The Most Common Method: Naegele's Rule

The standard way to calculate a due date is Naegele's Rule, which adds 280 days (40 weeks) to the first day of your last menstrual period. This is the method your doctor or midwife will use unless ultrasound shows a different date.

To use it yourself: write down the first day of your last period, add one year, subtract three months, then add seven days. If your last period started on January 10, you would calculate it as January 10 + 1 year = January 10 next year, minus 3 months = October 10, plus 7 days = October 17. That's your estimated due date.

This method assumes a 28-day cycle and ovulation on day 14. If your cycles are longer or shorter, your actual due date may shift by a week or more in either direction. The calculation also assumes you know exactly when your last period began—if you're uncertain, ultrasound dating becomes more reliable.

Key Takeaways

  • Naegele's Rule adds 280 days to the first day of your last menstrual period and is the standard calculation doctors use.
  • The formula is: last period date + 1 year − 3 months + 7 days = estimated due date.
  • Ultrasound dating in the first trimester is more accurate than menstrual dating if your cycle length varies or you don't remember your last period exactly.
  • Due dates are estimates; only about 5 percent of babies arrive on the exact date, and most arrive within two weeks before or after.
  • Cycle length, irregular periods, and the timing of ovulation all affect how accurate any calculation will be for your pregnancy.

Why Ultrasound Dating Is More Accurate Early On

An ultrasound performed between 8 and 13 weeks of pregnancy can measure the fetus and predict the due date within a margin of 3 to 5 days. This is more precise than Naegele's Rule because it doesn't depend on you remembering your cycle or assuming a standard 28-day pattern.

If you have an early ultrasound and it shows a different date than your calculation, your doctor will usually update your due date to match the ultrasound finding. Later ultrasounds (after 20 weeks) are less accurate for dating because fetal growth varies more widely by that point, so they're used mainly to check development, not to revise the due date.

If you didn't have an early ultrasound or can't remember your last period, your first ultrasound at any stage will still give your care team a baseline for tracking growth and planning care, even if the dating margin is wider.

Adjusting for Cycle Length and Ovulation Timing

Naegele's Rule works best if your menstrual cycle is exactly 28 days long. If your cycle is typically 30 days, add 2 extra days to your calculated due date. If it's 26 days, subtract 2 days. The difference exists because Naegele's Rule assumes ovulation happens on day 14 of a 28-day cycle, but it may happen earlier or later depending on your pattern.

If you track ovulation—through basal body temperature, ovulation predictor kits, or apps—you can calculate from ovulation instead. A pregnancy lasts about 266 days from ovulation, or 280 days from the first day of your last period. Adding 266 days to your ovulation date gives you another way to estimate the due date, though most providers still prefer the last-period method because it's easier to verify.

Irregular cycles make any calculation less reliable. If your periods come 25 days apart one month and 35 days apart the next, Naegele's Rule is a rough guess at best. Ultrasound dating becomes especially important in this situation.

What "Due Date" Actually Means

A due date is not a important date or a may provide. It's the middle of a range. Pregnancies naturally vary; babies born between 37 and 42 weeks are considered full-term, and most arrive sometime in that window. Only about 5 percent of babies are born on their exact due date.

Your due date is used to schedule routine monitoring, plan for labor induction if pregnancy goes past 42 weeks, and organize your life. It's not a prediction of when labor will start. Some pregnancies last 38 weeks, others 41. Both are normal.

If you go past your due date, your doctor will monitor you more closely and may discuss induction, but this doesn't mean something is wrong. Pregnancies that go to 41 or 42 weeks happen regularly and often end with healthy babies and uncomplicated births.

How to Calculate Weeks of Pregnancy

Once you know your due date, you can count backward to find how many weeks pregnant you are. Pregnancy is measured from the first day of your last menstrual period, not from conception. A pregnancy dated at 20 weeks means 20 weeks have passed since that first day, even though the fetus is only about 18 weeks old.

To find your current week: count the days from your last period to today, then divide by 7. If 140 days have passed, you're at 20 weeks. Your doctor will confirm this at your first ultrasound and may adjust it slightly based on what the scan shows.

Knowing your week of pregnancy helps you understand what to expect at each stage—when you might feel movement, when certain screening tests are offered, and when your baby's organs are developing. It also helps your care team flag any concerns that depend on how far along you are.

Tools and Apps for Due Date Calculation

Online due date calculators use the same Naegele's Rule formula and let you enter your last period date to get an when ready result. Most are accurate as long as you enter the correct date. Some also let you adjust for cycle length, which makes them slightly more personalized than the basic formula.

Pregnancy tracking apps store your due date and show you week-by-week development information. They don't change your medical due date, but they can help you keep track of milestones and prepare for appointments. Your doctor's office will have its own record based on your ultrasound or last period, so don't assume an app's date overrides what your care team tells you.

A straightforward calendar or spreadsheet works just as well if you prefer not to use an app. Write down your last period date, add 280 days, and mark that date. You can also count forward 40 weeks on a paper calendar if math isn't your preference.

When Your Due Date Might Change

If your first ultrasound shows a date more than 7 to 10 days different from your calculation, your doctor will usually update your due date to match the ultrasound. This happens fairly often and is not a sign of a problem—it just means your cycle was longer or shorter than average, or ovulation happened at a different time than expected.

After the first trimester, due dates are rarely changed based on ultrasound alone, because fetal growth becomes more variable and less reliable for dating. If you didn't have an early ultrasound and get one later in pregnancy, your care team may note the discrepancy but will usually stick with the original due date rather than shift it.

Your due date can also shift if you become pregnant again after a miscarriage or abortion. The new pregnancy is dated from the first day of the new cycle, not the old one, so the due date will be different even if conception happened close to the previous pregnancy.

Frequently Asked Questions

Can I calculate my due date if I don't remember my last period?

Yes, but less accurately. An ultrasound in the first trimester can estimate your due date within 3 to 5 days even if you don't know your last period. If you're past the first trimester, ultrasound is less precise for dating, but your care team can still use it along with other information to make a reasonable estimate.

What if my cycle is longer than 28 days?

Add the difference to your calculated due date. If your cycle is 32 days instead of 28, add 4 days to the date you get from Naegele's Rule. This accounts for the fact that ovulation happened later in your cycle than the standard formula assumes.

Is my due date the same as my baby's birth date?

No. Your due date is an estimate of when your pregnancy will reach 40 weeks. Most babies arrive within two weeks before or after that date. Only about 5 percent are born on the exact due date. Your baby's actual birth date will be whenever labor begins or induction is performed.

Can I change my due date after my first ultrasound?

Your doctor can, but you cannot. If your ultrasound shows a significantly different date than your calculation, your care team will update your medical record. This is normal and doesn't indicate a problem—it just refines the estimate based on what the scan shows.

What happens if I go past my due date?

Your doctor will monitor you more closely with non-stress tests and ultrasound to check the baby and placenta. Most pregnancies that go past 40 weeks are still healthy. If you reach 42 weeks, your doctor will typically discuss induction to reduce certain risks, but pregnancies at 41 weeks are common and usually end well without intervention.