Pregnancy Due Date Calculator

What this calculator does

A due date is the first real coordinate of a pregnancy — the day everything else gets measured against. This free calculator gives you that date the moment you enter the first day of your last period, along with the details that actually help you plan: how many weeks pregnant you are right now, which trimester you're in, an estimated conception date, and how many days are left to go.

It uses Naegele's rule, the 200-year-old method still used by obstetricians and built into hospital wheels and apps the world over: add 280 days (40 weeks) to the first day of your last menstrual period. The calculator also lets you adjust for your own cycle length, because the textbook version quietly assumes everyone has a tidy 28-day cycle, and most people don't. Everything runs in your browser — no account, no data leaves your device.

How to use it

1. Enter the first day of your last period (LMP). Not the day it ended — the day bleeding started. This is the single most important input, because pregnancy is dated from here.
2. Set your average cycle length. Leave it at 28 if you're not sure or your cycles are regular. If yours typically run longer or shorter, drag the slider; the due date shifts to match.
3. Read your timeline. You'll see your estimated due date up top, then a progress track showing where you are across the 40 weeks, your current weeks-and-days, your trimester, and your estimated conception date.

Because the math is live, you can see exactly how a longer cycle moves the date, or check on any future day how far along you'll be.

The formula behind it

Naegele's rule is elegantly simple. In its classic form you take the first day of the last period, subtract three months, add seven days, and add a year. Adding 280 days produces the same result and is what this calculator does internally.

A worked example: a last period starting January 1 gives a due date of October 8 the same year (January minus three months is October of the prior cycle; plus seven days; carried into the new year). Enter that date above and you'll see exactly that.

For cycles that aren't 28 days, the tool applies a simple, well-established adjustment: it adds the difference between your cycle and 28 to the due date. A 35-day cycle, for instance, means you likely ovulated about a week later than the textbook assumes, so the due date moves about a week later too.

Why pregnancy is counted from your last period

Here's the part that confuses nearly everyone: at the moment you conceive, you're already considered "two weeks pregnant." That's not a glitch. Conception is hard to pin down to a single day, but the start of a period is unmistakable, so doctors use the LMP as a reliable anchor and count gestational age from there.

A typical cycle ovulates around day 14, and conception happens within a day of that. So the 280-day pregnancy from your last period corresponds to about 266 days from actual conception. Those first two "phantom" weeks are why a positive pregnancy test often already reads "4 weeks" — you're four weeks from your last period, but only about two weeks from conception. The calculator's estimated conception date (LMP plus your cycle length minus 14) reflects this.

What "estimated" really means

The honest truth every provider will tell you: only about 1 in 20 babies actually arrives on their due date. A due date is the midpoint of a normal window, not a deadline. A full-term, perfectly healthy birth can happen any time in a roughly five-week span around it.

That's why the medical vocabulary has shifted away from a single "due date" toward describing where a birth falls:

• Early term: 37 weeks 0 days – 38 weeks 6 days
• Full term: 39 weeks 0 days – 40 weeks 6 days
• Late term: 41 weeks 0 days – 41 weeks 6 days
• Post term: 42 weeks 0 days and beyond

So if your baby comes a week or two on either side of the calculated date, nothing has gone wrong — that's the norm, not the exception.

When dating scans overrule the math

Naegele's rule rests on two assumptions that aren't always true: that you remember your exact LMP, and that you ovulate on day 14. If your cycles are irregular, if you conceived while breastfeeding or just after stopping birth control, or if you simply aren't sure of the date, the rule can be off by a week or more.

That's why an early ultrasound — usually in the first trimester — is considered the most accurate way to date a pregnancy. The sonographer measures the embryo (the crown-rump length) and, in those early weeks, every embryo grows at almost the same rate, so the measurement pins the age tightly. If the scan disagrees with your LMP-based date by more than about a week, your provider will usually adjust the official due date to the scan. Think of this calculator as your excellent first estimate, and the dating scan as the confirmation.

The trimesters, defined

Pregnancy is split into three trimesters. Using the boundaries set by the American College of Obstetricians and Gynecologists (ACOG):

The progress track in the calculator above is drawn to this 40-week scale, with a marker showing exactly where today falls and where each trimester begins.

A rough roadmap by trimester

While every pregnancy differs, the calendar this tool builds maps to a familiar arc:

• First trimester is the foundation phase. The big milestones are your first prenatal appointment and, often, that first dating ultrasound. It's also when many people choose to keep the news close while symptoms run highest.
• Second trimester is when planning gets real. The anatomy scan around 18–22 weeks is a major checkpoint, you may learn the sex if you choose, and it's the comfortable window for things like registering, setting up the nursery, and travel.
• Third trimester is the home stretch: more frequent checkups, packing a hospital bag, finalizing a birth plan, and watching for the signs of labor. Knowing your exact week — which the calculator keeps current every day — helps you know what's typical and when to call your provider.

How to count the weeks yourself

Once you have your LMP, you can track gestational age without any app. Count the completed weeks and days since the first day of your last period. If today is exactly eight weeks and three days after that date, you're "8 weeks 3 days," often written 8w3d. The calculator does this for you and refreshes every day, but the logic is just subtraction: days since LMP, divided by seven. This is also why providers talk in weeks-and-days rather than months — it's more precise, and "months pregnant" doesn't divide cleanly into 40 weeks.

A rough prenatal visit schedule

Your due date sets the cadence of your appointments. While every practice differs, a typical low-risk schedule looks like this:

• Weeks 4–28: roughly one visit a month.
• Weeks 28–36: about every two weeks.
• Weeks 36–birth: weekly.

Key checkpoints usually include a first-trimester dating scan, blood work and screening, the anatomy scan around 18–22 weeks, a glucose test around 24–28 weeks, and a Group B strep swab around 36 weeks. Knowing your week keeps you oriented on what's coming next.

IVF, IUI, and known conception dates

Naegele's rule estimates ovulation, but some people know the exact date conception happened — most often with IVF or timed fertility treatment. In that case the more accurate method is to count 266 days (38 weeks) from conception or egg retrieval, since you're skipping the two-week estimate baked into the LMP method. Embryo transfers are dated even more precisely based on the embryo's age at transfer. If you conceived this way, your clinic's date will be more reliable than any LMP calculator — use theirs.

Why a provider might move your due date

Your due date isn't necessarily permanent. If an early ultrasound measures the baby and the result differs from your LMP-based date by more than about a week, your provider will usually reassign the due date to the scan, because early measurements are so consistent across pregnancies. They generally won't change it for small differences, and they won't keep moving it with later scans (growth varies more as pregnancy progresses, so a third-trimester "big baby" reading doesn't shift the date). One date, set as early and accurately as possible, is the goal.

Due-date myths, debunked

• "A big bump means an early or big baby." Bump size reflects your body, muscle tone, and baby's position far more than timing or birth weight.
• "Second babies always come early." There's a slight tendency for first labors to run a bit longer, but it's not a rule you can bank on.
• "A full moon brings on labor." A popular belief with no solid evidence behind it.
• "Going past your date means something's wrong." Plenty of healthy babies arrive in week 41. Your provider will monitor you and discuss options if you go much past your date.

Signs your due date is near

As you approach the finish, watch for the classic signs that labor may be days away: the baby "dropping" lower into the pelvis (lightening), losing the mucus plug, more frequent and intense Braxton-Hicks tightenings, and a burst of nesting energy. True labor means regular contractions that get stronger, longer, and closer together — and of course, your water breaking. When in doubt, call your provider; that's what they're there for.

A second example

Say your last period started March 10 and your cycles run 32 days (four days longer than the textbook 28). The calculator adds 280 days plus the four-day adjustment, landing your due date in mid-December rather than early December — a meaningful shift that a plain 28-day calculator would miss. Try your own dates above and watch the date move as you change the cycle slider.

Dates worth putting on your calendar

One of the most useful things about knowing your due date is that it lets you mark the milestones ahead. Counting forward from your last period, these are the checkpoints most pregnancies share:

• ~Week 8–12: first prenatal visit and dating ultrasound.
• ~Week 12: end of the first trimester — many people share the news.
• ~Week 18–22: the anatomy scan, where you may learn the sex.
• ~Week 24–28: glucose screening for gestational diabetes.
• ~Week 28: start of the third trimester; visits become more frequent.
• ~Week 36: Group B strep swab; hospital bag should be ready.
• ~Week 37: baby is considered early term — full term begins at 39 weeks.

Drop these into your calendar the day you get your estimate and the whole pregnancy suddenly has a shape, instead of one far-off date at the end.

Using your date to get ready

Once you have a due date, you have a deadline to plan backward from. The smartest approach is to handle the essentials in the second trimester, while you have energy, rather than scrambling in the third. A few things worth having ready before the baby arrives — the items new parents reach for in the very first week:

• A reliable thermometer, so a midnight fever check doesn't become a panic.
• A feeding station stocked with bottles you trust.
• A sterilizer to keep that gear ready without living at the kitchen sink.

Save this page and check back any day of your pregnancy — the week count, trimester, and countdown all stay current. Your due date is the start of the story, not the whole plot, but it's a wonderful place to begin counting down.