trimester. An early ultrasound, a very different cycle length, or new fertility records can lead your provider to update your date. Later changes are less common and usually have a clear reason.

5. How accurate is a pregnancy calculator?

A pregnancy calculator gives an estimate, not an exact birth day. Only a small share of babies, often cited around 4 to 5 percent, are born on their exact due date. Most births happen within about two weeks before or after it, and 37 to 42 weeks is generally considered a normal term range.

6. What if I do not know my last period?

Do not guess. Book prenatal care and ask about a dating ultrasound. Early ultrasound measures development directly and is often more accurate than an uncertain LMP date.

7. How do I calculate a due date from conception?

Add 266 days to the conception date. This works best when ovulation or conception was tracked closely, such as with ovulation tests or fertility treatment.

8. How do I calculate an IVF due date?

For a day-3 transfer, add 263 days to the transfer date. For a day-5 blastocyst transfer, add 261 days. Share the transfer date and embryo age with your provider.

9. What is the difference between gestational age and fetal age?

Gestational age is counted from the first day of your last period. Fetal age is counted from conception. Gestational age is about two weeks longer, and it is the standard used for prenatal care and due dates.

10. When should I have my first prenatal visit?

Call as soon as pregnancy is confirmed. Many US providers aim for a first visit around 8 weeks, but booking can take time, so call early. Bring your LMP, cycle length, fertility or ultrasound records, medication list, and questions.

Sources

Information checked November 2, 2026; this article is for general information only and does not replace medical advice. For more from this site, visit our About Us page or Contact Us page.