Due Date Calculator

There are a few different ways to estimate when your baby will arrive, and the right method depends on what information you have available. The most common starting points are your last menstrual period, your known conception date, an ultrasound measurement, or IVF transfer records.

All of these methods work toward the same target: a full-term pregnancy is generally considered 40 weeks from the first day of your last menstrual period, or about 38 weeks from actual conception. That 40-week figure is the standard used by OBs, midwives, and hospitals across the country.

Whichever method you use, your healthcare provider will likely confirm or adjust the estimate at your first prenatal appointment, usually with an early ultrasound. Think of any calculator result as your best working guess until then.

The LMP method is by far the most widely used approach, and it's the default for most online calculators and clinical tools. Here's how it works: take the first day of your last menstrual period, add 40 weeks (or 280 days), and that's your estimated due date.

This method is based on Naegele's Rule, which has been the clinical standard for well over a century. The formula is straightforward: take the first day of your LMP, subtract 3 months, then add 7 days and 1 year. For example, if your LMP started on April 10, you'd subtract 3 months to get January 10, then add 7 days to land on January 17 of the following year as your EDD.

One thing worth understanding is why we count from the period rather than conception. Most people don't know the exact moment they conceived, but they do remember (or can look up) when their last period started. Since ovulation typically happens around day 14 of a 28-day cycle, the LMP method essentially adds those two pre-conception weeks into the total count.

This works well when your cycles are close to 28 days. If your cycle is significantly shorter or longer than that, the estimate may need adjustment, which we cover in a later section.

If you know when you conceived, whether because you tracked ovulation carefully, used ovulation predictor kits, or had timed intercourse, you can calculate your due date more directly. The math here is simple: add 266 days (38 weeks) to your conception date.

Why 266 instead of 280? Because conception-based counting skips those first two weeks of the menstrual cycle that happen before ovulation. Fertilization typically occurs within 24 hours of ovulation, so the fetal age clock starts from that point rather than from your last period.

For example, if you conceived on May 5, adding 266 days puts your due date around February 25 of the following year. You can also convert this to an LMP-equivalent date by subtracting 14 days from your conception date and then applying Naegele's Rule, which should give you the same result.

Keep in mind that sperm can survive in the reproductive tract for up to 5 days, so even if you know when you had sex, conception could have occurred a few days later. That small window of uncertainty is normal and usually only shifts the estimate by a day or two.

IVF pregnancies are a bit different because the timing of fertilization and transfer is precisely documented. That actually makes due date calculation more accurate for IVF patients than for those relying on period tracking. The key variable is whether you had a 3-day embryo transfer or a 5-day (blastocyst) transfer, since those two scenarios use different formulas.

In both cases, you're working backward from the transfer date to an equivalent "conception date," then adding 266 days (or the equivalent in weeks) to get your EDD. Your fertility clinic will typically give you this date directly, but it's helpful to understand how it's calculated.

An early ultrasound, usually done between 8 and 13 weeks, is one of the most reliable ways to confirm or fine-tune a due date. During this scan, the sonographer measures the embryo's crown-rump length (CRL), which is the distance from the top of the head to the bottom of the spine. That measurement is then compared against established growth charts to estimate gestational age.

The earlier the ultrasound is performed, the more accurate it tends to be. In the first trimester, embryo size is relatively uniform across pregnancies, so CRL measurements are a strong predictor of gestational age, usually accurate to within 5 to 7 days. By the second trimester, natural variation in fetal size increases, and ultrasound dating becomes less precise.

If your ultrasound-based date differs from your LMP-based date by more than a week (in the first trimester) or more than two weeks (in the second trimester), your provider may choose to adjust your official due date to match the ultrasound findings. This is completely normal and just means the scan gave a more reliable data point than period tracking alone.

Second and third trimester ultrasounds measure additional markers like femur length, head circumference, and abdominal circumference. These are more useful for tracking fetal growth than for pinning down a due date, but they still contribute to the overall clinical picture.

A 40-week pregnancy is divided into three trimesters, each with its own set of developments and milestones. Weeks are counted from the first day of your LMP, so weeks 1 and 2 technically occur before conception even happens. Actual fetal development begins around week 3, when fertilization occurs, and picks up rapidly from there.

Understanding the general timeline helps you know what to expect at each prenatal visit and gives you a framework for interpreting test results, ultrasound findings, and the physical changes you're experiencing. Here's a broad overview before we dive into the specifics.

• First trimester: Weeks 1 through 13
• Second trimester: Weeks 14 through 27
• Third trimester: Weeks 28 through 40 (and sometimes beyond)

Here's a clean summary of the main formulas used to estimate a due date, depending on your starting information:

All of these formulas target the same endpoint: 40 weeks of gestational age from the equivalent of the LMP date. The differences between them are just accounting for how many days of that 40-week window have already passed before the known starting point.

Seeing the math in action makes it a lot easier to apply. Here are a few worked examples using different methods.

Example 1: LMP Method
Last period started: March 15
Subtract 3 months: December 15
Add 7 days: December 22
Estimated due date: December 22 (of the following year)

Example 2: Conception Date Method
Known conception date: April 3
Add 266 days: December 25
(You can verify this by subtracting 14 days from April 3 to get March 20 as the LMP equivalent, then applying Naegele's Rule.)

Example 3: 5-Day IVF Transfer
Transfer date: February 10
Add 261 days: November 28

Example 4: 3-Day IVF Transfer
Transfer date: February 10
Add 263 days: November 30

Notice how the IVF examples produce dates just two days apart even though the transfer date is the same. That difference reflects the extra days the embryo spent developing in the lab before transfer.

These two terms sound interchangeable, but they're not, and mixing them up can cause real confusion when you're reading pregnancy literature or talking with your provider.

Gestational age is counted from the first day of your last menstrual period. It's the standard clinical measurement. When your doctor says you're "10 weeks pregnant," they mean 10 weeks of gestational age, even though conception didn't happen until around week 2 or 3 of that count.

Fetal age (sometimes called embryonic age or conceptional age) is counted from the actual date of fertilization. It runs about 2 weeks behind gestational age. So a fetus described as having a fetal age of 8 weeks would be at 10 weeks gestational age.

Most pregnancy apps, books, and clinical tools use gestational age as the default. If you ever read something that seems to describe a fetus as smaller or less developed than you'd expect for your week count, it may be using fetal age instead. When in doubt, ask your provider which measurement they're referencing.

Naegele's Rule assumes a textbook 28-day cycle with ovulation on day 14. That's a useful average, but plenty of people have shorter or longer cycles, and that matters for due date accuracy.

If your cycle is consistently shorter than 28 days, ovulation happens earlier than day 14, which means conception occurred earlier relative to your LMP. Your due date is probably a bit earlier than the standard LMP formula suggests. If your cycle is consistently longer than 28 days, the opposite is true: ovulation comes later, conception is later, and your actual due date is probably a few days to a week later than the formula indicates.

A rough adjustment: for every day your average cycle differs from 28, shift the due date by one day in the corresponding direction. So if your cycle is 35 days (7 days longer than 28), your adjusted due date would be about 7 days later than the LMP formula gives you.

This is one reason early ultrasound is so valuable. It measures actual fetal size rather than relying on period dates, so it naturally accounts for cycle length variation without needing manual adjustments.

People often get confused when they try to convert pregnancy weeks into months, and honestly, the confusion is understandable. The issue is that calendar months don't divide evenly into weeks.

A common misconception is that 40 weeks equals 10 months. Technically, 40 weeks is closer to 9 months and 1 week when you use standard calendar months (since most months have more than 4 weeks). But pregnancy is almost always discussed in weeks clinically, so the month count is mostly useful for casual conversation rather than medical tracking.

Here's a rough week-to-month guide:

• Month 1: Weeks 1 to 4
• Month 2: Weeks 5 to 8
• Month 3: Weeks 9 to 13
• Month 4: Weeks 14 to 17
• Month 5: Weeks 18 to 22
• Month 6: Weeks 23 to 27
• Month 7: Weeks 28 to 31
• Month 8: Weeks 32 to 35
• Month 9: Weeks 36 to 40

When someone asks how far along you are, answering in weeks is usually the most precise and least confusing option. Your provider will always use weeks, so getting comfortable with that framing early on makes everything easier to follow.

Your due date isn't set in stone from the moment it's calculated. Several things can prompt your provider to revise it, and that's a normal part of prenatal care rather than a cause for concern.

• Ultrasound measurements: If an early ultrasound suggests fetal size is significantly different from what the LMP date predicts, the ultrasound-based date usually takes precedence, especially in the first trimester.
• Irregular cycles: As covered above, non-standard cycle lengths can shift the true ovulation date, which affects conception timing and therefore the due date.
• Uncertain LMP: Not everyone remembers the exact start of their last period, especially with irregular cycles or recent hormonal contraceptive use. When the LMP is unclear, ultrasound becomes the primary dating tool.
• Multiple pregnancy: Twins, triplets, and higher-order multiples often arrive earlier than singletons. Providers typically plan for delivery before 38 weeks in twin pregnancies and earlier still for higher multiples.
• Assisted reproductive technology: IVF cycles provide very precise conception timing, which can actually make the due date more reliable than the LMP method for most natural conceptions.

Even with a well-established due date, labor can start anywhere from 3 weeks before to 2 weeks after the EDD and still be considered within the normal range. The due date is a target, not a deadline. Most providers consider 39 to 40 weeks the sweet spot for full-term delivery, but every pregnancy finds its own timeline.