Pregnancy Due Date Calculator

What the pregnancy due date calculator does

This pregnancy due date calculator estimates the day your baby is most likely to arrive. Enter the first day of your last menstrual period (LMP), and it gives you an estimated due date along with your current week of pregnancy. The standard method adds 280 days, which is 40 weeks, to that first day. So if your last period started on January 1, your estimated due date lands around October 8. It is free, works in any browser, and needs no signup.

Think of the result as a target, not a deadline. Only about 1 in 20 babies actually arrives on the exact estimated due date. Most births happen within the two weeks on either side of it, which is why doctors call it an estimate. This due date and pregnancy calculator gives you the same date a clinician would calculate by hand, instantly.

How to use it

1. Find the first day of your last menstrual period, not the day it ended.
2. Enter that date into the calculator.
3. If you know it, adjust your average cycle length, since the default assumes a 28-day cycle.
4. Read your estimated due date and your current gestational age in weeks and days.

If you conceived through IVF or know your exact conception or ovulation date, you can use that instead by counting forward 266 days (38 weeks), since conception typically happens about two weeks after the start of the last period.

The due date formula and a worked example

The classic approach is Naegele's rule, used by midwives and obstetricians for over two centuries. The formula is simple: due date = first day of LMP + 280 days. The shortcut version is to take the first day of your last period, add one year, subtract three months, and add seven days.

Worked example: if your LMP started on March 10, add a year (March 10 next year), subtract three months (December 10), and add seven days. The estimated due date is December 17. The calculator does exactly this, and it also adjusts when your cycle is longer or shorter than 28 days, because ovulation timing shifts the conception date.

Why 280 days? The figure comes from the observation that a typical pregnancy lasts about 40 weeks measured from the last period, or roughly nine calendar months plus a week. Because the count starts about two weeks before conception actually occurs, the baby spends closer to 38 weeks (266 days) developing. That two-week gap is the reason the gestational age the calculator reports always runs a little ahead of the "age since conception" you might expect, and it is completely standard in obstetric practice.

Trimesters and key milestones

Pregnancy is counted in weeks from the first day of your last period, which means you are considered "two weeks pregnant" before conception even happens. A full-term pregnancy is 37 to 42 weeks. Here is how the weeks map to trimesters and milestones, so the number from the due date calculator has context.

What can shift your estimated due date

The first estimate from your last period is a starting point, and several real factors can move it. Understanding them helps the number from this estimated due date pregnancy calculator make sense when your provider quotes a slightly different day.

If an early scan and your last period disagree by more than about five to seven days, most providers will redate the pregnancy to match the ultrasound. After that point the due date is generally not changed again, even if later scans show a larger or smaller baby, because growth differences do not mean the dating was wrong.

Common mistakes

• Using the last day of your period instead of the first. Naegele's rule counts from the first day of bleeding. Using the wrong day shifts your due date by several days.
• Ignoring cycle length. The 280-day rule assumes a 28-day cycle. If yours runs longer, ovulation is later, and the true due date moves back accordingly.
• Treating the date as exact. The estimate is a midpoint of a range. Babies arriving a week or two early or late are entirely normal.
• Confusing gestational age with conception age. Gestational age is counted from your last period and runs about two weeks ahead of the actual age since conception.
• Recalculating from an early scan and an LMP at the same time. An early ultrasound usually overrides the LMP estimate when the two disagree by more than a few days, so follow your provider's dated scan.

Tips and related tools

A few practical notes: an early dating ultrasound (before 14 weeks) is the most accurate way to set a due date, so use this estimated due date pregnancy calculator as a starting point and let your scan refine it. Keep a record of your last period date for your first appointment, and remember that twins and other multiples often arrive earlier than the single-baby estimate.

It is also worth planning a few dates backward from your estimate. Counting back from the due date, the start of the third trimester falls around 13 weeks before, the anatomy scan window sits near 20 weeks before, and the point at which many people start maternity leave is often a few weeks ahead of the date itself. Marking these on a calendar early takes the pressure off later, and you can recompute any of them quickly if a scan adjusts your due date.

For other date and number math, pair this with the Age Calculator to count exact days, weeks, and years between any two dates. The Percentage Calculator helps with weight-gain or budget percentages, the Average Calculator is handy for tracking measurements over time, and the Sales Tax Calculator is useful when budgeting for baby purchases. Explore the full set of online calculators for more.

This tool is for general information and planning. It does not replace medical advice. Always confirm your due date and any pregnancy concerns with a qualified healthcare provider.

Frequently asked questions

How does the pregnancy due date calculator work? It adds 280 days (40 weeks) to the first day of your last menstrual period using Naegele's rule, the same method clinicians use. It then shows your estimated due date and current week of pregnancy.

How accurate is the estimated due date? It is a reliable midpoint, but only about 5% of babies arrive on the exact date. Most births occur within two weeks before or after, and an early ultrasound gives the most precise estimate.

What date should I enter, the start or end of my last period? Enter the first day your last period started, not the day it ended. The 280-day count begins from that first day of bleeding.

Can I use my conception or IVF transfer date instead? Yes. From a known conception date, count forward 266 days. For IVF, subtract a few days based on whether you had a 3-day or 5-day embryo transfer.

Does cycle length change my due date? It can. The standard formula assumes a 28-day cycle. A longer cycle means later ovulation, so your due date moves back by roughly the number of extra days.

How many weeks pregnant am I? Count from the first day of your last period to today. The calculator shows this gestational age automatically in weeks and days, with 40 weeks marking full term.

Why am I "pregnant" before conception? Pregnancy is dated from the last period, which precedes ovulation by about two weeks. So at conception you are already counted as roughly two weeks pregnant.

Is this pregnancy due date calculator free and private? Yes. It is completely free, requires no signup, and runs in your browser, so the date you enter is never sent anywhere.