Due Date Calculator — Calculate Your Baby's Due Date

The moment you see two lines on a pregnancy test, one question comes to mind immediately: When will the baby arrive? A due date calculator helps you find your expected delivery date so you can plan and prepare for the big day.

But how does such a calculator actually work? How accurate is it? And what happens if the date changes after an ultrasound? In this article you'll find everything you need to know about pregnancy dating — from Naegele's classic formula to modern ultrasound measurement.

What Is a Due Date Calculator?

A due date calculator is a digital tool that estimates your expected delivery date based on information about your menstrual cycle or treatment dates for assisted reproduction. The calculator gives you an estimate of when your baby is likely to arrive, helping you, your midwife, and your doctor plan prenatal care.

For most pregnant people, the due date calculator is the first tool they use after a positive pregnancy test. It not only gives you a date to look forward to, but also helps you understand how far along you are and which week of pregnancy you're currently in.

The WHO and organizations like the AAP use the same calculation method — Naegele's rule — as the standard for establishing expected delivery dates.

How a Due Date Calculator Works

A due date calculator estimates your expected delivery date using a simple, well-established method that has been in use for over 200 years.

Naegele's Rule

The most common method for calculating a due date is called Naegele's rule, named after German obstetrician Franz Naegele (1778–1851). The formula is straightforward:

First day of last menstrual period + 280 days = expected due date

280 days equals 40 weeks, or approximately 9 months and 7 days. This calculation assumes a 28-day cycle and that ovulation occurs on day 14.

In practice:

1. Take the first day of your last period
2. Add 7 days
3. Subtract 3 months
4. Add 1 year

For example: if your last period started January 1, 2026, your due date would be October 8, 2026.

What If Your Cycle Is Irregular?

Not everyone has a 28-day cycle. A normal menstrual cycle can range from 21 to 35 days. If you have longer cycles, ovulation may have occurred later than day 14, and the due date shifts accordingly.

A good due date calculator accounts for your cycle length and adjusts the calculation. If you have a 35-day cycle, for example, 7 extra days are added, because ovulation likely happened around day 21 rather than day 14.

Calculate Your Due Date Step by Step

You really only need one piece of information to calculate your due date: the first day of your last period. Here's how:

Step 1: Find the Date of Your Last Period

Think back to when you last had your period. It's the first day of bleeding that counts, not the last. Many people use a cycle tracking app that makes it easy to find this date.

Step 2: Count 280 Days Forward

From the first day of your last period, count 280 days forward (40 weeks). That date is your estimated due date.

Step 3: Adjust for Cycle Length

If your cycle is shorter or longer than 28 days, adjust accordingly:

• Shorter cycle (e.g. 25 days): Subtract 3 days
• Longer cycle (e.g. 32 days): Add 4 days

Step 4: Use a Digital Calculator

Although you can do the math by hand, it's easier and more accurate to use a digital tool. Babysential's due date calculator does the calculation automatically and also shows you which pregnancy week you're in right now.

Once you have your due date, you can follow the entire pregnancy in our pregnancy timeline and read about development each week.

Ultrasound Dating and Due Date Adjustment

While Naegele's rule provides a good starting point, ultrasound is the most accurate method for determining a due date. Most prenatal programs include at least one ultrasound as part of standard care.

Early Ultrasound (Weeks 7–12)

An early ultrasound measures the fetus from crown to rump (CRL — Crown-Rump Length). This measurement is most accurate in the first trimester, because all fetuses grow at roughly the same rate in the early weeks. Accuracy is typically plus or minus 3–5 days.

Anatomy Scan (Weeks 18–20)

Most pregnant people are offered an anatomy scan around week 18–20. This scan has multiple purposes:

• Confirm or adjust the due date
• Examine fetal anatomy and development
• Check placenta position
• Measure amniotic fluid levels

Read more about what happens at an ultrasound in our article about ultrasound during pregnancy.

When Is the Due Date Changed?

The due date is typically adjusted if there is more than 7 days' difference between the calculated date (based on last period) and the ultrasound measurement. In that case, the ultrasound date becomes the official due date, because it is considered more reliable.

Getting a new due date can feel confusing, but it doesn't mean anything is wrong. It simply means ovulation happened at a slightly different time than assumed, or that your cycle was irregular that month.

IVF and Assisted Reproduction

For people who have conceived through assisted reproduction (IVF, ICSI, or frozen embryo transfer), the due date is calculated slightly differently.

How IVF Due Dates Are Calculated

With IVF, the exact date of fertilization is known, which allows for a more precise calculation:

• Fresh embryos (day 3 transfer): Egg retrieval date + 263 days
• Blastocyst transfer (day 5): Egg retrieval date + 261 days
• Frozen embryo transfer (FET): Transfer date + 261 days (day 5 embryo) or + 263 days (day 3 embryo)

The Difference from Natural Conception

The main advantage of IVF dating is that there's no uncertainty about ovulation timing. The date of egg retrieval or transfer is known with 100% certainty, making the due date calculation more accurate from the outset.

That said, most IVF pregnancies also include an early ultrasound to confirm the due date. If there's a significant difference between the IVF calculation and the ultrasound measurement, the provider will determine which date is most reliable.

Our due date calculator has a dedicated IVF mode where you can enter the egg retrieval or embryo transfer date.

How Accurate Is the Due Date?

Here's a fact that surprises many expectant parents: only about 4–5% of babies are born on their exact due date. So even if you circle the date on your calendar, there's a good chance your baby has other plans.

What the Numbers Show

According to research and birth statistics:

• About 4–5% are born on the exact due date
• About 80% are born within two weeks before or after the due date
• About 50% of first-time parents go past their due date
• Average pregnancy length for first-time parents is approximately 41 weeks and 1 day

The due date is best understood as the midpoint of a window. Most babies are born between week 37 and week 42, and all of this is considered a full-term pregnancy.

What Affects When Your Baby Actually Arrives?

Several factors play a role:

• Genetics: If your mother or sisters delivered early or late, you may have a similar tendency
• First or subsequent baby: First-time parents more often go slightly past their due date
• Baby's sex: Boys have a slight tendency to arrive a few days later than girls
• Parent's age and health: Various factors can influence pregnancy length

Post-Term — What Happens After the Due Date?

After the due date passes, most providers monitor the pregnancy more closely. According to standard obstetric guidelines:

• Week 41: Extra monitoring with non-stress test (NST) and possibly ultrasound
• Week 41+3 to 42: Evaluation of induction depending on your provider and individual circumstances

Most hospitals offer induction around week 42, but practices vary. Talk with your midwife or doctor about what applies at your care setting.

It's important to remember that while going past your due date can feel like a long wait, it is completely normal. Your baby is using those last days to gain weight and mature their lungs. As long as monitoring shows that you and your baby are doing well, there's no reason for concern.

The Gestational Wheel — What Is It?

You may have seen your midwife or doctor use a round, colorful wheel during a consultation. This is called a gestational wheel (or pregnancy wheel), and it's simply an analog version of a due date calculator.

The gestational wheel has two disks that rotate against each other. You set the date of your last period, and the wheel shows you:

• Expected due date
• Which pregnancy week you're in on any given date
• Approximate dates for key appointments and ultrasound scans

Today, digital due date calculators have largely replaced the physical wheel, but the principle is the same. Both use Naegele's 280-day rule as the basis for the calculation.

Pregnancy Week by Week

Once you've calculated your due date, it's natural to want to know what's happening in your body and with your baby along the way. Here's a brief overview of pregnancy's three trimesters:

First Trimester (Weeks 1–12)

The first weeks are a time of enormous internal changes, even if nothing shows on the outside yet. All the major organs form, and the heart begins to beat as early as week 8.

Many people experience nausea, fatigue, and mood swings. Read more about the early weeks in our week-by-week overview.

Second Trimester (Weeks 13–27)

Often called the "golden period" of pregnancy. Nausea eases for most people, energy returns, and the belly becomes visible. Around week 20 you can expect to feel the first kicks. This is also the period when many choose to tell family and friends about the pregnancy, and when the anatomy scan around week 18–20 confirms that everything looks good.

For many parents, this is the most worry-free part of pregnancy. You have energy to exercise, travel, and make preparations at home. Read our article on exercise during pregnancy for helpful tips.

Third Trimester (Weeks 28–40)

Now your baby grows quickly and your body prepares for birth. From week 37 onward, your baby is considered full term. Many use this period to pack the hospital bag, attend birth preparation classes, and make final practical preparations at home.

Toward the end of pregnancy, you may experience practice contractions (Braxton Hicks), sleep difficulties, and a feeling that the baby has "dropped" into the pelvis. All of this is normal and a sign that your body is preparing.

In week 12, the first major ultrasound typically takes place, and by the time you reach week 40, it's time to meet your little one. Every week brings something new, and it can be reassuring to know what to expect.

Frequently Asked Questions

How early can I calculate my due date?

You can calculate your due date as soon as you know your last period date. Many people use a due date calculator on the same day they get a positive pregnancy test. The calculation is equally valid whether you're at week 4 or week 20 — but the earlier you calculate, the longer you can enjoy the countdown.

Can the due date change during pregnancy?

Yes, the due date may be adjusted after an ultrasound, especially the early scan in the first trimester. If there's more than 7 days' difference between the calculated due date and the ultrasound measurement, the date is typically adjusted. After week 20, the due date is usually not changed, because measurements become less precise.

Is the due date calculator as accurate as ultrasound?

A due date calculator based on your last period has an accuracy of plus or minus 1–2 weeks. A first-trimester ultrasound is more accurate, with a margin of error of just 3–5 days. That said, the calculator is an excellent starting point that gives you a good estimate, especially if you have regular menstrual cycles.

What if I don't remember the date of my last period?

If you don't remember the exact date, your midwife or doctor can order an early ultrasound to estimate gestational age. Ultrasound measurement of fetal size in the first trimester is a reliable method for determining a due date, even without a known period date. Check whether you recorded the date in a cycle app or calendar.

Do pregnancy weeks count from conception or from the last period?

Pregnancy weeks are counted from the first day of your last period, not from conception. This means that in weeks 1 and 2 you are technically not yet pregnant — ovulation and conception typically occur around weeks 2–3. This method of counting is used by all healthcare services and is the global standard. When your provider says you are "8 weeks pregnant," that means 8 weeks since your last period, while the embryo is approximately 6 weeks old.

Can I use the due date calculator for a twin pregnancy?

Yes, calculating the due date works the same way for twin pregnancies. Naegele's rule and ultrasound measurement are used in the same way. What differs with twin pregnancies is that birth is typically planned a few weeks before the due date. For identical twins, delivery is generally recommended around week 36–37, and for fraternal twins around week 37–38, according to standard guidelines. The due date still serves as a reference point for monitoring throughout the pregnancy.

Tips for Remembering Your Due Date

Once you've calculated your due date, keep it easily accessible:

• Write it down in your phone, calendar, and maternity notes
• Share it with your partner so you can both follow the weeks together
• Create a profile on Babysential with your due date to get personalized weekly updates and reminders
• Remember that it's an estimate — don't book family visits from abroad on the exact date

Many parents also enjoy using a countdown app or creating a bump photo series documenting belly growth week by week. Whatever you choose, the important thing to remember is that the due date is an approximate timeframe, not a deadline.

Next Steps After Calculating Your Due Date

Now that you know roughly when your baby is coming, it's time to start preparing. Here are some useful steps:

1. Calculate your due date with Babysential's due date calculator
2. Follow your pregnancy week by week in the pregnancy timeline
3. Read about what's ahead in our pregnancy section
4. Prepare for your ultrasound with our article on ultrasound during pregnancy
5. Start with checklists to keep track of everything you need to do and buy
6. Read about nutrition in our guide to pregnancy nutrition

Sources

• WHO: Antenatal care recommendations — Guidance on pregnancy monitoring and care
• AAP/ACOG: Guidelines for induction and post-term management
• ACOG: Methods for estimating due date — Official guidelines for due date calculation
• CDC: Statistics on gestational length and births
• ACOG: Ultrasound in pregnancy — Routine ultrasound and due date confirmation