Pregnancy Due Date Calculator

What this calculator does

The Pregnancy Due Date Calculator takes the first day of your last menstrual period (LMP) and your average cycle length and works out five things at once: your estimated due date, your current gestational age in weeks and days, which trimester you are in, how many days remain until the due date, and an estimated conception date. There is no submit button. Enter the LMP, leave the cycle length at 28 or change it to match yours, and the result card appears underneath.

This is the standard pregnancy math that obstetric clinics and midwives use to date a pregnancy when there is no ultrasound yet. It is the same method behind the date your provider will quote at a first appointment, with one small upgrade: the tool adjusts for cycle length, so it does not assume everyone ovulates on day 14 of a textbook 28-day cycle.

Who this is for

The most common reason people open a tool like this is the obvious one: they just saw a positive test and want to know roughly when the baby is due. But there are other honest reasons too.

• Someone trying to conceive who wants to know how far along they would be if their last period turned into a pregnancy.
• A partner or family member trying to make sense of a "I'm 18 weeks" comment without doing the math by hand.
• People in early pregnancy who want a quick way to see when each trimester ends so they can plan around appointments, work, or travel.
• Anyone whose cycle is not 28 days who wants a date that reflects their actual ovulation timing rather than the textbook assumption.

If you are already past your first ultrasound, your provider's dating will be more accurate than anything an LMP-based tool can produce. Use this for orientation, not as a substitute for that ultrasound.

How to use it, step by step

1. Enter the first day of your last menstrual period. This is the day bleeding started, not the day it ended, and not a guess at when you ovulated. Use the date picker; it accepts any real calendar date.
2. Set your average cycle length in days. The field defaults to 28. It accepts values from 20 to 45. If you genuinely do not know your cycle length, leave it at 28. Most people fall somewhere between 21 and 35 days.
3. Read the result card. It updates instantly. You will see the estimated due date written out long-form (with the weekday), your current gestational age as "X weeks and Y days", the trimester as a badge, the days remaining as a plain number, and the estimated conception date.

That is the entire interaction. There is nothing to save, no account, no calendar that tracks the pregnancy over time. Bookmark the page if you want to come back to it; the numbers will update on their own each day because gestational age is recomputed from today's date.

How the math actually works

Two formulas are doing all the work.

Due date. The tool applies Naegele's rule with a cycle-length adjustment:

due date = LMP + 280 days + (cycle length − 28)

280 days is 40 weeks, the textbook length of a pregnancy measured from the LMP. The (cycle length − 28) term shifts the date to account for ovulation timing. The classic 40-week figure quietly assumes you ovulated 14 days after your period started; if your cycle is longer, you probably ovulated later, so the due date moves later by the same amount. A 32-day cycle adds 4 days. A 25-day cycle subtracts 3.

Gestational age. Today's date minus your LMP, in days, then split into completed weeks and leftover days:

weeks = floor(daysPregnant / 7)
days = daysPregnant mod 7

So "13 weeks and 2 days" means 93 full days have passed since the first day of your last period. Because pregnancy is dated from the LMP by convention, you are counted as roughly two weeks pregnant around the time conception actually happened. This is intentional and matches how every clinic in the world keeps the numbers consistent.

Trimester. Read directly off the gestational age in weeks:

• First Trimester: weeks 0 through 12
• Second Trimester: weeks 13 through 26
• Third Trimester: week 27 onward

Different sources draw the trimester lines in slightly different places. Some put the third trimester at 28 weeks, some at 27. This tool uses the 13/27 split, which is one of the standard conventions, and that is the boundary the badge in the result card actually reflects.

Days remaining. Due date minus today, rounded up to a whole number of days, and floored at zero. Once you reach your due date, this stays at 0 rather than going negative.

Estimated conception date. LMP plus 14 days, adjusted for cycle length the same way the due date is:

conception = LMP + 14 days + (cycle length − 28)

This is an approximation of ovulation, not a measurement. The "+14 days" piece quietly assumes a 14-day luteal phase (the time between ovulation and the next period), which is the standard textbook assumption but not universally true; some people run shorter or longer luteal phases, and real ovulation can drift by several days even in a regular cycle.

A worked example

Suppose your LMP was March 1 and your cycle is 30 days. The adjustment is 30 − 28 = +2 days.

• Due date: March 1 + 280 days + 2 days = December 8 (the tool will spell out the weekday).
• Estimated conception: March 1 + 14 + 2 = March 17.
• If today is June 1: 92 days have passed since LMP. 92 ÷ 7 is 13 weeks with 1 day left, so you are 13 weeks and 1 day. That puts you in the Second Trimester. Days remaining: 190.

Change the cycle length to 26 and the due date moves to December 4; change it to 35 and it moves to December 13. The shift is one day per day of cycle difference, in either direction.

Common pitfalls

• Entering the wrong LMP. Use the day bleeding started, not the day it ended, and not the day you think you ovulated. If you are not sure of the exact day, the result will be off by however many days you are off.
• Leaving cycle length at 28 when yours is not. If your cycle is consistently 32 days, the unadjusted 40-week date is about 4 days too early. The field is right there; use it.
• Reading "2 weeks pregnant" as "you conceived two weeks ago". Gestational age is measured from the LMP, so it is always ahead of time-since-conception by about two weeks. A "6 weeks pregnant" reading means conception happened around 4 weeks ago.
• Trusting an LMP date with an irregular cycle. The adjustment helps with consistent long or short cycles. It does not help when ovulation timing varies from month to month. A first-trimester ultrasound is the only reliable answer in that case.
• Expecting the baby on the due date. Only about 5% of babies arrive on the exact date. A full-term birth runs anywhere from 37 to 42 weeks. Think of the due date as the middle of a normal delivery window, not a deadline.

When not to use this tool

This is a quick estimator built on a single formula. It is not the right tool for several real situations:

• IVF pregnancies. Due dates from IVF are calculated from the embryo transfer date and the age of the embryo at transfer (day 3 vs day 5 makes a difference). Use your fertility clinic's date.
• You don't know your LMP. If your last period is fuzzy or you were not tracking, the tool cannot help. A first-trimester dating ultrasound measures the embryo directly and is accurate to within a few days.
• You are past your first ultrasound. Once a sonographer has measured the embryo or fetus, that date is more reliable than any LMP-based calculation. Use the date from your provider.
• Medical decisions. Anything that depends on knowing exactly how far along you are, like medication dosing, prenatal testing windows, or induction timing, should come from your provider, not a web calculator.

If the result looks wrong

A few things to check before assuming the tool is broken:

• Did you enter the LMP in the correct year? The date picker reflects your device's locale, but a wrong year is the most common cause of a wildly off result.
• Did you put the LMP in the right field? It is the first field, labeled "First Day of Last Period".
• Is the cycle length sensible? Values below 20 or above 45 are rejected by the field. If you typed something extreme, clear it and try again.
• If your ultrasound dating differs from this by less than a week, that is normal. If it differs by more than a week, your provider will usually go with the ultrasound.

Adjacent concepts worth knowing

Gestational age vs. fetal age. Gestational age is dated from the LMP (what this tool shows). Fetal age is dated from conception and is about two weeks less. When you see a pregnancy described as "8 weeks", that is almost always gestational age.

Trimesters vs. weeks. Most modern obstetric care tracks weeks, not trimesters. Tests and milestones are scheduled by week (the first ultrasound around 8–12 weeks, the anatomy scan around 18–22, glucose screening around 24–28, group B strep around 36). The trimester badge is a friendly summary; the week count is the number your provider will actually use.

Term definitions. "Early term" is 37 to 38 weeks 6 days, "full term" is 39 to 40 weeks 6 days, "late term" is 41 to 41 weeks 6 days, and "post term" is 42 weeks and beyond. Anything before 37 weeks is preterm. Most providers do not let a pregnancy go beyond 42 weeks.

Ovulation. The estimated conception date here is really an ovulation estimate. If you want to plan around the fertile window directly, the Ovulation Calculator is the better tool for that question.

A note on accuracy and what this is not

Every date this tool produces is an estimate built on two assumptions: that your LMP is correct, and that you ovulated at a predictable point in your cycle. Both of those can be off in normal, healthy pregnancies. The 5% figure for babies born on their exact due date is well established; the spread on either side is wide enough that you should treat the due date as a midpoint, not a target. This is informational, not medical advice. Anything that matters should be confirmed with a qualified healthcare provider.