Week by Week Due Date Calculator

What Is the Week by Week Due Date Calculator?

A week by week due date calculator works backward from your current gestational age. Enter how many weeks and days pregnant you are today, and it finds your LMP-equivalent date, adds 280 days, and returns your estimated due date — plus your trimester and a full pregnancy timeline.

Gestational age is counted from the first day of your last menstrual period, which is why the first two “weeks of pregnancy” happen before conception. This tool is built for the common situation where you know your current week — perhaps an ultrasound report just told you “12w3d” — but you never pinned down your LMP, or your cycles are irregular enough that LMP math feels shaky.

Once you know your week, the natural next step is learning what is happening inside it. Our pregnancy week by week guide walks through all 40+ weeks with baby size comparisons, development milestones, common symptoms, nutrition tips, and the tests typically scheduled — bookmark your current week and check back as you grow.

How to Use the Week by Week Calculator

1. Find your gestational age. Check your most recent ultrasound report (written like “12w3d”) or ask your clinician — or count forward in 7-day blocks from your last period’s first day.
2. Enter the weeks and the extra days. Both matter: 12w0d and 12w6d are nearly a week apart in due-date math.
3. Read your LMP-equivalent date. This is the “as if” last period that matches your current age — handy for forms and other calculators.
4. Read your estimated due date. The tool adds the standard 280 days (40 weeks) to the LMP equivalent.
5. Check your trimester and countdown. You will see which trimester you are in and how many days remain until your EDD.
6. Explore your week. Jump to your week in the guide linked above to see what your baby is doing right now.

Not sure of your current week yet? Our how many weeks pregnant calculator figures it out from your last period or conception date first.

How the Calculation Works

The logic is the classic Naegele’s Rule run in reverse. Normally the rule goes forward: add 280 days to the last menstrual period to get the due date. Here, because you supply the answer’s middle step — your current gestational age — the calculator first rewinds the calendar to reconstruct the LMP that would produce that age today, then runs the standard 280-day addition. The result is identical to entering that LMP directly into a regular due date calculator.

Pregnancy is then divided the way clinicians divide it: first trimester weeks 1–13, second trimester weeks 14–27, and third trimester weeks 28–40 and beyond. Full term officially spans 39 weeks to 40 weeks and 6 days, though the normal birth window stretches from 37 to 42 weeks. And a useful mental conversion: 40 weeks is about 9.2 calendar months, which is why “nine months” is only ever an approximation.

The 280-day rule itself is one of medicine’s oldest back-of-the-envelope formulas. In the early 1800s, the German obstetrician Franz Naegele popularized the method of adding nine months and seven days to the last menstrual period — arithmetic that lands on the same date as adding 280 days. It assumes a textbook 28-day cycle with ovulation on day 14, which is why it works best as a starting estimate and why dating by ultrasound or a known current week often improves on it.

Worked Example

Suppose today is October 5, 2026 and your scan report says you are 12 weeks and 3 days pregnant:

• Gestational age in days: 12 × 7 + 3 = 87 days.
• LMP-equivalent: October 5, 2026 minus 87 days = Friday, July 10, 2026.
• Estimated due date: July 10, 2026 plus 280 days = Friday, April 16, 2027.
• Trimester: first trimester (weeks 1–13).
• Days remaining: 280 − 87 = 193 days to go.

The same math works at any point: at 28 weeks and 0 days on the same date, the LMP equivalent would be Monday, March 23, 2026, the EDD Monday, December 28, 2026, and you would be opening the third trimester with 84 days remaining. One caution — the calculator trusts the week you enter, so double-check whether your report reads “12w3d” or “13w3d” before treating the output as final.

Once you have your dates, they become useful everywhere: plug the LMP equivalent into any standard due date calculator to double-check the result, write the EDD on medical forms and in your pregnancy journal, and use the remaining-days count to plan milestones like the anatomy scan (around 20 weeks) or the start of maternity leave.

How Accurate Is It?

The calculation itself is exact — the uncertainty lives entirely in the week you type in. If your gestational age comes from a first-trimester ultrasound, this is among the most reliable dating you can get; early scans measure the embryo when growth is most uniform, usually within a few days. An age counted from a well-remembered LMP with regular cycles is nearly as good. An age guessed from vague dates or a late-pregnancy scan carries more wobble, and the due date inherits all of it.

Keep expectations honest: studies estimate only about 5% of babies arrive on their exact due date, and most arrive within the 37–42 week window. Think of the EDD as the midpoint of a range, not an appointment. If ultrasound dating and your LMP disagree by more than a few days in early pregnancy, clinicians generally trust the scan — which is exactly the number to enter here.

It helps to know how the dating methods rank. A first-trimester ultrasound is the gold standard, accurate to within a few days. A well-remembered LMP with regular 28-day cycles comes next. Known conception or IVF transfer dates are also excellent, since ovulation day is certain. Late-pregnancy ultrasounds and fundal-height measurements are the least precise for dating — useful for tracking growth, but not the numbers you want anchoring your due date.

When to Talk to Your Doctor

Use this page for planning and curiosity, and let your prenatal appointments set the official dates — your clinician will confirm or adjust your dating at your first visits, sometimes with a dating ultrasound. Between appointments, call your provider promptly if you experience vaginal bleeding, severe abdominal pain or cramping, persistent severe headache, vision changes, sudden swelling of the face or hands, leaking fluid, or — once you are feeling regular movement — a noticeable decrease in your baby’s kicks.

None of these are reasons to panic; most turn out to be nothing serious, and calling early is always the right move. Bring your calculated dates and any scan reports to each visit so you and your clinician are always working from the same timeline.